Inoculation with Azospirillum lipoferum or Azotobacter chroococcum Reinforces Maize Growth by Improving Physiological Activities Under Saline Conditions

Pseudomonas sp. G31 and Azotobacter sp. PBC2 Changed Structure of Bacterial Community and Modestly Promoted Growth of Oilseed Rape

Oilseed rape is one of the most important oilseed crops, requiring high levels of nitrogen fertilization. Excessive nitrogen use, however, leads to numerous negative environmental impacts, spurring the search for sustainable, environmentally friendly alternatives to reduce reliance on mineral nitrogen fertilizers. One promising approach involves plant-growth-promoting bacteria (PGPB), which can support oilseed rape growth and lessen the need for traditional nitrogen fertilizers. This study evaluates a selected microbial consortium comprising bacterial isolates obtained from soil: Pseudomonas sp. G31 and Azotobacter sp. PBC2 (P1A). The applied PGPB significantly increased seed yield (a 27.12% increase) and, in the initial phase of the study, reduced the ammonium nitrogen content in the soil (a 20.18% decrease). Metataxonomic analyses were performed using Next-Generation Sequencing (NGS) technology by Illumina. Although P1A did not significantly affect alpha diversity, it altered the relative abundance of some dominant soil microorganisms. In the BBCH 75 phase, the P1A consortium increased the abundance of bacteria of Firmicutes phylum, including the genera Bacillus and Paenibacillus, which was considered a beneficial change. In summary, the Pseudomonas sp. G31 and Azotobacter sp. PBC2 consortium increased seed yield and was found to be part of the native rhizosphere community of oilseed rape, making it a promising candidate for commercialization.

Metabolomics combined with physiology reveal how white clover (Trifolium repens L.) respond to 6PPD stress

As a ubiquitous tire antioxidant, N-(1,3-Dimethyl-butyl)-N'-phenyl-p-phenylene- diamine (6PPD) exists widely in various environmental media and has been detected at high levels in the environment. However, the effects of 6PPD on plants are still poorly understood. In this study, a hydroponic experiment was carried out to investigate the response of white clover (Trifolium repens L.) stressed by 6PPD on physiology and metabolomics. The results indicated that the length of stem and root, as well as biomass were significantly reduced after 500 μg L-1 6PPD treatment. Photosynthetic performances including photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), transpiration rate (Tr) and chlorophyll content of leaves decreased in all treatments except 500 μg L-1 of 6PPD. The malondialdehyde (MDA) content in the shoot of white clover increased by 66.33 % when exposed to 500 μg L-1 of 6PPD compared to control group (CK). Hydrogen peroxide and superoxide anion presented a U-shape trend and began to increase at 500 μg L-1. Besides, peroxidase and catalase significantly decreased compared to CK after exposure to 500 μg L-1. Metabolic analysis of clover showed that 6PPD treatment induced changes in 10 metabolic pathways of white clover. Metabolites were significantly down-regulated after exposure to 500 μg L-1 in shoot, while significantly down-regulated in all treatment groups except 500 μg L-1 in root. These findings may provide a novel perspective for phytotoxicity assessment and phytoremediation of 6PPD.

Restoration of degraded seagrass meadows: Effects of plant growth-promoting rhizobacteria (PGPR) inoculation on Zostera marina growth, rhizosphere microbiome and ecosystem functionality

The utilization of plant growth-promoting rhizobacteria (PGPR) holds great promise for the restoration of damaged terrestrial plant ecosystems. However, there is a significant knowledge gap regarding the application of PGPR in rehabilitating aquatic ecosystems. In this study, we conducted a mesocosm experiment to investigate the effects of Raoultella ornithinolytica F65, Pantoea cypripedii G84, Klebsiella variicola G85, Novosphingobium profundi G86, and Klebsiella pneumoniae I109 on eelgrass (Zostera marina L.), which is a crucial marine angiosperm. The application of these strains resulted in a significant increase in the new leaf area of eelgrass, with improvements of 55.4%, 14.4%, 39.1%, 20.6%, and 55.7% observed, respectively. Moreover, PGPR inoculation enhanced shoot biomass, rhizome elongation, leaf carbon and nitrogen content, as well as photosynthetic pigments. Furthermore, it stimulated enzymatic activities within the rhizosphere soil and positively influenced its physicochemical properties. The Illumina Miseq sequencing results revealed a positive shift in the bacterial community, leading to an enrichment of functional groups associated with nitrogen fixation and degradation of aromatic compounds. These findings underscore the significant potential of PGPR as a transformative tool for enhancing seagrass growth and survival, offering innovative strategies for the restoration of degraded seagrass meadows. This research not only advances our understanding of microbial-plant interactions in aquatic ecosystems but contributes to the broader goals of ecosystem revitalization and biodiversity conservation.

Screening and Evaluation of Salt-Tolerant Wheat Germplasm Based on the Main Morphological Indices at the Germination and Seedling Stages

The successful screening and evaluation of salt-tolerant germplasm at the germination and seedling stages is of great importance for promoting the breeding of wheat varieties with salt tolerance. In this study, 70 wheat varieties bred in different regions were evaluated for salt tolerance through hydroponic exposure to different concentrations of salt. The relative water absorption, water absorption rate, dehiscence rate, germination rate, and germination index of seeds, and plant height, root length, stem diameter, and biomass of seedlings were determined at the germination and seedling stages of wheat, and the salt tolerance was identified and evaluated using multivariate statistical analysis. The germination ability and seedling growth potential of wheat germplasms decreased with the aggravation of salt stress. Based on the comprehensive salt tolerance index at the germination stage, our study identified 35 varieties to be salt-tolerant. There were nine varieties further screened for having strong salt tolerance according to the comprehensive salt tolerance index at the germination and seedling stages. SN41, Emam, YN301, and JM262 were superior in salt-tolerance, and YM39, LM30, JM60, YN999, and SD29 were salt-tolerant. Our study suggests that the biomass of seedlings can be used as a key parameter for assessing wheat germplasm's ability to withstand salt. Our results can provide some basic materials for cultivating new germplasm with salt tolerance and excavating the related genes of wheat.

