Serratia liquefaciens KM4 Improves Salt Stress Tolerance in Maize by Regulating Redox Potential, Ion Homeostasis, Leaf Gas Exchange and Stress-Related Gene Expression

Enhancing spinach (Spinacia oleracea L.) resilience in pesticide-contaminated soil: Role of pesticide-tolerant Ciceribacter azotifigens and Serratia marcescens in root architecture, leaf gas exchange attributes and antioxidant response restoration

This study unveils the detoxification potential of insecticide-tolerant plant beneficial bacteria (PBB), i.e., Ciceribacter azotifigens SF1 and Serratia marcescens SRB1, in spinach treated with fipronil (FIP), profenofos (PF) and chlorantraniliprole (CLP) insecticides. Increasing insecticide doses (25-400 μg kg-1 soil) significantly curtailed germination attributes and growth of spinach cultivated at both bench-scale and in greenhouse experiments. Profenofos at 400 μg kg-1 exhibited maximum inhibitory effects and reduced germination by 55%; root and shoot length by 78% and 81%, respectively; dry matter accumulation in roots and shoots by 79% and 62%, respectively; leaf number by 87% and leaf area by 56%. Insecticide application caused morphological distortion in root tips/surfaces, increased levels of oxidative stress, and cell death in spinach. Application of insecticide-tolerant SF1 and SRB1 strains relieved insecticide pressure resulting in overall improvement in growth and physiology of spinach grown under insecticide stress. Ciceribacter azotifigens improved germination rate (10%); root biomass (53%); shoot biomass (25%); leaf area (10%); Chl-a (45%), Chl-b (36%) and carotenoid (48%) contents of spinach at 25 μg CLP kg-1 soil. PBB inoculation reinvigorated the stressed spinach and modulated the synthesis of phytochemicals, proline, malondialdehyde (MDA), superoxide anions (O2•-), and hydrogen peroxide (H2O2). Scanning electron microscopy (SEM) revealed recovery in root tip morphology and stomatal openings on abaxial leaf surfaces of PBB-inoculated spinach grown with insecticides. Ciceribacter azotifigens inoculation significantly increased intrinsic water use efficiency, transpiration rate, vapor pressure deficit, intracellular CO2 concentration, photosynthetic rate, and stomatal conductance in spinach exposed to 25 μg FIP kg-1. Also, C. azotifigens and S. marcescens modulated the antioxidant defense systems of insecticide-treated spinach. Bacterial strains were strongly colonized to root surfaces of insecticide-stressed spinach seedlings as revealed under SEM. The identification of insecticide-tolerant PBBs such as C. azotifigens and S. marcescens hold the potential for alleviating abiotic stress to spinach, thereby fostering enhanced and safe production within polluted agroecosystems.

PGPR-Enabled bioremediation of pesticide and heavy metal-contaminated soil: A review of recent advances and emerging challenges

The excessive usage of agrochemicals, including pesticides, along with various reckless human actions, has ensued discriminating prevalence of pesticides and heavy metals (HMs) in crop plants and the environment. The enhanced exposure to these chemicals is a menace to living organisms. The pesticides may get bioaccumulated in the food chain, thereby leading to several deteriorative changes in the ecosystem health and a rise in the cases of some serious human ailments including cancer. Further, both HMs and pesticides cause some major metabolic disturbances in plants, which include oxidative burst, osmotic alterations and reduced levels of photosynthesis, leading to a decline in plant productivity. Moreover, the synergistic interaction between pesticides and HMs has a more serious impact on human and ecosystem health. Various attempts have been made to explore eco-friendly and environmentally sustainable methods of improving plant health under HMs and/or pesticide stress. Among these methods, the employment of PGPR can be a suitable and effective strategy for managing these contaminants and providing a long-term remedy. Although, the application of PGPR alone can alleviate HM-induced phytotoxicities; however, several recent reports advocate using PGPR with other micro- and macro-organisms, biochar, chelating agents, organic acids, plant growth regulators, etc., to further improve their stress ameliorative potential. Further, some PGPR are also capable of assisting in the degradation of pesticides or their sequestration, reducing their harmful effects on plants and the environment. This present review attempts to present the current status of our understanding of PGPR's potential in the remediation of pesticides and HMs-contaminated soil for the researchers working in the area.

Regulatory mechanisms of plant rhizobacteria on plants to the adaptation of adverse agroclimatic variables

The mutualistic plant rhizobacteria which improve plant development and productivity are known as plant growth-promoting rhizobacteria (PGPR). It is more significant due to their ability to help the plants in different ways. The main physiological responses, such as malondialdehyde, membrane stability index, relative leaf water content, photosynthetic leaf gas exchange, chlorophyll fluorescence efficiency of photosystem-II, and photosynthetic pigments are observed in plants during unfavorable environmental conditions. Plant rhizobacteria are one of the more crucial chemical messengers that mediate plant development in response to stressed conditions. The interaction of plant rhizobacteria with essential plant nutrition can enhance the agricultural sustainability of various plant genotypes or cultivars. Rhizobacterial inoculated plants induce biochemical variations resulting in increased stress resistance efficiency, defined as induced systemic resistance. Omic strategies revealed plant rhizobacteria inoculation caused the upregulation of stress-responsive genes-numerous recent approaches have been developed to protect plants from unfavorable environmental threats. The plant microbes and compounds they secrete constitute valuable biostimulants and play significant roles in regulating plant stress mechanisms. The present review summarized the recent developments in the functional characteristics and action mechanisms of plant rhizobacteria in sustaining the development and production of plants under unfavorable environmental conditions, with special attention on plant rhizobacteria-mediated physiological and molecular responses associated with stress-induced responses.

Serratia spp. as plant growth-promoting bacteria alleviating salinity, drought, and nutrient imbalance stresses

In agricultural environments, plants are often exposed to abiotic stresses including temperature extremes, salt stress, drought, and heavy metal soil contamination, which leads to significant economic losses worldwide. Especially salt stress and drought pose serious challenges since they induce ionic toxicity, osmotic stress, and oxidative stress in plants. A potential solution can be the application of bacteria of the Serratia spp. known to promote plant growth under normal conditions Thus the mini-review aims to summarize the current knowledge on plant growth promotion by Serratia spp. (under the conditions of salinity stress, drought, and nutrient deficit) and highlight areas for development in the field. So far, it has been proven that Serratia spp. strains exhibit a variety of traits contributing to enhanced plant growth and stress tolerance, such as phytohormone production, ACC deaminase activity, nitrogen fixation, P and Zn solubilization, antioxidant properties improvement, and modulation of gene expression. Nevertheless, further research on Serratia spp. is needed, especially on two subjects: elucidating its mechanisms of action on plants at the molecular level and the effects of Serratia spp. on the indigenous soil and plant microbiota and, particularly, the rhizosphere. In both cases, it is advisable to use omics techniques to gain in-depth insights into the issues. Additionally, some strains of Serratia spp. may be phytopathogens, therefore studies to rule out this possibility are recommended prior to field trials. It is believed that by improving said knowledge the potential of Serratia spp. to stimulate plant growth will increase and strains from the genus will serve as an eco-friendly biofertilizer in sustainable agriculture more often.

Bacillus cereus G2 alleviate salt stress in Glycyrrhiza uralensis Fisch. by balancing the downstream branches of phenylpropanoids and activating flavonoid biosynthesis

The salinity environment is one of the biggest threats to Glycyrrhiza uralensis Fisch. (G. uralensis) growth, resulting from the oxidative stress caused by excess reactive oxygen species (ROS). Flavonoids are the main pharmacodynamic composition and help maintain ROS homeostasis and mitigate oxidative damage in G. uralensis in the salinity environment. To investigate whether endophytic Bacillus cereus G2 can improve the salt-tolerance of G. uralensis through controlling flavonoid biosynthesis, the transcriptomic and physiological analysis of G. uralensis treated by G2 in the saline environment was conducted, focused on flavonoid biosynthesis-related pathways. Results uncovered that salinity inhibited flavonoids synthesis by decreasing the activities of phenylalanine ammonialyase (PAL) and 4-coumarate-CoA ligase (4CL) (42% and 39%, respectively) due to down-regulated gene Glyur000910s00020578 at substrate level, and then decreasing the activities of chalcone isomerase (CHI) and chalcone synthase (CHS) activities (50% and 42%, respectively) due to down-regulated genes Glyur006062s00044203 and Glyur000051s00003431, further decreasing isoliquiritigenin content by 53%. However, salt stress increased liquiritin content by 43%, which might be a protective mechanism of salt-treated G. uralensis seedlings. Interestingly, G2 enhanced PAL activity by 27% whereas reduced trans-cinnamate 4-monooxygenase (C4H) activity by 43% which could inhibit lignin biosynthesis but promote flavonoid biosynthesis of salt-treated G. uralensis at the substrate level. G2 decreased shikimate O-hydroxycinnamoyltransferase (HCT) activity by 35%, increased CHS activity by 54% through up-regulating the gene Glyur000051s00003431 encoding CHS, and increased CHI activity by 72%, thereby decreasing lignin (34%) and liquiritin (24%) content, but increasing isoliquiritigenin content (35%), which could mitigate oxidative damage and changed salt-tolerance mechanism of G. uralensis.

Unveiling the dynamic relationship of viruses and/or symbiotic bacteria with plant resilience in abiotic stress

In the ecosphere, plants interact with environmental biotic and abiotic partners, where unbalanced interactions can induce unfavourable stress conditions. Abiotic factors (temperature, water, and salt) are primarily required for plants healthy survival, and any change in their availability is reflected as a stress signal. In certain cases, the presence of infectious pathogens such as viruses, bacteria, fungi, protozoa, nematodes, and insects can also create stress conditions in plants, leading to the emergence of disease or deficiency symptoms. While these symptoms are often typical of abiotic or biotic stress, however, there are instances where they can intensify under specific conditions. Here, we primarily summarize the viral interactions with plants during abiotic stress to understand how these associations are linked together during viral pathogenesis. Secondly, focus is given to the beneficial effects of root-associated symbiotic bacteria in fulfilling the basic needs of plants during normal as well as abiotic stress conditions. The modulations of plant functional proteins, and their occurrence/cross-talk, with pathogen (virus) and symbiont (bacteria) molecules are also discussed. Furthermore, we have highlighted the biochemical and systematic adaptations that develop in plants due to bacterial symbiosis to encounter stress hallmarks. Lastly, directions are provided towards exploring potential rhizospheric bacteria to maintain plant-microbes ecosystem and manage abiotic stress in plants to achieve better trait health in the horticulture crops.