Enhanced antioxidant phytochemicals and catalase activity of celery by-products by a combined strategy of selenium and PGPB under restricted N supply

Introduction

The reduction of N supplied combined with the use of biostimulants can be an efficient strategy that allows sustainable agriculture to achieve better economic, nutritional and environmental goals without reducing production. Moreover, the industrial processing of celery generates large amounts of waste. Therefore the purpose of this study was improve crop management strategies to reduce nitrate pollution while turning crop waste into value-added products for others sectors.

Methods

Consequently, in this work twelve treatments were examined: three N nitrogen content in the nutrient solution (100% control, 60%, and 30%) combined with the inoculation of the roots with Azotobacter salinestris, and foliar application selenium solution (8 μM, Na2SeO4). The celery parts from plants grown under limited N dose showed a higher antioxidant activity and TPC (total phenolic compounds) content.

Results and discussion

The antioxidant activity increased 28% in leaves and 41% in by-products and TPC improved 27% in leaves and 191% in by-products respect to the control. Besides, a significant reduction of β-carotene content (56%, 11% and 43% in petioles, leaves and by-products respect to the control, respectively) was obtained in plants fed with restricted dose of N. The catalase activity was not affected by N dose. The inoculation of the plants with Azotobacter, together with a reduced N dose, achieved a greater accumulation of all the parameters studied. This accumulation was maximum when Se was applied to the leaves compared with the control and depending on the celery part: TPC (121-450%); antioxidant activity (60-68%), of catalase activity (59% - 158%), and of pigments content (50-90%). These findings can boost the valorization of celery by-products as excellent source of bioactive compounds.

The Biotechnological Potential of Plant Growth-Promoting Rhizobacteria Isolated from Maize (Zea mays L.) Cultivations in the San Martin Region, Peru

Maize (Zea mays L.) is an essential commodity for global food security and the agricultural economy, particularly in regions such as San Martin, Peru. This study investigated the plant growth-promoting characteristics of native rhizobacteria isolated from maize crops in the San Martin region of Peru with the aim of identifying microorganisms with biotechnological potential. Soil and root samples were collected from maize plants in four productive zones in the region: Lamas, El Dorado, Picota, and Bellavista. The potential of twelve bacterial isolates was evaluated through traits, such as biological nitrogen fixation, indole acetic acid (IAA) production, phosphate solubilization, and siderophore production, and a completely randomized design was used for these assays. A completely randomized block design was employed to assess the effects of bacterial strains and nitrogen doses on maize seedlings. The B3, B5, and NSM3 strains, as well as maize seeds of the yellow hard 'Advanta 9139' variety, were used in this experiment. Two of these isolates, B5 and NSM3, exhibited outstanding characteristics as plant growth promoters; these strains were capable of nitrogen fixation, IAA production (35.65 and 26.94 µg mL-1, respectively), phosphate solubilization (233.91 and 193.31 µg mL-1, respectively), and siderophore production (34.05 and 89.19%, respectively). Furthermore, molecular sequencing identified the NSM3 isolate as belonging to Sporosarcina sp. NSM3 OP861656, while the B5 isolate was identified as Peribacillus sp. B5 OP861655. These strains show promising potential for future use as biofertilizers, which could promote more sustainable agricultural practices in the region.

Strategies for combating plant salinity stress: the potential of plant growth-promoting microorganisms

Global climate change and the decreasing availability of high-quality water lead to an increase in the salinization of agricultural lands. This rising salinity represents a significant abiotic stressor that detrimentally influences plant physiology and gene expression. Consequently, critical processes such as seed germination, growth, development, and yield are adversely affected. Salinity severely impacts crop yields, given that many crop plants are sensitive to salt stress. Plant growth-promoting microorganisms (PGPMs) in the rhizosphere or the rhizoplane of plants are considered the "second genome" of plants as they contribute significantly to improving the plant growth and fitness of plants under normal conditions and when plants are under stress such as salinity. PGPMs are crucial in assisting plants to navigate the harsh conditions imposed by salt stress. By enhancing water and nutrient absorption, which is often hampered by high salinity, these microorganisms significantly improve plant resilience. They bolster the plant's defenses by increasing the production of osmoprotectants and antioxidants, mitigating salt-induced damage. Furthermore, PGPMs supply growth-promoting hormones like auxins and gibberellins and reduce levels of the stress hormone ethylene, fostering healthier plant growth. Importantly, they activate genes responsible for maintaining ion balance, a vital aspect of plant survival in saline environments. This review underscores the multifaceted roles of PGPMs in supporting plant life under salt stress, highlighting their value for agriculture in salt-affected areas and their potential impact on global food security.

Calcium Hexacyanoferrate Nanozyme Enhances Plant Stress Resistance by Oxidative Stress Alleviation and Heavy Metal Removal

Oxidative damage, exacerbated by the excessive accumulation of reactive oxygen species (ROS), profoundly inhibits both crop growth and yield. Herein, a biocompatible nanozyme, calcium hexacyanoferrate nanoparticles (CaHCF NPs), targeting ROS is developed, to mitigate oxidative damage and sequestrate heavy metal ions during plant growth. Uniquely, CaHCF NPs feature multifaced enzyme-like activities, involving superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione peroxidase, thiol peroxidase, and ascorbate peroxidase, which enable them to neutralize excessive ROS. Furthermore, CaHCF NPs promote calcium-cadmium exchange process, diminishing the uptake of heavy metals. Importantly, 120 µg mL-1 of CaHCF NPs alleviate the inhibitory effects of hydrogen peroxide and cadmium chloride on Arabidopsis and tomato. The activities of SOD, POD, and CAT increase by 46.2%, 74.4%, and 48.3%, respectively, meanwhile the glutathione level rises by 72.4% in Arabidopsis under cadmium stress. Moreover, CaHCF NPs boost the expression of genes associated with antioxidation, heavy metal detoxification, nutrient transport, and stress resistance. These findings unveil the significant potential of nanoplatforms equipped with nanozymes in alleviating oxidative stress in plants, which not only regulate crop growth but also substantially ameliorate yield and quality, heralding a new era in agricultural nanotechnology.

Abiotic/Biotic Stress and Substrate Dictated Metabolic Diversity of Azotobacter Chroococcum: Synthesis of Alginate, Antifungal n-Alkanes, Lactones, and Indoles

The current paper deals with new metabolites of different groups produced by Azotobacter chroococcum XU1. The strain's metabolic diversity is strongly altered by different factors, and some secondary metabolites are being reported for the first time for this species. As an abiotic/biotic stress response, the strain produced a broad spectrum of indole ring-containing compounds, n-alkanes (eicosane, heneicosane, docosane, tetracosane, and hexacosane), alkanes (7-hexyl eicosane and 2-methyloctacosane), saturated fatty acids (hexanoic and octanoic acids), esters (hexadecanoic acid methyl and pentadecanoic acid-14-methyl-methyl esters), and amides (9-Octadecenamide, (Z)- and 13-Docosenamide, (Z)-). Furthermore, to mitigate the abiotic stress the strain actively produced exopolysaccharide (EPS) to biosorb the Na+ ions. Apart from these metabolites, A. chroococcum XU1 synthesized lactones, namely 1,5-d-gluconolactone and d, l-mevalonic acid lactone in response to carbon source modification.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-024-01212-x.

Azolla filiculoides extract improved salt tolerance in wheat (Triticum aestivum L.) is associated with prompting osmostasis, antioxidant potential and stress-interrelated genes

The growth and productivity of crop plants are negatively affected by salinity-induced ionic and oxidative stresses. This study aimed to provide insight into the interaction of NaCl-induced salinity with Azolla aqueous extract (AAE) regarding growth, antioxidant balance, and stress-responsive genes expression in wheat seedlings. In a pot experiment, wheat kernels were primed for 21 h with either deionized water or 0.1% AAE. Water-primed seedlings received either tap water, 250 mM NaCl, AAE spray, or AAE spray + NaCl. The AAE-primed seedlings received either tap water or 250 mM NaCl. Salinity lowered growth rate, chlorophyll level, and protein and amino acids pool. However, carotenoids, stress indicators (EL, MDA, and H2O2), osmomodulators (sugars, and proline), antioxidant enzymes (CAT, POD, APX, and PPO), and the expression of some stress-responsive genes (POD, PPO and PAL, PCS, and TLP) were significantly increased. However, administering AAE contributed to increased growth, balanced leaf pigments and assimilation efficacy, diminished stress indicators, rebalanced osmomodulators and antioxidant enzymes, and down-regulation of stress-induced genes in NaCl-stressed plants, with priming surpassing spray in most cases. In conclusion, AAE can be used as a green approach for sustaining regular growth and metabolism and remodelling the physio-chemical status of wheat seedlings thriving in salt-affected soils.

Biofortification and Valorization of Celery byproducts Using Selenium and PGPB under Reduced Nitrogen Regimes

Due to climate change and exacerbated population growth, the search for new sustainable strategies that allow for greater food productivity and that provide greater nutritional quality has become imperative. One strategy for addressing this problem is the combined use of fertilization with a reduced dose of nitrogen and biostimulants. Celery processing produces a large amount of waste with its concomitant pollution. Therefore, it is necessary to address the valorization of its byproducts. Our results revealed reductions in the biomass, Na, P, Mn, B, sugars, and proteins in the byproducts and increased lipid peroxidation, Fe (all celery parts), and K (byproducts) when the N supplied was reduced. Plants inoculated with Azotobacter salinestris obtained a greater biomass, a higher accumulation of K (byproducts), a build-up of sugars and proteins, reduced concentrations of P, Cu, Mn, B, Fe (petioles), and Zn (byproducts), and reduced lipid peroxidation. The application of Se at 8 μM reinforced the beneficial effect obtained after inoculation with Azotobacter salinestris. In accordance with our results, edible celery parts are recommended as an essential ingredient in the daily diet. Furthermore, the valorization of celery byproducts with health-promoting purposes should be considered.