Growth and protein response of rice plant with plant growth-promoting rhizobacteria inoculations under salt stress conditions

Soil salinity has been one of the significant barriers to improving rice production and quality. According to reports, Bacillus spp. can be utilized to boost plant development in saline soil, although the molecular mechanisms behind the interaction of microbes towards salt stress are not fully known. Variations in rice plant protein expression in response to salt stress and plant growth-promoting rhizobacteria (PGPR) inoculations were investigated using a proteomic method and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Findings revealed that 54 salt-responsive proteins were identified by mass spectrometry analysis (LC-MS/MS) with the Bacillus spp. interaction, and the proteins were functionally classified as gene ontology. The initial study showed that all proteins were labeled by mass spectrometry analysis (LC-MS/MS) with Bacillus spp. interaction; the proteins were functionally classified into six groups. Approximately 18 identified proteins (up-regulated, 13; down-regulated, 5) were involved in the photosynthetic process. An increase in the expression of eight up-regulated and two down-regulated proteins in protein synthesis known as chaperones, such as the 60 kDa chaperonin, the 70 kDa heat shock protein BIP, and calreticulin, was involved in rice plant stress tolerance. Several proteins involved in protein metabolism and signaling pathways also experienced significant changes in their expression. The results revealed that phytohormones regulated the manifestation of various chaperones and protein abundance and that protein synthesis played a significant role in regulating salt stress. This study also described how chaperones regulate rice salt stress, their different subcellular localizations, and the activity of chaperones.

Enhancing the Biological Control of Mite Species Infesting Olive Trees through Application of Predatory Mite Agistemus exsertus Gonzalez (Acari: Stigmaeidae) and Eco-Friendly Natural Compounds

Olive (Olea europaea L.) is a subtropical tree cultivated in arid, dry and temperate regions. Olive orchards in Al-Jouf of Saudi Arabia are the largest worldwide and currently face harmful pest infestation. The present study aimed at evaluating the efficiency of the predatory mite Agistemus exsertus Gonzalez (Acari: Stigmaeidae) and the exogenously applied melatonin (MT), glycine betaine (GB) and 5-aminolevulinic acid (ALA) as eco-friendly approaches for enhancing the biological control of four mite species (Tegolophus hassani, Oxycenus niloticus, Aceria olivi and Tetranychus urticae) infesting olive trees in Al-Jouf under laboratory and field conditions. Field experiment was conducted on 6-year-old Manzanillo olive trees grown in a private orchard farm in Al-Jouf during two seasons, 2020 and 2021. Results revealed that A. exsertus developed successfully from egg to adult. The females of T. hassani, O. niloticus, A. olivi, and T. urticae required 7.36, 8.89, 9.98 and 8.38 days, respectively, to develop from egg to adult at 28°C and 65 ± 5% relative humidity. O. niloticus was the most preferred prey of A. exsertus. The net reproductive rate (R0 ) was 42.1, 38.7, 34.6 and 36.8 females/female/generation, the intrinsic rate of increase (rm ) was 0.27, 0.26, 0.23 and 0.20 females/female/day, and the mean generation time (T) was 16.2, 17.1, 18.6 and 17.2 days when a predator consumed T. hassani, O. niloticus, A. olivi and T. urticae, respectively. The adult female consumed daily about 114 O. niloticus, 105 A. olivi, 95 T. hassani and 15.2 T. urticae individuals, respectively. A. exsertus proved to be an effective biocontrol agent against mites infesting olive trees. In addition, the exogenous application of 1 mM MT, 15 mM GB and 25 mg/L ALA, alone or in combination, caused significant mortality for the four mites. Application of these natural compounds, alone or in combination, also significantly enhanced the growth, relative water content, relative chlorophyll, content of flavonoid and nutrients, antioxidant enzymes activities, stress-related genes expression and fruit yield and quality of the infested olive trees compared to non-treated infested trees. This study is the first that demonstrates the efficiency of these eco-friendly approaches for controlling mites infesting olive trees, and could be used as a replacement for the harmful chemical acaricides.

SNAC3 Transcription Factor Enhances Arsenic Stress Tolerance and Grain Yield in Rice (Oryza sativa L.) through Regulating Physio-Biochemical Mechanisms, Stress-Responsive Genes, and Cryptochrome 1b

Arsenic (As) is one of the toxic heavy metal pollutants found in the environment. An excess of As poses serious threats to plants and diminishes their growth and productivity. NAC transcription factors revealed a pivotal role in enhancing crops tolerance to different environmental stresses. The present study investigated, for the first time, the functional role of SNAC3 in boosting As stress tolerance and grain productivity in rice (Oryza sativa L.). Two SNAC3-overexpressing (SNAC3-OX) and two SNAC3-RNAi transgenic lines were created and validated. The wild-type and transgenic rice plants were exposed to different As stress levels (0, 25, and 50 µM). The results revealed that SNAC3 overexpression significantly improved rice tolerance to As stress and boosted grain yield traits. Under both levels of As stress (25 and 50 µM), SNAC3-OX rice lines exhibited significantly lower levels of oxidative stress biomarkers and OsCRY1b (cryptochrome 1b) expression, but they revealed increased levels of gas exchange characters, chlorophyll, osmolytes (soluble sugars, proteins, proline, phenols, and flavonoids), antioxidant enzymes (SOD, CAT, APX, and POD), and stress-tolerant genes expression (OsSOD-Cu/Zn, OsCATA, OsCATB, OsAPX2, OsLEA3, OsDREB2B, OsDREB2A, OsSNAC2, and OsSNAC1) in comparison to wild-type plants. By contrast, SNAC3 suppression (RNAi) reduced grain yield components and reversed the aforementioned measured physio-biochemical and molecular traits. Taken together, this study is the first to demonstrate that SNAC3 plays a vital role in boosting As stress resistance and grain productivity in rice through modulating antioxidants, photosynthesis, osmolyte accumulation, and stress-related genes expression, and may be a useful candidate for further genetic enhancement of stress resistance in many crops.

Seed Priming Modulates Physiological and Agronomic Attributes of Maize (Zea mays L.) under Induced Polyethylene Glycol Osmotic Stress

Drought and osmotic stresses are major threats to agricultural crops as they affect plants during their life cycle. The seeds are more susceptible to these stresses during germination and establishment of seedlings. To cope with these abiotic stresses, various seed priming techniques have broadly been used. The present study aimed to assess seed priming techniques under osmotic stress. Osmo-priming with chitosan (1 and 2%), hydro-priming with distilled water, and thermo-priming at 4 °C were used on the physiology and agronomy of Zea mays L. under polyethylene glycol (PEG-4000)-induced osmotic stress (-0.2 and -0.4 MPa). The vegetative response, osmolyte content, and antioxidant enzymes of two varieties (Pearl and Sargodha 2002 White) were studied under induced osmotic stress. The results showed that seed germination and seedling growth were inhibited under osmotic stress and germination percentage, and the seed vigor index was enhanced in both varieties of Z. mays L. with chitosan osmo-priming. Osmo-priming with chitosan and hydro-priming with distilled water modulated the level of photosynthetic pigments and proline, which were reduced under induced osmotic stress; moreover, the activities of antioxidant enzymes were improved significantly. In conclusion, osmotic stress adversely affects the growth and physiological attributes; on the contrary, seed priming ameliorated the stress tolerance resistance of Z. mays L. cultivars to PEG-induced osmotic stress by activating the natural antioxidation enzymatic system and accumulating osmolytes.

Bacterial-Mediated Salinity Stress Tolerance in Maize (Zea mays L.): A Fortunate Way toward Sustainable Agriculture

Sustainable agriculture is threatened by salinity stress because of the low yield quality and low crop production. Rhizobacteria that promote plant growth modify physiological and molecular pathways to support plant development and reduce abiotic stresses. The recent study aimed to assess the tolerance capacity and impacts of Bacillus sp. PM31 on the growth, physiological, and molecular responses of maize to salinity stress. In comparison to uninoculated plants, the inoculation of Bacillus sp. PM31 improved the agro-morphological traits [shoot length (6%), root length (22%), plant height (16%), fresh weight (39%), dry weight (29%), leaf area (11%)], chlorophyll [Chl a (17%), Chl b (37%), total chl (22%)], carotenoids (15%), proteins (40%), sugars (43%), relative water (11%), flavonoids (22%), phenols (23%), radical scavenging capacity (13%), and antioxidants. The Bacillus sp. PM31-inoculated plants showed a reduction in the oxidative stress indicators [electrolyte leakage (12%), H2O2 (9%), and MDA (32%)] as compared to uninoculated plants under salinity and increased the level of osmolytes [free amino acids (36%), glycine betaine (17%), proline (11%)]. The enhancement of plant growth under salinity was further validated by the molecular profiling of Bacillus sp. PM31. Moreover, these physiological and molecular mechanisms were accompanied by the upregulation of stress-related genes (APX and SOD). Our study found that Bacillus sp. PM31 has a crucial and substantial role in reducing salinity stress through physiological and molecular processes, which may be used as an alternative approach to boost crop production and yield.

The Contribution of PGPR in Salt Stress Tolerance in Crops: Unravelling the Molecular Mechanisms of Cross-Talk between Plant and Bacteria

Soil salinity is a major abiotic stress in global agricultural productivity with an estimated 50% of arable land predicted to become salinized by 2050. Since most domesticated crops are glycophytes, they cannot be cultivated on salt soils. The use of beneficial microorganisms inhabiting the rhizosphere (PGPR) is a promising tool to alleviate salt stress in various crops and represents a strategy to increase agricultural productivity in salt soils. Increasing evidence underlines that PGPR affect plant physiological, biochemical, and molecular responses to salt stress. The mechanisms behind these phenomena include osmotic adjustment, modulation of the plant antioxidant system, ion homeostasis, modulation of the phytohormonal balance, increase in nutrient uptake, and the formation of biofilms. This review focuses on the recent literature regarding the molecular mechanisms that PGPR use to improve plant growth under salinity. In addition, very recent -OMICs approaches were reported, dissecting the role of PGPR in modulating plant genomes and epigenomes, opening up the possibility of combining the high genetic variations of plants with the action of PGPR for the selection of useful plant traits to cope with salt stress conditions.