Application of prohexadione-calcium priming affects Brassica napus L. seedlings by regulating morph-physiological characteristics under salt stress

Salinity stress imposes severe constraints on plant growth and development. Here, we explored the impacts of prohexadione-calcium (Pro-Ca) on rapeseed growth under salt stress. We designed a randomized block design pot experiment using two rapeseed varieties, 'Huayouza 158R' and 'Huayouza 62'. We conducted six treatments, S0: non-primed + 0 mM NaCl, Pro-Ca+S0: Pro-Ca primed + 0 mM NaCl, S100: non-primed + 100 mM NaCl, Pro-Ca+S100: Pro-Ca primed + 100 mM NaCl, S150: non-primed + 150 mM NaCl, Pro-Ca+S150: Pro-Ca primed + 150 mM NaCl. The morphophysiological characteristics, and osmoregulatory and antioxidant activities were compared for primed and non-primed varieties. Our data analysis showed that salt stress induced morph-physiological traits and significantly reduced the antioxidant enzyme activities in both rapeseed varieties. The Pro-Ca primed treatment significantly improved seedlings, root, and shoot morphological traits and accumulated more dry matter biomass under salt stress. Compared to Huayouza 158R, Huayouza 62 performed better with the Pro-Ca primed treatment. The Pro-Ca primed treatment significantly enhanced chlorophyll content, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and actual photochemical quantum efficiency (ФPSII). Furthermore, the Pro-Ca primed treatment also improved ascorbic acid (ASA) content, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) activity, and stimulated the accumulation of soluble proteins. These findings strongly suggested that the Pro-Ca primed treatment may effectively counteract the negative impacts of salinity stress by regulating the morph-physiological and antioxidant traits.

Toxicity of bisphenol A and p-nitrophenol on tomato plants: Morpho-physiological, ionomic profile, and antioxidants/defense-related gene expression studies

Bisphenol A (BPA) and p-nitrophenol (PNP) are emerging contaminants of soils due to their wide presence in agricultural and industrial products. Thus, the present study aimed to integrate morpho-physiological, ionic homeostasis, and defense- and antioxidant-related genes in the response of tomato plants to BPA or PNP stress, an area of research that has been scarcely studied. In this work, increasing the levels of BPA and PNP in the soil intensified their drastic effects on the biomass and photosynthetic pigments of tomato plants. Moreover, BPA and PNP induced osmotic stress on tomato plants by reducing soluble sugars and soluble proteins relative to control. The soil contamination with BPA and PNP treatments caused a decline in the levels of macro- and micro-elements in the foliar tissues of tomatoes while simultaneously increasing the contents of non-essential micronutrients. The Fourier transform infrared analysis of the active components in tomato leaves revealed that BPA influenced the presence of certain functional groups, resulting in the absence of some functional groups, while on PNP treatment, there was a shift observed in certain functional groups compared to the control. At the molecular level, BPA and PNP induced an increase in the gene expression of polyphenol oxidase and peroxidase, with the exception of POD gene expression under BPA stress. The expression of the thaumatin-like protein gene increased at the highest level of PNP and a moderate level of BPA without any significant effect of both pollutants on the expression of the tubulin (TUB) gene. The comprehensive analysis of biochemical responses in tomato plants subjected to BPA and PNP stress illustrates valuable insights into the mechanisms underlying tolerance to these pollutants.

Halo-alkaliphilic microbes as an effective tool for heavy metal pollution abatement and resource recovery: challenges and future prospects

The current study presents an overview of heavy metals bioremediation from halo-alkaline conditions by using extremophilic microorganisms. Heavy metal remediation from the extreme environment with high pH and elevated salt concentration is a challenge as mesophilic microorganisms are unable to thrive under these polyextremophilic conditions. Thus, for effective bioremediation of extreme systems, specialized microbes (extremophiles) are projected as potential bioremediating agents, that not only thrive under such extreme conditions but are also capable of remediating heavy metals from these environments. The physiological versatility of extremophiles especially halophiles and alkaliphiles and their enzymes (extremozymes) could conveniently be harnessed to remediate and detoxify heavy metals from the high alkaline saline environment. Bibliometric analysis has shown that research in this direction has found pace in recent years and thus this review is a timely attempt to highlight the importance of halo-alkaliphiles for effective contaminant removal in extreme conditions. Also, this review systematically presents insights on adaptive measures utilized by extremophiles to cope with harsh environments and outlines the role of extremophilic microbes in industrial wastewater treatment and recovery of metals from waste with relevant examples. Further, the major challenges and way forward for the effective applicability of halo-alkaliphilic microbes in heavy metals bioremediation from extremophilic conditions are also highlighted.

Potential Impacts of Certain N2-Fixing Bacterial Strains and Mineral N Doses for Enhancing the Growth and Productivity of Maize Plants

The enhancing effect of N2-fixing bacterial strains in the presence of mineral N doses on maize plants in pots and field trials was investigated. The OT-H1 of 10 isolates maintained the total nitrogen, nitrogenase activities, IAA production, and detection of NH3 in their cultures. In addition, they highly promoted the germination of maize grains in plastic bags compared to the remainder. Therefore, OT-H1 was subjected for identification and selected for further tests. Based on their morphological, cultural, and biochemical traits, they belonged to the genera Azotobacter. The genomic sequences of 16S rRNA were, thus, used to confirm the identification of the genera. Accordingly, the indexes of tree and similarity for the related bacterial species indicated that genera were exactly closely linked to Azotoacter salinestris strain OR512393. In pot (35 days) and field (120 days) trials, the efficiencies of both A. salinestris and Azospirillum oryzea SWERI 111 (sole/dual) with 100, 75, 50, and 25% mineral N doses were evaluated with completely randomized experimental design and three repetitions. Results indicated that N2-fixing bacteria in the presence of mineral N treatment showed pronounced effects compared to controls. A high value of maize plants was also noticed through increasing the concentration of mineral N and peaked at a dose of 100%. Differences among N2-fixing bacteria were insignificant and were observed for A. oryzea with different mineral N doses. Thus, the utilization of A. oryzea and A. salinestris in their dual mix in the presence of 75 followed by 50% mineral N was found to be the superior treatments, causing the enhancement of vegetative growth and grain yield parameters of maize plants. Additionally, proline and the enzyme activities of both polyphenol oxidase (PPO) and peroxidase (PO) of maize leaves were induced, and high protein contents of maize grains were accumulated due to the superior treatments. The utilization of such N2-fixing bacteria was, therefore, found to be effective at improving soil fertility and to be an environmentally safe strategy instead, or at least with low doses, of chemical fertilizers.

Algal Bio-Stimulants Enhance Salt Tolerance in Common Bean: Dissecting Morphological, Physiological, and Genetic Mechanisms for Stress Adaptation

Salinity adversely affects the plant's morphological characteristics, but the utilization of aqueous algal extracts (AE) ameliorates this negative impact. In this study, the application of AE derived from Chlorella vulgaris and Dunaliella salina strains effectively reversed the decline in biomass allocation and water relations, both in normal and salt-stressed conditions. The simultaneous application of both extracts in salt-affected soil notably enhanced key parameters, such as chlorophyll content (15%), carotene content (1%), photosynthesis (25%), stomatal conductance (7%), and transpiration rate (23%), surpassing those observed in the application of both AE in salt-affected as compared to salinity stress control. Moreover, the AE treatments effectively mitigated lipid peroxidation and electrolyte leakage induced by salinity stress. The application of AE led to an increase in GB (6%) and the total concentration of free amino acids (47%) by comparing with salt-affected control. Additionally, salinity stress resulted in an elevation of antioxidant enzyme activities, including superoxide dismutase, ascorbate peroxidase, catalase, and glutathione reductase. Notably, the AE treatments significantly boosted the activity of these antioxidant enzymes under salinity conditions. Furthermore, salinity reduced mineral contents, but the application of AE effectively counteracted this decline, leading to increased mineral levels. In conclusion, the application of aqueous algal extracts, specifically those obtained from Chlorella vulgaris and Dunaliella salina strains, demonstrated significant efficacy in alleviating salinity-induced stress in Phaseolus vulgaris plants.

Organic amendments improve salinity-induced osmotic and oxidative stress tolerance in Okra (Abelmoschus esculentus (L.)Moench)

AIMS

Salinity adversely affects okra [Abelmoschus esculentus (L.) Moench] plants by inducing osmotic and oxidative stresses. This study was designed to enhance salinity-induced osmotic and oxidative stress tolerance in okra plants by applying organic amendments.

METHODS

The effects of different organic amendments (municipal solid waste compost, farmyard manure (FYM) and press mud) on osmotic potential, water use efficiency, activities of antioxidant enzymes, total soluble sugar, total soluble proline, total soluble protein and malondialdehyde (MDA) contents of okra plants grown under saline conditions (50 mM sodium chloride) were evaluated in a pot experiment. The organic amendments were applied each at the rate of 5% and 10% per pot or in various combinations (compost + FYM, FYM + press mud and compost + press mud each at the rate of 2.5% and 5% per pot).

RESULTS

As compared to control, high total soluble sugar (60.41), total soluble proline (33.88%) and MDA (51%) contents and increased activities of antioxidant enzymes [superoxide dismutase (83.54%), catalase (78.61%), peroxidase (53.57%] in salinity-stressed okra plants, were indicative of oxidative stress. Salinity significantly reduced the osmotic potential (41.78%) and water use efficiency (4.75%) of okra plants compared to control. Under saline conditions, 5% (farmyard manure + press mud) was the most effective treatment, which significantly improved osmotic potential (27.05%), total soluble sugar (4.20%), total soluble protein (73.62%) and total soluble proline (23.20%) contents and superoxide dismutase activity (32.41%), compared to saline soil. Application of 2.5% (FYM + press mud), 5% press mud, and 10% compost significantly reduced MDA content (27%) and improved activities of catalase (38.64%) and peroxidase (48.29%), respectively, compared to saline soil, thus facilitated to alleviate oxidative stress in okra plants.

CONCLUSIONS

Using organic amendments (municipal solid waste compost, farmyard manure and press mud) was a cost-effective approach to improve salinity-induced osmotic and oxidative stress tolerance in okra plants.

Microbial Exudates as Biostimulants: Role in Plant Growth Promotion and Stress Mitigation

Microbes hold immense potential, based on the fact that they are widely acknowledged for their role in mitigating the detrimental impacts of chemical fertilizers and pesticides, which were extensively employed during the Green Revolution era. The consequence of this extensive use has been the degradation of agricultural land, soil health and fertility deterioration, and a decline in crop quality. Despite the existence of environmentally friendly and sustainable alternatives, microbial bioinoculants encounter numerous challenges in real-world agricultural settings. These challenges include harsh environmental conditions like unfavorable soil pH, temperature extremes, and nutrient imbalances, as well as stiff competition with native microbial species and host plant specificity. Moreover, obstacles spanning from large-scale production to commercialization persist. Therefore, substantial efforts are underway to identify superior solutions that can foster a sustainable and eco-conscious agricultural system. In this context, attention has shifted towards the utilization of cell-free microbial exudates as opposed to traditional microbial inoculants. Microbial exudates refer to the diverse array of cellular metabolites secreted by microbial cells. These metabolites enclose a wide range of chemical compounds, including sugars, organic acids, amino acids, peptides, siderophores, volatiles, and more. The composition and function of these compounds in exudates can vary considerably, depending on the specific microbial strains and prevailing environmental conditions. Remarkably, they possess the capability to modulate and influence various plant physiological processes, thereby inducing tolerance to both biotic and abiotic stresses. Furthermore, these exudates facilitate plant growth and aid in the remediation of environmental pollutants such as chemicals and heavy metals in agroecosystems. Much like live microbes, when applied, these exudates actively participate in the phyllosphere and rhizosphere, engaging in continuous interactions with plants and plant-associated microbes. Consequently, they play a pivotal role in reshaping the microbiome. The biostimulant properties exhibited by these exudates position them as promising biological components for fostering cleaner and more sustainable agricultural systems.