Metal-Resistant PGPR Strain Azospirillum brasilense EMCC1454 Enhances Growth and Chromium Stress Tolerance of Chickpea (Cicer arietinum L.) by Modulating Redox Potential, Osmolytes, Antioxidants, and Stress-Related Gene Expression

Heavy metal stress, including from chromium, has detrimental effects on crop growth and yields worldwide. Plant growth-promoting rhizobacteria (PGPR) have demonstrated great efficiency in mitigating these adverse effects. The present study investigated the potential of the PGPR strain Azospirillum brasilense EMCC1454 as a useful bio-inoculant for boosting the growth, performance and chromium stress tolerance of chickpea (Cicer arietinum L.) plants exposed to varying levels of chromium stress (0, 130 and 260 µM K₂Cr₂O₇). The results revealed that A. brasilense EMCC1454 could tolerate chromium stress up to 260 µM and exhibited various plant growth-promoting (PGP) activities, including nitrogen fixation, phosphate solubilization, and generation of siderophore, trehalose, exopolysaccharide, ACC deaminase, indole acetic acid, and hydrolytic enzymes. Chromium stress doses induced the formation of PGP substances and antioxidants in A. brasilense EMCC1454. In addition, plant growth experiments showed that chromium stress significantly inhibited the growth, minerals acquisition, leaf relative water content, biosynthesis of photosynthetic pigments, gas exchange traits, and levels of phenolics and flavonoids of chickpea plants. Contrarily, it increased the concentrations of proline, glycine betaine, soluble sugars, proteins, oxidative stress markers, and enzymatic (CAT, APX, SOD, and POD) and non-enzymatic (ascorbic acid and glutathione) antioxidants in plants. On the other hand, A. brasilense EMCC1454 application alleviated oxidative stress markers and significantly boosted the growth traits, gas exchange characteristics, nutrient acquisition, osmolyte formation, and enzymatic and non-enzymatic antioxidants in chromium-stressed plants. Moreover, this bacterial inoculation upregulated the expression of genes related to stress tolerance (CAT, SOD, APX, CHS, DREB2A, CHI, and PAL). Overall, the current study demonstrated the effectiveness of A. brasilense EMCC1454 in enhancing plant growth and mitigating chromium toxicity impacts on chickpea plants grown under chromium stress circumstances by modulating the antioxidant machinery, photosynthesis, osmolyte production, and stress-related gene expression.

Enhancing Stevia rebaudiana growth and yield through exploring beneficial plant-microbe interactions and their impact on the underlying mechanisms and crop sustainability

Stevia rebaudiana is an important medicinal plant which represents the most important sugar substitute in many countries. Poor seed germination of this plant is a critical problem that affects the final yield and the availability of the products in the market. Continuous cropping without supplying soil nutrients is also a serious issue as it results in declining soil fertility. This review highlights the important use of beneficial bacteria for the enhancement of Stevia rebaudiana growth and its dynamic interactions in the phyllosphere, rhizosphere, and endosphere. Fertilizers can increase crop yield and preserve and improve soil fertility. There is a rising concern that prolonged usage of chemical fertilizers may have negative impacts on the ecosystem of the soil. On the other hand, soil health and fertility are improved by plant growth-promoting bacteria which could eventually increase plant growth and productivity. Accordingly, a biocompatible strategy involving beneficial microorganisms inoculation is applied to boost plant growth and reduce the negative effects of chemical fertilizers. Plants benefit extensively from endophytic bacteria, which promote growth and induce resistance to pathogens and stresses. Additionally, several plant growth-promoting bacteria are able to produce amino acids, polyamines, and hormones that can be used as alternatives to chemicals. Therefore, understanding the dynamic interactions between bacteria and Stevia can help make the favorable bacterial bio-formulations, use them more effectively, and apply them to Stevia to improve yield and quality.

Organic Amendments Improved the Productivity and Bio-Fortification of Fine Rice by Improving Physiological Responses and Nutrient Homeostasis under Salinity Stress

Salinity stress (SS) is major abiotic stress that is seriously limiting crop production across the globe. The application of organic amendments (OA) mitigate the effects of salinity and improves soil health and crop production on a sustainable basis. However, limited studies are conducted to determine the impact of farmyard manure (FYM) and press mud (PM) on the performance of rice crop. Therefore, we performed this study to determine the impacts of FYM and PM on the growth, physiological and biochemical attributes, yield, and grain bio-fortification of rice crop under SS. The experiment was comprised of SS levels; control, 6 and 12 dS m^-1 SS and OA; control, FYM: 5%, press mud 5% and combination of FYM (5%) + PM (5%). Soil salinity imposed deleterious impacts on the growth, yield, and grain quality of rice, however, OA appreciably offset the deleterious impacts of SS and improved the growth, yield, and grain bio-fortification of rice crop. The combined application of FYM + PM improved the growth and yield of rice through an increase in chlorophyll contents, leaf water contents, anti-oxidant activities (ascorbate peroxidise: APX; catalase: CAT, peroxidise: POD and ascorbic acid: AsA), K⁺ accumulation and decrease in Na⁺/K⁺ ratio, electrolyte leakage, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), Na⁺ accumulation. Moreover, the combined application of FYM + PM significantly improved the grain protein (5.84% and 12.90%), grain iron (40.95% and 42.37%), and grain zinc contents (36.81% and 50.93%) at 6 and 12 dS m^-1 SS. Therefore, this study suggested that the application of FYM and PM augmented the growth, yield, physiology, biochemistry, and grain bio-fortification of rice and proved to be a good practice for better rice production in salt-affected soils.

Burkholderia cepacia CS8 improves phytoremediation potential of Calendula officinalis for tannery solid waste polluted soil

Microbes have shown potential for the bioremediation of tannery waste polluted soil. During our previous study, it was observed that heavy metal resistant Burkholderia cepacia CS8 augmented growth and phytoremediation capability of an ornamental plant. Objective of the present research work was to evaluate the capability of B. cepacia CS8 assisted Calendula officinalis plants for the phytoremediation of tannery solid waste (TSW) polluted soil. The TSW treatment significantly reduced growth attributes and photosynthetic pigments in C. officinalis. However, supplementation of B. cepacia CS8 which exhibited substantial tolerance to the TSW amended soil, augmented growth traits, carotenoid, proline, and antioxidant enzymes level in C. officinalis under toxic and nontoxic regimes. Inoculation of B. cepacia CS8 augmented plant growth (shoot length 13%, root length 11%), physiological attributes (chlorophyll a 14%, chlorophyll b 17%), antioxidant enzyme activities (peroxidase 24%, superoxide dismutase 31% and catalase 19%), improved proline 36%, phenol 32%, flavonoids 14% and declined malondialdehyde (MDA) content 15% and hydrogen peroxide (H2O2) level 12% in C. officinalis at TSW10 stress compared with relevant un-inoculated plants of TSW10 treatment. Moreover, B. cepacia CS8 application enhanced labile metals in soil and subsequent metal uptake, such as Cr 19%, Cd 22%, Ni 35%, Fe 18%, Cu 21%, Pb 34%, and Zn 30%, respectively in C. officinalis plants subjected to TSW10 stress than that of analogous un-inoculated treatment. Higher plant stress tolerance and improved phytoremediation potential through microbial inoculation will assist in the retrieval of agricultural land in addition to the renewal of native vegetation.

Application of Plant Growth-Promoting Bacteria from Cape Verde to Increase Maize Tolerance to Salinity

Salinity constitutes a major abiotic factor that negatively affects crop productivity. Inoculation with plant growth-promoting bacteria (PGPB) is proven to increase plant tolerance to abiotic stresses and enhance plant growth, development and productivity. The present study aims to increase the resilience of crops to salinity using bacteria from the microbiome of plants growing in saline environments. For that, the halotolerance of bacteria present in the roots of natural plants growing on Sal Island, which is characterized by its arid environment and maritime influence, was determined, with some strains having extreme halotolerance. Their ability to produce plant growth-promoting traits was evaluated, with most strains increasing indole acetic acid (26-418%), siderophore (>300%) and alginate (2-66%) production and phosphate solubilization (13-100%) under salt stress. The strains evidencing the best performance were inoculated in maize (Zea mays L.) plants and their influence on plant growth and biochemical status was evaluated. Results evidenced bacterial ability to especially increase proline (55-191%), whose osmotic, antioxidant and protein-protecting properties reduced protein damage in salt-stressed maize plants, evidencing the potential of PGPB to reduce the impact of salinity on crops. Enhanced nutrition, phytohormone production and osmolyte synthesis along with antioxidant response all contribute to increasing plant tolerance to salt stress.

Grapevine (Vitis vinifera) responses to salt stress and alkali stress: transcriptional and metabolic profiling

BACKGROUND

Soil salinization and alkalization are widespread environmental problems that limit grapevine (Vitis vinifera L.) growth and yield. However, little is known about the response of grapevine to alkali stress. This study investigated the differences in physiological characteristics, chloroplast structure, transcriptome, and metabolome in grapevine plants under salt stress and alkali stress.

RESULTS

We found that grapevine plants under salt stress and alkali stress showed leaf chlorosis, a decline in photosynthetic capacity, a decrease in chlorophyll content and Rubisco activity, an imbalance of Na⁺ and K⁺, and damaged chloroplast ultrastructure. Fv/Fm decreased under salt stress and alkali stress. NPQ increased under salt stress whereas decreased under alkali stress. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed the differentially expressed genes (DEGs) induced by salt stress and alkali stress were involved in different biological processes and have varied molecular functions. The expression of stress genes involved in the ABA and MAPK signaling pathways was markedly altered by salt stress and alkali stress. The genes encoding ion transporter (AKT1, HKT1, NHX1, NHX2, TPC1A, TPC1B) were up-regulated under salt stress and alkali stress. Down-regulation in the expression of numerous genes in the 'Porphyrin and chlorophyll metabolism', 'Photosynthesis-antenna proteins', and 'Photosynthesis' pathways were observed under alkali stress. Many genes in the 'Carbon fixation in photosynthetic organisms' pathway in salt stress and alkali stress were down-regulated. Metabolome showed that 431 and 378 differentially accumulated metabolites (DAMs) were identified in salt stress and alkali stress, respectively. L-Glutamic acid and 5-Aminolevulinate involved in chlorophyll synthesis decreased under salt stress and alkali stress. The abundance of 19 DAMs under salt stress related to photosynthesis decreased. The abundance of 16 organic acids in salt stress and 22 in alkali stress increased respectively.