Harnessing the action mechanisms of microbial endophytes for enhancing plant performance and stress tolerance: current understanding and future perspectives

Microbial endophytes are microorganisms that reside within plant tissues without causing any harm to their hosts. These microorganisms have been found to confer a range of benefits to plants, including increased growth and stress tolerance. In this review, we summarize the recent advances in our understanding of the mechanisms by which microbial endophytes confer abiotic and biotic stress tolerance to their host plants. Specifically, we focus on the roles of endophytes in enhancing nutrient uptake, modulating plant hormones, producing secondary metabolites, and activating plant defence responses. We also discuss the challenges associated with developing microbial endophyte-based products for commercial use, including product refinement, toxicology analysis, and prototype formulation. Despite these challenges, there is growing interest in the potential applications of microbial endophytes in agriculture and environmental remediation. With further research and development, microbial endophyte-based products have the potential to play a significant role in sustainable agriculture and environmental management.

Morpho-physiological responses and growth indices of triticale to drought and salt stresses

Salinity and drought are two major abiotic stresses challenging global crop production and food security. In this study, the effects of individual and combined effects of drought (at different phenological stages) and salt stresses on growth, morphology, and physiology of triticale were evaluated. For this purpose, a 3 x 4 factorial design in three blocks experiment was conducted. The stress treatments included three levels of salinity (0, 50, and 100 mM NaCl) and four levels of drought (regular irrigation as well as irrigation disruption at heading, flowering, and kernel extension stages). The stresses, individual as well as combined, caused a significant decrease in chlorophyll contents, total dry matter, leaf area index, relative water content, and grain yield of triticale. In this regard, the highest reduction was recorded under combined stresses of 100 mM NaCl and drought stress at flowering. However, an increase in soluble sugars, leaf free proline, carotenoid contents, and electrolyte leakage was noted under stress conditions compared to the control. In this regard, the highest increase in leaf free proline, soluble sugars, and carotenoid contents were noted under the combination of severe salinity and drought stress imposed at the flowering stage. Investigating the growth indices in severe salinity and water deficit stress in different phenological stages shows the predominance of ionic stress over osmotic stress under severe salinity. The highest grain yield was observed under non-saline well-watered conditions whereas the lowest grain yield was recorded under severe salinity and drought stress imposed at the flowering stage. In conclusion, the flowering stage was more sensitive than the heading and kernel extension stages in terms of water deficit. The impact of salinity and water deficit was more pronounced on soluble sugars and leaf free proline; so, these criteria can be used as physiological indicators for drought and salinity tolerance in triticale.

Interactive effects of Pseudomonas putida and salicylic acid for mitigating drought tolerance in canola (Brassica napus L.)

This research was designed to analyze the interactive effects of Pseudomonas putida and salicylic acid on the growth of canola in stress and non-stress conditions. Salicylic acid is a phenolic derivative, that has a direct involvement in various plant stages like growth, and inflorescence. While Pseudomonas putida is a drought-tolerant strain having plant growth-promoting characteristics like phosphate solubilization, indole acetic acid, and catalase production. Combined application of Pseudomonas putida and salicylic acid has the ability to develop stress tolerance in plants and also improve growth of plants. They have significant (p < 0.05) effects on germination and morphological, physiological, and biochemical parameters. The plants that received the co-application of Pseudomonas putida and salicylic acid gave more significant results than their alone application. They showed enhanced germination percentage, germination index, promptness index and, seedling vigor index by 19%, 18%, 34% and, 27%, respectively. There was a substantial increase of 25%, 27%, and 39% in shoot length, root length, and leaf area, respectively. The synergistic effect of both treatments has caused a 14% and 12% increase in the Canola plants' relative water content and membrane stability index respectively. A substantial increase of 18% in proline content was observed by the inoculation of Pseudomonas putida, whereas proline content was increased by 28% by the exogenous application of salicylic acid. The content of flavonoids (39%) and phenol (40%) was significantly increased by the co-application. The increase in superoxide dismutase (46%), ascorbate peroxidase (43%), and glutathione (19%) were also significant. The present research demonstrated that the combined application of Pseudomonas putida and salicylic acid induces drought tolerance in canola and significantly improves its growth.

The effects of temperature, salt, and phosphate on biofilm and exopolysaccharide production by Azotobacter spp

Inoculation of agriculturally important biofilms to plants under stress conditions has been of great interest in recent years. Therefore, in this study, biofilm- and exopolysaccharide (EPS)-forming ability of Azotobacter spp. was examined under different temperatures, NaCl concentrations, and phosphate levels. Azotobacter strains formed varying levels of biofilm and EPS depending on the tested factors. The pattern of biofilm formation was similar to that of EPS production under the conditions tested. Biofilm and EPS production at 28 °C was consistently higher than at either 18 or 37 °C. Biofilm production significantly increased in A. chroococcum strains (SBS2, SBS4, and SBS12) and A. vinelandii SBS6 with increasing salinity. Furthermore, a strong negative correlation was observed between biofilm or EPS production and increasing phosphate concentrations. Higher phosphate concentrations decreased biofilm and EPS production. In conclusion, contrary to temperature and phosphate effect, salinity differently affected biofilm and EPS production by Azotobacter strains.

Improved effects of combined application of nitrogen-fixing bacteria Azotobacter beijerinckii and microalgae Chlorella pyrenoidosa on wheat growth and saline-alkali soil quality

Soil salinization seriously affects crop yield and soil productivity. The application of bacteria and microalgae has been considered as a promising strategy to alleviate soil salinization. However, the effect of bacteria-microalgae symbiosis on saline-alkali land is still unclear. This study evaluated the effects of Azotobacter beijerinckii, Chlorella pyrenoidosa, and their combined application on the wheat growth and saline-alkali soil improvement. The results showed that, among all the treatments, A. beijerinckii + live C. pyrenoidosa combined inoculation group (BA) had the best effect on increasing wheat plant biomass, improving salt tolerance, and improving soil fertility. The dry weight of wheat plant in the BA group increased by 66.7%, 17.4%, and 35.0%, respectively, compared with the control group (CK), A. beijerinckii inoculation group (B), and live C. pyrenoidosa inoculation group (A). The total nitrogen content of wheat plant in the BA group increased by 69.5%, 76.7%, and 71.1%, compared with the CK, B, and A group. The proline content of wheat plant in the BA group was 100% higher than that in the CK group. The N/P ratio and K/Na ratio of wheat plant increased by 157% and 12.9% in the BA group compared with the CK group, respectively, which was more conducive to alleviating nitrogen limitation and salt stress. The A. beijerinckii + live C. pyrenoidosa inoculation treatment better reduced soil pH and improved the availability of phosphorus in soil. This study illustrated the comprehensive application prospects of bacteria-microalgae interactions on wheat growth promotion and soil improvement in saline-alkali land, and provided a new effective strategy for improving saline-alkali soil quality and increasing crop productivity.

Protective effects of chitosan based salicylic acid nanocomposite (CS-SA NCs) in grape (Vitis vinifera cv. 'Sultana') under salinity stress

Salinity is one of the most important abiotic stresses that reduce plant growth and performance by changing physiological and biochemical processes. In addition to improving the crop, using nanomaterials in agriculture can reduce the harmful effects of environmental stresses, particularly salinity. A factorial experiment was conducted in the form of a completely randomized design with two factors including salt stress at three levels (0, 50, and 100 mM NaCl) and chitosan-salicylic acid nanocomposite at three levels (0, 0.1, and 0.5 mM). The results showed reductions in chlorophylls (a, b, and total), carotenoids, and nutrient elements (excluding sodium) while proline, hydrogen peroxide, malondialdehyde, total soluble protein, soluble carbohydrate, total antioxidant, and antioxidant enzymes activity increased with treatment chitosan-salicylic acid nanocomposite (CS-SA NCs) under different level NaCl. Salinity stress reduced Fm', Fm, and Fv/Fm by damage to photosynthetic systems, but treatment with CS-SA NCs improved these indices during salinity stress. In stress-free conditions, applying the CS-SA NCs improved the grapes' physiological, biochemical, and nutrient elemental balance traits. CS-SA NCs at 0.5 mM had a better effect on the studied traits of grapes under salinity stress. The CS-SA nanoparticle is a biostimulant that can be effectively used to improve the grape plant yield under salinity stress.

Integrated Metabolomics and Morpho-Biochemical Analyses Reveal a Better Performance of Azospirillum brasilense over Plant-Derived Biostimulants in Counteracting Salt Stress in Tomato

Increased soil salinity is one of the main concerns in agriculture and food production, and it negatively affects plant growth and crop productivity. In order to mitigate the adverse effects of salinity stress, plant biostimulants (PBs) have been indicated as a promising approach. Indeed, these products have a beneficial effect on plants by acting on primary and secondary metabolism and by inducing the accumulation of protective molecules against oxidative stress. In this context, the present work is aimed at comparatively investigating the effects of microbial (i.e., Azospirillum brasilense) and plant-derived biostimulants in alleviating salt stress in tomato plants by adopting a multidisciplinary approach. To do so, the morphological and biochemical effects were assessed by analyzing the biomass accumulation and root characteristics, the activity of antioxidant enzymes and osmotic stress protection. Furthermore, modifications in the metabolomic profiles of both leaves and root exudates were also investigated by ultra-high performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/QTOF-MS). According to the results, biomass accumulation decreased under high salinity. However, the treatment with A. brasilense considerably improved root architecture and increased root biomass by 156% and 118% in non-saline and saline conditions, respectively. The antioxidant enzymes and proline production were enhanced in salinity stress at different levels according to the biostimulant applied. Moreover, the metabolomic analyses pointed out a wide set of processes being affected by salinity and biostimulant interactions. Crucial compounds belonging to secondary metabolism (phenylpropanoids, alkaloids and other N-containing metabolites, and membrane lipids) and phytohormones (brassinosteroids, cytokinins and methylsalicylate) showed the most pronounced modulation. Overall, our results suggest a better performance of A. brasilense in alleviating high salinity than the vegetal-derived protein hydrolysates herein evaluated.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Combined Proteomic and Metabolomic Analysis of the Molecular Mechanism Underlying the Response to Salt Stress during Seed Germination in Barley