CONCLUSIONS

Our findings suggested that alkali stress had more adverse effects on grapevine leaves, chloroplast structure, ion balance, and photosynthesis than salt stress. Transcriptional and metabolic profiling showed that there were significant differences in the effects of salt stress and alkali stress on the expression of key genes and the abundance of pivotal metabolites in grapevine plants.

Synergistic effects of Azospirillum brasilense and Bacillus cereus on plant growth, biochemical attributes and molecular genetic regulation of steviol glycosides biosynthetic genes in Stevia rebaudiana

The current study aimed to scale up the favorable bio-stimulants for enhancing the growth and breeding strategies of Stevia rebaudiana to increase sugar productivity. Inoculation of 45-day-old S. rebaudiana plantlets with Bacillus cereus and Azospirillum brasilense alone or in combination for 30 days allowed comparisons among their effects on enhancement and improvement of plant growth, production of bioactive compounds and expression of steviol glycoside genes. B. cereus SrAM1 isolated from surface-sterilized Stevia rebaudiana leaves was molecularly identified using 16s rRNA and tested for its ability to promote plant growth. Beneficial endophytic B. cereus SrAM1 induced all plant growth-promoting traits, except solubilization of phosphate, therefore it showed high effectiveness in the promotion of growth and production of bioactive compounds. Treatment of plants with B. cereus SrAM1 alone revealed carbohydrates content of 278.99 mg/g, total soluble sugar of 114.17 mg/g, total phenolics content of 34.05 mg gallic acid equivalent (GAE)/g dry weight) and total antioxidants activity of 32.33 mg (A.A)/g dry weight). Thus, plantlets inoculated with B. cereus SrAM1 alone exhibited the greatest responses in physiological and morphological parameters, but plantlets inoculated with B. cereus SrAM1 + A. brasilense showed a maximal upregulation of genes responsible for the biosynthesis of steviol glycosides (Kaurene oxidase, ent-KO; UDP-dependent glycosyl transferases of UGT85C2, UGT74G1, UGT76G1). Taken together, the used bacterial strains, particularly B. cereus SrAM1 could significantly improve the growth of S. rebaudiana via dynamic interactions in plants.

Isolation of bacterial strains from compost teas and screening of their PGPR properties on potato plants

The beneficial effect of compost and compost tea on plant growth and protection is mainly associated with the microbial diversity and the presence of bacteria with plant growth-promoting effect. PGPR are considered as eco-friendly bio-fertilizers that may reduce the use of chemical pesticides and fertilizers. Three composts (AT, A10, and A30) were previously prepared from industrial wastes (olive mill wastewater, olive pomace, coffee ground, and phosphogypsum). In the present study, we isolated three bacterial strains from the compost teas. The phylogenetic identification of these bacterial strains (B.AT, B.A10, and B.A30) showed that they correspond to Serratia liquefaciens (B.AT and B.A10) and Achromobacter spanius (B.A30) species. A further characterization of the PGPR traits of these bacteria showed that they produce siderophore, exopolysaccharides, and IAA. Their effect on potato plant growth, yields, and tuber quality was performed under field culture conditions. Results showed that these strains can be characterized as PGPR, the best effect on potato plant growth was observed with Serratia liquefaciens (B.AT), the best yield and tuber quality was observed with Serratia liquefaciens (B.A10) while bacterial treatment with Achromobacter spanius (B.A30) is a Cd-tolerant PGPR.

Stress-Tolerant Endophytic Isolate Priestia aryabhattai BPR-9 Modulates Physio-Biochemical Mechanisms in Wheat (Triticum aestivum L.) for Enhanced Salt Tolerance

In efforts to improve plant productivity and enhance defense mechanisms against biotic and abiotic stresses, endophytic bacteria have been used as an alternative to chemical fertilizers and pesticides. In the current study, 25 endophytic microbes recovered from plant organs of Triticum aestivum L. (wheat) were assessed for biotic (phyto-fungal pathogens) and abiotic (salinity, drought, and heavy metal) stress tolerance. Among the recovered isolates, BPR-9 tolerated maximum salinity (18% NaCl), drought (15% PEG-6000), and heavy metals (µg mL^-1): Cd (1200), Cr (1000), Cu (1000), Pb (800), and Hg (30). Based on phenotypic and biochemical characteristics, as well as 16S rDNA gene sequencing, endophytic isolate BPR-9 was recognized as Priestia aryabhattai (accession no. OM743254.1). This isolate was revealed as a powerful multi-stress-tolerant crop growth promoter after extensive in-vitro testing for plant growth-promoting attributes, nutrient (phosphate, P; potassium, K; and zinc, Zn) solubilization efficiency, extracellular enzyme (protease, cellulase, amylase, lipase, and pectinase) synthesis, and potential for antagonistic activity against important fungal pathogens viz. Alternaria solani, Rhizoctonia solani, Fusarium oxysporum, and Ustilaginoidea virens. At elevated salt levels, increases were noted in indole-3-acetic acid; siderophores; P, K, and Zn-solubilization; ACC deaminase; and ammonia synthesized by Priestia aryabhattai. Additionally, under in-vitro plant bioassays, wheat seedlings inoculated with P. aryabhattai experienced superior growth compared to non-inoculated seedlings in high salinity (0-15% NaCl) environment. Under NaCl stress, germination rate, plant length, vigor indices, and leaf pigments of wheat seedlings significantly increased following P. aryabhattai inoculation. Furthermore, at 2%-NaCl, B. aryabhattai greatly and significantly (p ≤ 0.05) decreased relative leaf water content, membrane damage, and electrolyte leakage compared with the non-inoculated control. Catalase, superoxide dismutase, and peroxidase activity increased by 29, 32, and 21%, respectively, in wheat seedlings exposed to 2% NaCl and inoculated with the bacteria. The present findings demonstrate that endophytic P. aryabhattai strains might be used in the future as a multi-stress reducer and crop growth promoter in agronomically important crops including cereals.

Bacillus thuringiensis PM25 ameliorates oxidative damage of salinity stress in maize via regulating growth, leaf pigments, antioxidant defense system, and stress responsive gene expression

Soil salinity is the major abiotic stress that disrupts nutrient uptake, hinders plant growth, and threatens agricultural production. Plant growth-promoting rhizobacteria (PGPR) are the most promising eco-friendly beneficial microorganisms that can be used to improve plant responses against biotic and abiotic stresses. In this study, a previously identified B. thuringiensis PM25 showed tolerance to salinity stress up to 3 M NaCl. The Halo-tolerant Bacillus thuringiensis PM25 demonstrated distinct salinity tolerance and enhance plant growth-promoting activities under salinity stress. Antibiotic-resistant Iturin C (ItuC) and bio-surfactant-producing (sfp and srfAA) genes that confer biotic and abiotic stresses were also amplified in B. thuringiensis PM25. Under salinity stress, the physiological and molecular processes were followed by the over-expression of stress-related genes (APX and SOD) in B. thuringiensis PM25. The results detected that B. thuringiensis PM25 inoculation substantially improved phenotypic traits, chlorophyll content, radical scavenging capability, and relative water content under salinity stress. Under salinity stress, the inoculation of B. thuringiensis PM25 significantly increased antioxidant enzyme levels in inoculated maize as compared to uninoculated plants. In addition, B. thuringiensis PM25-inoculation dramatically increased soluble sugars, proteins, total phenols, and flavonoids in maize as compared to uninoculated plants. The inoculation of B. thuringiensis PM25 significantly reduced oxidative burst in inoculated maize under salinity stress, compared to uninoculated plants. Furthermore, B. thuringiensis PM25-inoculated plants had higher levels of compatible solutes than uninoculated controls. The current results demonstrated that B. thuringiensis PM25 plays an important role in reducing salinity stress by influencing antioxidant defense systems and abiotic stress-related genes. These findings also suggest that multi-stress tolerant B. thuringiensis PM25 could enhance plant growth by mitigating salt stress, which might be used as an innovative tool for enhancing plant yield and productivity.

Halotolerant Bacillus altitudinis WR10 improves salt tolerance in wheat via a multi-level mechanism

Soil salinity is an important abiotic stress factor that seriously affects the crop growth and yield. Use of plant-derived microorganisms is a promising strategy to alleviate salt stress. In a previous study, the endophytic strain Bacillus altitudinis WR10 isolated from wheat roots showed high salt resistance. In this study, we investigated the efficacy of WR10 in improving the salt tolerance of wheat and its potential mechanisms using a hydroponic test. Under salt stress, WR10 inoculation significantly increased the lengths and dry weights of the roots and shoots, indicating that WR10 improves wheat salt tolerance at the seedling stage. WR10 inoculation significantly reduced Na⁺ accumulation and enhanced K⁺, P, and Ca²⁺ uptake in salt-stressed plants, which can be attributed to the upregulated gene expression of H⁺-ATPase as well as the P-solubilizing and biofilm-producing characteristics of WR10. At the transcriptional level, L-ascorbate peroxidase (APX), glutathione (GSH) synthetase related to GSH biosynthesis, and phenylpropanoid biosynthesis genes (CYP73A, 4CL, and CAD) were significantly upregulated, whereas those of GSH metabolism genes (glutathione S-transferase and gamma-glutamyltranspeptidase) were significantly downregulated in WR10-applied wheat roots under salt stress. These changes increased the APX activity and GSH levels and resulted in a decrease in hydrogen peroxide levels. Additionally, a decrease in proline content was observed in WR10-inoculated plants under salt stress because of WR10-induced upregulation of proline dehydrogenase gene expression. These results provide supporting evidence that WR10 improves wheat salt tolerance via more than one mechanism and open a window of opportunity for WR10 application in salinized soil.

Characterization of Siccibacter sp. Strain C2 a Novel Rhizobacterium that Enhances Tolerance of Barley to Salt Stress

Plant growth promoting rhizobacteria (PGPR) arouse an increasing interest as an eco-friendly solution for improving crop tolerance to environmental stresses. In this study, we report the characterization of a novel halotolerant PGPR strain (named C2) identified in a screen of rhizospheric bacterial isolates from southeast of Tunisia. Phylogenetic analysis showed that strain C2 is most likely affiliated to the genus Siccibacter with Siccibacter turicensis as the closest species (98.19%). This strain was able to perform phosphate solubilization and production of indole acetic acid (IAA), siderophores, hydrogen cyanide (HCN), as well as different hydrolytic enzymes (proteases, amylases, cellulases, and lipases). The potential of strain C2 in enhancing salt stress tolerance of Hordeum vulgare was also investigated. Our greenhouse inoculation assays showed that strain C2 promotes barley growth in the presence of 400 mM NaCl by increasing biomass, root length, and chlorophyll contents. It has a positive effect on the photosynthetic efficiency, concomitantly with lower intercellular CO₂ contents, compared to non-inoculated plants. Moreover, barley inoculation with strain C2 under salt stress, resulted in higher accumulation of proline and soluble sugars and alleviate the oxidative stress by decreasing hydrogen peroxide and malondialdehyde contents. Remarkably, this positive effect corroborates with a significant activation in the expression of a subset of barley stress responsive genes, including HVA1, HvDREB1, HvWRKY38 and HvP5CS. In summary, Siccibacter sp. strain C2 is able to enhance barley salt stress tolerance and should be leveraged in developing sustainable practices for cereal crop production.

Unraveling the potential of pesticide-tolerant Pseudomonas sp. augmenting biological and physiological attributes of Vigna radiata (L.) under pesticide stress

In the agricultural domain, chemical pesticides are repetitively and harshly used to kill harmful pests, but they often pose a serious threat to microbial diversity, soil fertility and agricultural output. To deal with these problems, pesticide-tolerant plant growth promoting (PGP) rhizobacterial strains are often used to combat pesticidal toxicity. Here, Pseudomonas sp. PGR-11 (accession no. OM348534), recovered from a Vigna radiata (L.) rhizosphere, produced various growth regulating (GR) substances, including indole-3-acetic acid (IAA; 82.5 ± 9.2 μg mL^-1), enzyme 1-aminocyclopropane 1-carboxylate (ACC) deaminase (μM α-ketobutyrate mg^-1 protein h^-1), siderophores and ammonia. Strain PGR-11 grew well when cultured in growth medium with added metalaxyl (MTXL; 1200 μg mL^-1), carbendazim (CBZM; 800 μg mL^-1) and tebuconazole (TBZL; 1600 μg mL^-1). Pseudomonas sp. synthesized PGP substances even in the presence of increasing doses of pesticides. The phytotoxicity of the tested pesticides was assessed both in vitro and under pot-house conditions using a Vigna radiata (L.) crop. Increasing concentrations of chemical pesticides negatively impacted the growth, physiological and biochemical features. However, pesticide-tolerant Pseudomonas sp. relieved the toxicity and improved the biological attributes of the plant. Bio-inoculated plants showed significant enhancement in germination attributes, dry biomass, symbiotic features and yield features when compared to un-inoculated ones. Furthermore, with 100 μg metalaxyl kg^-1 soil, strain PGR-11 increased the chl-a, chl-b, total chlorophyll, carotenoids, SPAD index, photosystem efficiency (Fv/Fm), PSII quantum yield (FPSII), photochemical quenching (qP) and non-photochemical quenching (NpQ) content by 12, 19, 16, 27, 34, 41, 26, 29 and 33%, respectively, over un-inoculated but pesticide-treated plants. Additionally, inoculation of Pseudomonas sp. with 100 μg tebuconazole kg^-1 soil caused a significant (p ≤ 0.05) enhancement in transpiration rate (E), stomatal conductance (g _s), photosynthetic rate (P _N), vapor pressure deficit (kPa) and internal CO₂ concentration (C _i) of 19, 26, 23, 28 and 34%, respectively. Conclusively, the power to tolerate abnormally high pesticide concentration, the capacity to produce/secrete PGP substances even in a pesticide-stressed medium and the potential for improving/increasing the growth and physiology of plants by pesticide detoxification makes Pseudomonas sp. PGR-11 a fascinating choice for augmenting the productivity of V. radiata (L.) even in pesticide-stressed soils. The current findings will be helpful for exploring pesticide-tolerant ACC-deaminase-positive microbial strains as gifted entities for the environmental bioremediation of pesticides.

Salt Stress Tolerance-Promoting Proteins and Metabolites under Plant-Bacteria-Salt Stress Tripartite Interactions

The rapid increase in soil salinization has impacted agricultural output and poses a threat to food security. There is an urgent need to focus on improving soil fertility and agricultural yield, both of which are severely influenced by abiotic variables such as soil salinity and sodicity. Abiotic forces have rendered one-third of the overall land unproductive. Microbes are the primary answer to the majority of agricultural production’s above- and below-ground problems. In stressful conditions, proper communication between plants and beneficial microbes is critical for avoiding plant cell damage. Many chemical substances such as proteins and metabolites synthesized by bacteria and plants mediate communication and stress reduction. Metabolites such as amino acids, fatty acids, carbohydrates, vitamins, and lipids as well as proteins such as aquaporins and antioxidant enzymes play important roles in plant stress tolerance. Plant beneficial bacteria have an important role in stress reduction through protein and metabolite synthesis under salt stress. Proper genomic, proteomic and metabolomics characterization of proteins and metabolites’ roles in salt stress mitigation aids scientists in discovering a profitable avenue for increasing crop output. This review critically examines recent findings on proteins and metabolites produced during plant-bacteria interaction essential for the development of plant salt stress tolerance.

Exogenous Caffeine (1,3,7-Trimethylxanthine) Application Diminishes Cadmium Toxicity by Modulating Physio-Biochemical Attributes and Improving the Growth of Spinach (Spinacia oleracea L.)

Leafy vegetables usually absorb and retain heavy metals more readily than most of the other crop plants, and thus contribute ≥70% of the total cadmium (Cd) intake of humans. Caffeine mediates plant growth and has proved to be beneficial against pathogens and insects. Therefore, it was hypothesized that foliar applications of caffeine could alter metabolism and reduce Cd toxicity in spinach (Spinacia oleracea L.). Seven-day old spinach seedlings were provided with Cd (0, 50, and 100 µM) stress. Caffeine (0, 5, or 10 mM) foliar spray was given twice at after 20 days of seeds germination with an interval of one week. In results, Cd stress reduced photosynthetic pigments biosynthesis, increased oxidative stress, imbalanced nutrient retention, and inhibited plant growth. On the other hand, the caffeine-treated spinach plants showed better growth owing to the enhanced biosynthesis of chlorophylls, better oxidative defense systems, and lower accumulation and transport of Cd within the plant tissues. Furthermore, caffeine application enhanced the accumulation of the proline and ascorbic acid, but reduced MDA and H2O2 contents and Cd in plant leaves, and ultimately improved mineral nutrition of spinach plants exposed to different Cd regimes. In conclusion, exogenous application of caffeine significantly diminishes Cd stress by modulating physiological, biochemical, and growth attributes of spinach (Spinacia oleracea L.)

Bacillus mycoides PM35 Reinforces Photosynthetic Efficiency, Antioxidant Defense, Expression of Stress-Responsive Genes, and Ameliorates the Effects of Salinity Stress in Maize

Soil salinity is one of the abiotic constraints that imbalance nutrient acquisition, hampers plant growth, and leads to potential loss in agricultural productivity. Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can alleviate the adverse impacts of salt stress by mediating molecular, biochemical, and physiological status. In the present study, the bacterium Bacillus mycoides PM35 showed resistance up to 3 M NaCl stress and exhibited plant growth-promoting features. Under salinity stress, the halo-tolerant bacterium B. mycoides PM35 showed significant plant growth-promoting traits, such as the production of indole acetic acid, siderophore, ACC deaminase, and exopolysaccharides. Inoculation of B. mycoides PM35 alleviated salt stress in plants and enhanced shoot and root length under salinity stress (0, 300, 600, and 900 mM). The B. mycoides PM35 alleviated salinity stress by enhancing the photosynthetic pigments, carotenoids, radical scavenging capacity, soluble sugars, and protein content in inoculated maize plants compared to non-inoculated plants. In addition, B. mycoides PM35 significantly boosted antioxidant activities, relative water content, flavonoid, phenolic content, and osmolytes while reducing electrolyte leakage, H₂O₂, and MDA in maize compared to control plants. Genes conferring abiotic stress tolerance (CzcD, sfp, and srfAA genes) were amplified in B. mycoides PM35. Moreover, all reactions are accompanied by the upregulation of stress-related genes (APX and SOD). Our study reveals that B. mycoides PM35 is capable of promoting plant growth and increasing agricultural productivity.

PGPR-Mediated Salt Tolerance in Maize by Modulating Plant Physiology, Antioxidant Defense, Compatible Solutes Accumulation and Bio-Surfactant Producing Genes

Salinity stress is a barrier to crop production, quality yield, and sustainable agriculture. The current study investigated the plant growth promotion, biochemical and molecular characterization of bacterial strain Enterobacter cloacae PM23 under salinity stress (i.e., 0, 300, 600, and 900 mM). E. cloacae PM23 showed tolerance of up to 3 M NaCl when subjected to salinity stress. Antibiotic-resistant Iturin C (ItuC) and bio-surfactant-producing genes (sfp and srfAA) were amplified in E. cloacae PM23, indicating its multi-stress resistance potential under biotic and abiotic stresses. Moreover, the upregulation of stress-related genes (APX and SOD) helped to mitigate salinity stress and improved plant growth. Inoculation of E. cloacae PM23 enhanced plant growth, biomass, and photosynthetic pigments under salinity stress. Bacterial strain E. cloacae PM23 showed distinctive salinity tolerance and plant growth-promoting traits such as indole-3-acetic acid (IAA), siderophore, ACC deaminase, and exopolysaccharides production under salinity stress. To alleviate salinity stress, E. cloacae PM23 inoculation enhanced radical scavenging capacity, relative water content, soluble sugars, proteins, total phenolic, and flavonoid content in maize compared to uninoculated (control) plants. Moreover, elevated levels of antioxidant enzymes and osmoprotectants (Free amino acids, glycine betaine, and proline) were noticed in E. cloacae PM23 inoculated plants compared to control plants. The inoculation of E. cloacae PM23 significantly reduced oxidative stress markers under salinity stress. These findings suggest that multi-stress tolerant E. cloacae PM23 could enhance plant growth by mitigating salt stress and provide a baseline and ecofriendly approach to address salinity stress for sustainable agriculture.

Halotolerant Rhizobacteria for Salinity-Stress Mitigation: Diversity, Mechanisms and Molecular Approaches

Agriculture is the best foundation for human livelihoods, and, in this respect, crop production has been forced to adopt sustainable farming practices. However, soil salinity severely affects crop growth, the degradation of soil quality, and fertility in many countries of the world. This results in the loss of profitability, the growth of agricultural yields, and the step-by-step decline of the soil nutrient content. Thus, researchers have focused on searching for halotolerant and plant growth-promoting bacteria (PGPB) to increase soil fertility and productivity. The beneficial bacteria are frequently connected with the plant rhizosphere and can alleviate plant growth under salinity stress through direct or indirect mechanisms. In this context, PGPB have attained a unique position. The responses include an increased rate of photosynthesis, high production of antioxidants, osmolyte accumulation, decreased Na+ ions, maintenance of the water balance, a high germination rate, and well-developed root and shoot elongation under salt-stress conditions. Therefore, the use of PGPB as bioformulations under salinity stress has been an emerging research avenue for the last few years, and applications of biopesticides and biofertilizers are being considered as alternative tools for sustainable agriculture, as they are ecofriendly and minimize all kinds of stresses. Halotolerant PGPB possess greater potential for use in salinity-affected soil as sustainable bioinoculants and for the bioremediation of salt-affected soil.

Mulberry based zinc nano-particles mitigate salinity induced toxic effects and improve the grain yield and zinc bio-fortification of wheat by improving antioxidant activities, photosynthetic performance, and accumulation of osmolytes and hormones

Salinity stress (SS) is a challenging abiotic stress that limits crop growth and productivity. Sustainable and cost effective methods are needed to improve crop production and decrease the deleterious impacts of SS. Zinc (Zn) nano-particles (NPs) have emerged as an important approach to regulating plant tolerance against SS. However, the mechanisms of SS tolerance mediated by Zn-NPs are not fully explained. Thus, this study was performed to explore the role of Zn-NPs (seed priming and foliar spray) in reducing the deleterious impacts of SS on wheat plants. The study comprised different SS levels: control, 6 and 12 dS m^-1, and different Zn-NPs treatments: control, seed priming (40 ppm), foliar spray (20 ppm), and their combination. Salinity stress markedly reduced plant growth, biomass, and grain yield. This was associated with enhanced electrolyte leakage (EL), malondialdehyde (MDA), hydrogen peroxide (H₂O₂), sodium (Na), chloride (Cl) accumulation, reduced photosynthetic pigments, relative water contents (RWC), photosynthetic rate (Pn), transpiration rate (Tr), stomata conductance (Gs), water use efficiency (WUE), free amino acids (FAA), total soluble protein (TSP), indole acetic acid (IAA), gibberellic acid (GA), and nutrients (Ca, Mg, K, N, and P). However, the application of Zn-NPs significantly improved the yield of the wheat crop, which was associated with reduced abscisic acid (ABA), MDA, H₂O₂ concentration, and EL, owing to improved antioxidant activities, and an increase in RWC, Pn, Tr, WUE, and the accumulation of osmoregulating compounds (proline, soluble sugars, TSP, and FAA) and hormones (GA and IAA). Furthermore, Zn-NPs contrasted the salinity-induced uptake of toxic ions (Na and Cl) and increased the uptake of Ca, K, Mg, N, and P. Additionally, Zn-NPs application substantially increased the wheat grain Zn bio-fortification. Our results support previous findings on the role of Zn-NPs in wheat growth, yield, and grain Zn bio-fortification, demonstrating that beneficial effects are obtained under normal as well as adverse conditions, thanks to improved physiological activity and the accumulation of useful compounds. This sets the premise for general use of Zn-NPs in wheat, to which aim more experimental evidence is intensively being sought. Further studies are needed at the genomic, transcriptomic, proteomic, and metabolomic level to better acknowledge the mechanisms of general physiological enhancement observed with Zn-NPs application.

Understanding the potential of root microbiome influencing salt-tolerance in plants and mechanisms involved at the transcriptional and translational level

Soil salinity severely affects plant growth and development and imparts inevitable losses to crop productivity. Increasing the concentration of salts in the vicinity of plant roots has severe consequences at the morphological, biochemical, and molecular levels. These include loss of chlorophyll, decrease in photosynthetic rate, reduction in cell division, ROS generation, inactivation of antioxidative enzymes, alterations in phytohormone biosynthesis and signaling, and so forth. The association of microorganisms, viz. plant growth-promoting rhizobacteria, endophytes, and mycorrhiza, with plant roots constituting the root microbiome can confer a greater degree of salinity tolerance in addition to their inherent ability to promote growth and induce defense mechanisms. The mechanisms involved in induced stress tolerance bestowed by these microorganisms involve the modulation of phytohormone biosynthesis and signaling pathways (including indole acetic acid, gibberellic acid, brassinosteroids, abscisic acid, and jasmonic acid), accumulation of osmoprotectants (proline, glycine betaine, and sugar alcohols), and regulation of ion transporters (SOS1, NHX, HKT1). Apart from this, salt-tolerant microorganisms are known to induce the expression of salt-responsive genes via the action of several transcription factors, as well as by posttranscriptional and posttranslational modifications. Moreover, the potential of these salt-tolerant microflora can be employed for sustainably improving crop performance in saline environments. Therefore, this review will briefly focus on the key responses of plants under salinity stress and elucidate the mechanisms employed by the salt-tolerant microorganisms in improving plant tolerance under saline environments.

Plants Saline Environment in Perception with Rhizosphere Bacteria Containing 1-Aminocyclopropane-1-Carboxylate Deaminase

Soil salinity stress has become a serious roadblock for food production worldwide since it is one of the key factors affecting agricultural productivity. Salinity and drought are predicted to cause considerable loss of crops. To deal with this difficult situation, a variety of strategies have been developed, including plant breeding, plant genetic engineering, and a wide range of agricultural practices, including the use of plant growth-promoting rhizobacteria (PGPR) and seed biopriming techniques, to improve the plants' defenses against salinity stress, resulting in higher crop yields to meet future human food demand. In the present review, we updated and discussed the negative effects of salinity stress on plant morphological parameters and physio-biochemical attributes via various mechanisms and the beneficial roles of PGPR with 1-Aminocyclopropane-1-Carboxylate(ACC) deaminase activity as green bio-inoculants in reducing the impact of saline conditions. Furthermore, the applications of ACC deaminase-producing PGPR as a beneficial tool in seed biopriming techniques are updated and explored. This strategy shows promise in boosting quick seed germination, seedling vigor and plant growth uniformity. In addition, the contentious findings of the variation of antioxidants and osmolytes in ACC deaminase-producing PGPR treated plants are examined.

In Silico Study of the RSH (RelA/SpoT Homologs) Gene Family and Expression Analysis in Response to PGPR Bacteria and Salinity in Brassica napus

Among several mechanisms involved in the plant stress response, synthesis of guanosine tetra and pentaphosphates (alarmones), homologous to the bacterial stringent response, is of crucial importance. Plant alarmones affect, among others, photosynthetic activity, metabolite accumulation, and nutrient remobilization, and thus regulate plant growth and development. The plant RSH (RelA/SpoT homolog) genes, that encode synthetases and/or hydrolases of alarmones, have been characterized in a limited number of plant species, e.g., Arabidopsis thaliana, Oryza sativa, and Ipomoea nil. Here, we used dry-to-wet laboratory research approaches to characterize RSH family genes in the polyploid plant Brassica napus. There are 12 RSH genes in the genome of rapeseed that belong to four types of RSH genes: 6 RSH1, 2 RSH2, 3 RSH3, and 1 CRSH. BnRSH genes contain 13-24 introns in RSH1, 2-6 introns in RSH2, 1-6 introns in RSH3, and 2-3 introns in the CRSH genes. In the promoter regions of the RSH genes, we showed the presence of regulatory elements of the response to light, plant hormones, plant development, and abiotic and biotic stresses. The wet-lab analysis showed that expression of BnRSH genes is generally not significantly affected by salt stress, but that the presence of PGPR bacteria, mostly of Serratia sp., increased the expression of BnRSH significantly. The obtained results show that BnRSH genes are differently affected by biotic and abiotic factors, which indicates their different functions in plants.

Rhizosphere Bacteria in Plant Growth Promotion, Biocontrol, and Bioremediation of Contaminated Sites: A Comprehensive Review of Effects and Mechanisms

Agriculture in the 21st century is facing multiple challenges, such as those related to soil fertility, climatic fluctuations, environmental degradation, urbanization, and the increase in food demand for the increasing world population. In the meanwhile, the scientific community is facing key challenges in increasing crop production from the existing land base. In this regard, traditional farming has witnessed enhanced per acre crop yields due to irregular and injudicious use of agrochemicals, including pesticides and synthetic fertilizers, but at a substantial environmental cost. Another major concern in modern agriculture is that crop pests are developing pesticide resistance. Therefore, the future of sustainable crop production requires the use of alternative strategies that can enhance crop yields in an environmentally sound manner. The application of rhizobacteria, specifically, plant growth-promoting rhizobacteria (PGPR), as an alternative to chemical pesticides has gained much attention from the scientific community. These rhizobacteria harbor a number of mechanisms through which they promote plant growth, control plant pests, and induce resistance to various abiotic stresses. This review presents a comprehensive overview of the mechanisms of rhizobacteria involved in plant growth promotion, biocontrol of pests, and bioremediation of contaminated soils. It also focuses on the effects of PGPR inoculation on plant growth survival under environmental stress. Furthermore, the pros and cons of rhizobacterial application along with future directions for the sustainable use of rhizobacteria in agriculture are discussed in depth.

Plant growth promoting bacteria for combating salinity stress in plants - Recent developments and prospects: A review

Soil salinity has emerged as a great threat to the agricultural ecosystems throughout the globe. Many continents of the globe are affected by salinity and crop productivity is severely affected. Anthropogenic activities leading to the degradation of agricultural land have also accelerated the rate of salinization in arid and semi-arid regions. Several approaches are being evaluated for remediating saline soil and restoring their productivity. Amongst these, utilization of plant growth promoting bacteria (PGPB) has been marked as a promising tool. This greener approach is suitable for simultaneous reclamation of saline soil and improving the productivity. Salt-tolerant PGPB utilize numerous mechanisms that affect physiological, biochemical, and molecular responses in plants to cope with salt stress. These mechanisms include osmotic adjustment by ion homeostasis and osmolyte accumulation, protection from free radicals by the formation of free radicals scavenging enzymes, oxidative stress responses and maintenance of growth parameters by the synthesis of phytohormones and other metabolites. As salt-tolerant PGPB elicit better plant survival under salinity, they are the potential candidates for enhancing agricultural productivity. The present review focuses on the various mechanisms used by PGPB to improve plant health under salinity. Recent developments and prospects to facilitate better understanding on the functioning of PGPB for ameliorating salt stress in plants are emphasized.

Exogenously Applied Gibberellic Acid Enhances Growth and Salinity Stress Tolerance of Maize through Modulating the Morpho-Physiological, Biochemical and Molecular Attributes

Soil salinity is the major limiting factor restricting plant growth and development. Little is known about the comparative and combined effects of gibberellic acid (GA3) seed priming and foliar application on maize under salt stress. The current study determined the impact of different application methods of GA3 on morpho-physiological, biochemical and molecular responses of maize seedlings under three salinity stress treatments (no salinity, moderate salinity-6 dS m-1, and severe salinity-12 dS m-1). The GA3 treatments consisted of control, hydro-priming (HP), water foliar spray (WFS), HP + WFS, seed priming with GA3 (GA3P, 100 mg L-1), foliar spray with GA3 (GA3FS, 100ppm) and GA3P + GA3FS. Salt stress particularly at 12 dS m-1 reduced the length of shoots and roots, fresh and dry weights, chlorophyll, and carotenoid contents, K+ ion accumulation and activities of antioxidant enzymes, while enhanced the oxidative damage and accumulation of the Na+ ion in maize plants. Nevertheless, the application of GA3 improved maize growth, reduced oxidative stress, and increased the antioxidant enzymes activities, antioxidant genes expression, and K+ ion concentration under salt stress. Compared with control, the GA3P + GA3FS recorded the highest increase in roots and shoots length (19-37%), roots fresh and dry weights (31-43%), shoots fresh and dry weights (31-47%), chlorophyll content (21-70%), antioxidant enzymes activities (73.03-150.74%), total soluble protein (13.05%), K+ concentration (13-23%) and antioxidants genes expression levels under different salinity levels. This treatment also reduced the H2O2 content, and Na+ ion concentration. These results indicated that GA3P + GA3FS could be used as an effective tool for improving the maize growth and development, and reducing the oxidative stress in salt-contaminated soils.

Genomic Analysis of Serratia plymuthica MBSA-MJ1: A Plant Growth Promoting Rhizobacteria That Improves Water Stress Tolerance in Greenhouse Ornamentals

Water stress decreases the health and quality of horticulture crops by inhibiting photosynthesis, transpiration, and nutrient uptake. Application of plant growth promoting rhizobacteria (PGPR) can increase the growth, stress tolerance, and overall quality of field and greenhouse grown crops subjected to water stress. Here, we evaluated Serratia plymuthica MBSA-MJ1 for its ability to increase plant growth and quality of Petunia × hybrida (petunia), Impatiens walleriana (impatiens), and Viola × wittrockiana (pansy) plants recovering from severe water stress. Plants were treated weekly with inoculum of MBSA-MJ1, and plant growth and quality were evaluated 2 weeks after recovery from water stress. Application of S. plymuthica MBSA-MJ1 increased the visual quality and shoot biomass of petunia and impatiens and increased the flower number of petunia after recovery from water stress. In addition, in vitro characterizations showed that MBSA-MJ1 is a motile bacterium with moderate levels of antibiotic resistance that can withstand osmotic stress. Further, comprehensive genomic analyses identified genes putatively involved in bacterial osmotic and oxidative stress responses and the synthesis of osmoprotectants and vitamins that could potentially be involved in increasing plant water stress tolerance. This work provides a better understanding of potential mechanisms involved in beneficial plant-microbe interactions under abiotic stress using a novel S. plymuthica strain as a model.

Facile Bioself-Assembled Crystals in Plants Promote Photosynthesis and Salt Stress Resistance

Salty soil is a global problem that has adverse effects on plants. We demonstrate that bioself-assembled molybdenum-sulfur (Mo-S) crystals formed by the foliar application of MoCl₅ and cysteine augment the photosynthesis of plants treated with 200 mM salt for 7 days by promoting Ca²⁺ signal transduction and free radical scavenging. Reductions in glutathione and phytochelatins were attributed to the biosynthesized Mo-S crystals. Plants embedded with the Mo-S crystals and exposed to salty soil exhibited carbon assimilation rates, photosynthesis rates (Fv/Fm), and electron transport rates (ETRs) that were increased by 40%, 63-173%, and 50-78%, respectively, compared with those of plants without Mo-S crystals. Increased compatible osmolyte levels and decreased levels of oxidative damage, stomatal conductance (0.63-0.42 mmol m² s^-1), and transpiration (22.9-15.3 mmol m² s^-1), free radical scavenging, and calcium-dependent protein kinase, and Ca²⁺ signaling pathway activation were evidenced by transcriptomics and metabolomics. The bioself-assembled crystals originating from ions provide a method for protecting plant development under adverse conditions.

Transcriptomics of Biostimulation of Plants Under Abiotic Stress

Plant biostimulants are compounds, living microorganisms, or their constituent parts that alter plant development programs. The impact of biostimulants is manifested in several ways: via morphological, physiological, biochemical, epigenomic, proteomic, and transcriptomic changes. For each of these, a response and alteration occur, and these alterations in turn improve metabolic and adaptive performance in the environment. Many studies have been conducted on the effects of different biotic and abiotic stimulants on plants, including many crop species. However, as far as we know, there are no reviews available that describe the impact of biostimulants for a specific field such as transcriptomics, which is the objective of this review. For the commercial registration process of products for agricultural use, it is necessary to distinguish the specific impact of biostimulants from that of other legal categories of products used in agriculture, such as fertilizers and plant hormones. For the chemical or biological classification of biostimulants, the classification is seen as a complex issue, given the great diversity of compounds and organisms that cause biostimulation. However, with an approach focused on the impact on a particular field such as transcriptomics, it is perhaps possible to obtain a criterion that allows biostimulants to be grouped considering their effects on living systems, as well as the overlap of the impact on metabolism, physiology, and morphology occurring between fertilizers, hormones, and biostimulants.

Improved salt tolerance of Chenopodium quinoa Willd. contributed by Pseudomonas sp. strain M30-35

Background

Plant-growth-promoting rhizobacteria (PGPR) can promote plant growth and enhance plant tolerance to salt stress. Pseudomonas sp. strain M30-35 might confer abiotic stress tolerance to its host plants. We evaluated the effects of M30-35 inoculation on the growth and metabolite accumulation of Chenopodium quinoa Willd. during salt stress growth conditions.

Methods

The effects of M30-35 on the growth of C. quinoa seedlings were tested under salt stress. Seedling growth parameters measured included chlorophyll content, root activity, levels of plant- phosphorus (P), and saponin content.

Results

M30-35 increased biomass production and root activity compared to non-inoculated plants fertilized with rhizobia and plants grown under severe salt stress conditions. The photosynthetic pigment content of chlorophyll a and b were higher in M30-35-inoculated C. quinoa seedlings under high salt stress conditions compared to non-inoculated seedlings. The stability of P content was also maintained. The content of saponin, an important secondary metabolite in C. quinoa, was increased by the inoculation of M30-35 under 300 mM NaCl conditions.

Conclusion

Inoculation of M30-35 rescues the growth diminution of C. quinoa seedlings under salt stress.

Salicylic Acid Improves Boron Toxicity Tolerance by Modulating the Physio-Biochemical Characteristics of Maize (Zea mays L.) at an Early Growth Stage

The boron (B) concentration surpasses the plant need in arid and semi-arid regions of the world, resulting in phyto-toxicity. Salicylic acid (SA) is an endogenous signaling molecule responsible for stress tolerance in plants and is a potential candidate for ameliorating B toxicity. In this study, the effects of seed priming with SA (0, 50, 100 and 150 µM for 12 h) on the growth, pigmentation and mineral concentrations of maize (Zea mays L.) grown under B toxicity were investigated. One-week old seedlings were subjected to soil spiked with B (0, 15 and 30 mg kg−1 soil) as boric acid. Elevating concentrations of B reduced the root and shoot length, but these losses were significantly restored in plants raised from seeds primed with 100 µM of SA. The B application decreased the root and shoot fresh/dry biomasses significantly at 30 mg kg−1 soil. The chlorophyll and carotenoid contents decreased with increasing levels of B, while the contents of anthocyanin, H2O2, ascorbic acid (ASA) and glycinebetaine (GB) were enhanced. The root K and Ca contents were significantly increased, while a reduction in the shoot K contents was recorded. The nitrate concentration was significantly higher in the shoot as compared to the root under applied B toxic regimes. However, all of these B toxicity effects were diminished with 100 µM SA applications. The current study outcomes suggested that the exogenously applied SA modulates the response of plants grown under B toxic conditions, and hence could be used as a plant growth regulator to stimulate plant growth and enhance mineral nutrient uptake under B-stressed conditions.

Peptone-Induced Physio-Biochemical Modulations Reduce Cadmium Toxicity and Accumulation in Spinach (Spinacia oleracea L.)

The accumulation of cadmium (Cd) in edible plant parts and fertile lands is a worldwide problem. It negatively influences the growth and productivity of leafy vegetables (e.g., spinach, Spinacia oleracea L.), which have a high tendency to radially accumulate Cd. The present study investigated the influences of peptone application on the growth, biomass, chlorophyll content, gas exchange parameters, antioxidant enzymes activity, and Cd content of spinach plants grown under Cd stress. Cd toxicity negatively affected spinach growth, biomass, chlorophyll content, and gas exchange attributes. However, it increased malondialdehyde (MDA), hydrogen peroxide (H₂O₂), electrolyte leakage (EL), proline accumulation, ascorbic acid content, Cd content, and activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) in spinach plants. The exogenous foliar application of peptone increased the growth, biomass, chlorophyll content, proline accumulation, and gas exchange attributes of spinach plants. Furthermore, the application of peptone decreased Cd uptake and levels of MDA, H₂O₂, and EL in spinach by increasing the activity of antioxidant enzymes. This enhancement in plant growth and photosynthesis might be due to the lower level of Cd accumulation, which in turn decreased the negative impacts of oxidative stress in plant tissues. Taken together, the findings of the study revealed that peptone is a promising plant growth regulator that represents an efficient approach for the phytoremediation of Cd-polluted soils and enhancement of spinach growth, yield, and tolerance under a Cd-dominant environment.

Biofilm forming rhizobacteria enhance growth and salt tolerance in sunflower plants by stimulating antioxidant enzymes activity

Salinity stress is one of the major environmental stresses that impose global socio-economic impacts, as well as hindering crop productivity. Halotolerant plant growth-promoting rhizobacteria (PGPR) having potential to cope with salinity stress can be employed to counter this issue in eco-friendly way. In the present investigation, halotolerant PGPR strains, AP6 and PB5, were isolated from saline soil and characterized for their biochemical, molecular and physiological traits. Sequencing of 16 S rRNA gene and comparative analysis confirmed the taxonomic affiliation of AP6 with Bacillus licheniformis and PB5 with Pseudomonas plecoglossicida. The study was carried out in pots with different levels of induced soil salinity viz. 0, 5, 10 and 15 dSm^-1 to evaluate the potential of bacterial inoculants in counteracting salinity stress in sunflower at different plant growth stages (30, 45 and 60 days after sowing). Both the bacterial inoculants were capable of producing indole acetic acid and biofilm, solubilizing inorganic rock phosphate, and also expressed ACC deaminase activity. The PGPR inoculated plants showed significantly higher fresh and dry biomass, plant height, root length and yield plant^-1. Ameliorative significance of applied bacterial inoculants was also evidenced by mitigating oxidative stress through upregulation of catalase (CAT), superoxide dismutase (SOD) and guaiacol peroxidase (GPX) antioxidant enzymes. Increase in photosynthetic pigments, gas exchange activities and nutrient uptake are crucial salt stress adaptations, which were enhanced with the inoculation of salt tolerant biofilm producing PGPR in sunflower plants. Although increase in salinity stress levels has gradually decreased the plant's output compared to non-salinized plants, the plants inoculated with PGPR confronted salinity stress in much better way than uninoculated plants. Owing to the wide action spectrum of these bacterial inoculants, it was concluded that these biofilm PGPR could serve as effective bioinoculants and salinity stress alleviator for sunflower (oil seed crop) by increasing crop productivity in marginalized agricultural systems.

Plant-Growth-Promoting Bacteria Mitigating Soil Salinity Stress in Plants

Soil deterioration has led to problems with the nutrition of the world’s population. As one of the most serious stressors, soil salinization has a negative effect on the quantity and quality of agricultural production, drawing attention to the need for environmentally friendly technologies to overcome the adverse effects. The use of plant-growth-promoting bacteria (PGPB) can be a key factor in reducing salinity stress in plants as they are already introduced in practice. Plants having halotolerant PGPB in their root surroundings improve in diverse morphological, physiological, and biochemical aspects due to their multiple plant-growth-promoting traits. These beneficial effects are related to the excretion of bacterial phytohormones and modulation of their expression, improvement of the availability of soil nutrients, and the release of organic compounds that modify plant rhizosphere and function as signaling molecules, thus contributing to the plant’s salinity tolerance. This review aims to elucidate mechanisms by which PGPB are able to increase plant tolerance under soil salinity.

When Salt Meddles Between Plant, Soil, and Microorganisms

In extreme environments, the relationships between species are often exclusive and based on complex mechanisms. This review aims to give an overview of the microbial ecology of saline soils, but in particular of what is known about the interaction between plants and their soil microbiome, and the mechanisms linked to higher resistance of some plants to harsh saline soil conditions. Agricultural soils affected by salinity is a matter of concern in many countries. Soil salinization is caused by readily soluble salts containing anions like chloride, sulphate and nitrate, as well as sodium and potassium cations. Salinity harms plants because it affects their photosynthesis, respiration, distribution of assimilates and causes wilting, drying, and death of entire organs. Despite these life-unfavorable conditions, saline soils are unique ecological niches inhabited by extremophilic microorganisms that have specific adaptation strategies. Important traits related to the resistance to salinity are also associated with the rhizosphere-microbiota and the endophytic compartments of plants. For some years now, there have been studies dedicated to the isolation and characterization of species of plants' endophytes living in extreme environments. The metabolic and biotechnological potential of some of these microorganisms is promising. However, the selection of microorganisms capable of living in association with host plants and promoting their survival under stressful conditions is only just beginning. Understanding the mechanisms of these processes and the specificity of such interactions will allow us to focus our efforts on species that can potentially be used as beneficial bioinoculants for crops.

Fertigation of Ajwain (Trachyspermum ammi L.) with Fe-Glutamate Confers Better Plant Performance and Drought Tolerance in Comparison with FeSO4

Shortage of fresh water limits crop yield. Different ways including the use of chemicals are being employed for the improvement in yield through induction of plant performance. In the present study, ajwain plants grown under water stress and normal irrigation conditions were fertigated with Fe-chelated glutamate (Fe-Glu), as a foliar spray for the induction of plant performance in comparison with FeSO4. Water shortage adversely affected the plant growth and seed yield, associated with decreased uptake of water and nutrients, along with perturbations in different physio-biochemical attributes. On the other hand, Fe-Glu and FeSO4 fertigation improved plant performance under water stress and normal irrigation conditions. Fe-Glu and FeSO4 fertigation ameliorated the adverse effects of water stress on biomass and seed production, improved water and nutrients uptake, increased the accumulation of essential amino acids, leaf chlorophyll and carotenoids, and reduced the lipid peroxidation due to the induction of antioxidative mechanisms. Fertigation of Fe-Glu and FeSO4 also improved Fe uptake and conferred better mobility and availability of Fe for plants when applied in chelated form. Overall, a significant improvement in ajwain performance under water stress and normal irrigation conditions was recorded due to the fertigation of Fe-Glu as compared with FeSO4.

Foliar Spray of Fe-Asp Confers Better Drought Tolerance in Sunflower as Compared with FeSO4: Yield Traits, Osmotic Adjustment, and Antioxidative Defense Mechanisms

Different techniques are being employed to reduce the adverse effects of water stress on seed yield and quality of crop plants. The current study aimed to improve the water stress tolerance of field-grown sunflower by foliar-supplied ecofriendly iron-chelated aspartate (Fe-Asp) in comparison with FeSO4. Water stress decreased the plant growth and yield, accompanied with disturbed water relations, nutrient acquisition, accumulation of amino acids, and antioxidative defense mechanisms. However, lipid peroxidation, total anthocyanin, and photosynthetic pigments were increased. Fertigation of FeSO4 and Fe-Asp as foliar sprays proved effective to reduce the negativities of limited irrigation on biomass production and seed yield, accompanied with a reduction in lipid peroxidation and improvements in water relations, antioxidative defense mechanisms, and leaf photosynthetic pigments. In comparison with FeSO4, foliary applied Fe-Asp better improved the plant water relations with more accumulation of essential amino acids and nutrient acquisition, especially leaf aspartate (Asp) and Fe accumulation which showed better translocation. Overall, foliary applied Fe-Asp proved better for induction of drought tolerance in sunflower plants as compared with FeSO4. The study recommended the use of the ecofriendly Fe-Asp as a foliar spray for better growth and production of sunflower under limited irrigation.

Mechanistic Insights of the Interaction of Plant Growth-Promoting Rhizobacteria (PGPR) With Plant Roots Toward Enhancing Plant Productivity by Alleviating Salinity Stress

Agriculture plays an important role in a country's economy. The sector is challenged by many stresses, which led to huge loss in plant productivity worldwide. The ever-increasing population, rapid urbanization with shrinking agricultural lands, dramatic change in climatic conditions, and extensive use of agrochemicals in agricultural practices that caused environmental disturbances confront mankind of escalating problems of food security and sustainability in agriculture. Escalating environmental problems and global hunger have led to the development and adoption of genetic engineering and other conventional plant breeding approaches in developing stress-tolerant varieties of crops. However, these approaches have drawn flaws in their adoption as the process of generating tolerant varieties takes months to years in bringing the technology from the lab to the field. Under such scenario, sustainable and climate-smart agricultural practices that avail bacterial usage open the avenues in fulfilling the incessant demand for food for the global population. Ensuring stability on economic fronts, bacteria minimizes plant salt uptake by trapping ions in their exopolysaccharide matrix besides checking the expression of Na+/H+ and high-affinity potassium transporters. Herein we describe information on salinity stress and its effect on plant health as well as strategies adopted by plant growth-promoting rhizobacteria (PGPR) in helping plants to overcome salinity stress and in mitigating loss in overall plant productivity. It is believed that acquisition of advanced knowledge of plant-beneficial PGPR will help in devising strategies for sustainable, environment-friendly, and climate-smart agricultural technologies for adoption in agriculture to overcome the constrained environmental conditions.

Low Doses of Cuscuta reflexa Extract Act as Natural Biostimulants to Improve the Germination Vigor, Growth, and Grain Yield of Wheat Grown under Water Stress: Photosynthetic Pigments, Antioxidative Defense Mechanisms, and Nutrient Acquisition

The present study was conducted to investigate the effects of Cuscuta reflexa extract (CRE) on the activities of germination enzymes, seed germination vigor, biomass production, physio-biochemical attributes, and seed yield of water-stressed wheat plants. Different levels of CRE (0, 10, 20, 30, 40, and 50%), including water soaking, were used as seed priming. Water stress negatively affected the seed germination, germination enzyme activities, growth, yield, and different physio-biochemical attributes of wheat plants. Low doses of CRE (10, 20, and 30%) ameliorated the adverse effects of water stress on seed germination attributes, and activities of germination enzymes, but negative impacts were recorded at higher doses (40 and 50%) of CRE. Water-stressed wheat plants grown from seeds pre-treated with low doses of CRE also showed better growth and yield as compared with non-treated ones, and that was associated with an improvement in water relations, photosynthetic pigments, nutrient acquisition, reduced lipid peroxidation, and better antioxidative defense mechanisms. The maximum increase in seed yield was 14.77 and 12.32%, found in plants grown from seeds treated with 20% and 10% CRE, respectively. In conclusion, it is suggested that using low doses of CRE as seed priming can contribute to better wheat yield under water stress, especially in semi-arid and arid areas.