Salt stress is a major abiotic stress factor affecting crop production, and understanding of the response mechanisms of seed germination to salt stress can help to improve crop tolerance and yield. The differences in regulatory pathways during germination in different salt-tolerant barley seeds are not clear. Therefore, this study investigated the responses of different salt-tolerant barley seeds during germination to salt stress at the proteomic and metabolic levels. To do so, the proteomics and metabolomics of two barley seeds with different salt tolerances were comprehensively examined. Through comparative proteomic analysis, 778 differentially expressed proteins were identified, of which 335 were upregulated and 443 were downregulated. These proteins, were mainly involved in signal transduction, propanoate metabolism, phenylpropanoid biosynthesis, plant hormones and cell wall stress. In addition, a total of 187 salt-regulated metabolites were identified in this research, which were mainly related to ABC transporters, amino acid metabolism, carbohydrate metabolism and lipid metabolism; 72 were increased and 112 were decreased. Compared with salt-sensitive materials, salt-tolerant materials responded more positively to salt stress at the protein and metabolic levels. Taken together, these results suggest that salt-tolerant germplasm may enhance resilience by repairing intracellular structures, promoting lipid metabolism and increasing osmotic metabolites. These data not only provide new ideas for how seeds respond to salt stress but also provide new directions for studying the molecular mechanisms and the metabolic homeostasis of seeds in the early stages of germination under abiotic stresses.

Ferric Oxide Colloid: Towards Green Nano-Fertilizer for Tomato Plant with Enhanced Vegetative Growth and Immune Response Against Fusarium Wilt Disease

Global food crisis due to climate change, pandemic COVID-19 outbreak, and Russia-Ukraine conflict leads to catastrophic consequences; almost 10 percent of the world’s population go to bed hungry daily. Narrative solution for green agriculture with high vegetation and crop yield is mandatory; novel nanomaterials can improve plant immunity and restrain plant diseases. Iron is fundamental nutrient element; it plays vital role in enzyme activity and RNA synthesis; furthermore it is involved in photosynthesis electron-transfer chains. This study reports on the facile synthesis of colloidal ferric oxide nanoparticles as novel nano-fertilizer to promote vegetation and to suppress Fusarium wilt disease in tomato plant. Disease index, protection percent, photosynthetic pigments, and metabolic indicators of resistance in plant as response to induction of systemic resistance (SR) were recorded. Results illustrated that Fe₂O₃ NPs had antifungal activity against F. oxysporum. Fe₂O₃ NPs (at 20 µg/mL) was the best treatment and reduced percent disease indexes by 15.62 and gave highly protection against disease by 82.15% relative to untreated infected plants. Fe₂O₃ NPs treatments in either (non-infected or infected) plants showed improvements in photosynthetic pigments, osmolytes, and antioxidant enzymes activity. The beneficial effects of the synthesized Fe₂O₃ NPs were extended to increase not only photosynthetic pigments, osmolytes contents but also the activities of peroxidase (POD), polyphenol oxidase (PPO), catalase (CAT) and superoxide dismutase (SOD), enzymes of the healthy and infected tomato plants in comparison with control. For, peroxidase and polyphenol oxidase activities it was found that, application of Fe₂O₃ NPs (10 µg/mL) on challenged plants offered the best treatments which increased the activities of POD by (34.4%) and PPO by (31.24%). On the other hand, application of Fe₂O₃ NPs (20 µg/mL) on challenged plants offered the best treatments which increased the activities of CAT by (30.9%), and SOD by (31.33%).

Inoculation with Azospirillum brasilense and/or Pseudomonas geniculata reinforces flax (Linum usitatissimum) growth by improving physiological activities under saline soil conditions

BACKGROUND

Salinized soils negatively affect plant growth, so it has become necessary to use safe and eco-friendly methods to mitigate this stress. In a completely randomized design, a pot experiment was carried out to estimate the influence of the inoculation with endophytic bacterial isolates Azospirillum brasilense, Pseudomonas geniculata and their co-inoculation on growth and metabolic aspects of flax (Linum usitatissimum) plants that already grown in salinized soil.

RESULTS

The results observed that inoculation of salinity-stressed flax plants with the endophytes A. brasilense and P. geniculata (individually or in co-inoculation) increases almost growth characteristics (shoot and root lengths, fresh and dry weights as well as number of leaves). Moreover, contents of chlorophylls and carotenoids pigments, soluble sugars, proteins, free proline, total phenols, ascorbic acid, and potassium (K⁺) in flax plants grown in salinized soil were augmented because of the inoculation with A. brasilense and P. geniculata. Oppositely, there are significant decreases in free proline, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and sodium (Na⁺) contents. Regarding antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), the inoculation with the tested endophytes led to significant enhancements in the activities of antioxidant enzymes in stressed flax plants.

CONCLUSIONS

The results of this work showed that the use of the endophytic bacterial isolates Azospirillum brasilense, Pseudomonas geniculata (individually or in co-inoculation) could be regarded as an uncommon new model to alleviate salinity stress, especially in salinized soils.

Salt-Tolerant PGPR Confer Salt Tolerance to Maize Through Enhanced Soil Biological Health, Enzymatic Activities, Nutrient Uptake and Antioxidant Defense

Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can improve soil enzyme activities, which are indicators of the biological health of the soil, and can overcome the nutritional imbalance in plants. A pot trial was executed to evaluate the effect of inoculation of different salt-tolerant PGPR strains in improving soil enzyme activities. Three different salinity levels (original, 5, and 10 dS m-1) were used and maize seeds were coated with the freshly prepared inocula of ten different PGPR strains. Among different strains, inoculation of SUA-14 (Acinetobacter johnsonii) caused a maximum increment in urease (1.58-fold), acid (1.38-fold), and alkaline phosphatase (3.04-fold) and dehydrogenase (72%) activities as compared to their respective uninoculated control. Acid phosphatase activities were found to be positively correlated with P contents in maize straw (r = 0.96) and grains (r = 0.94). Similarly, a positive correlation was found between alkaline phosphatase activities and P contents in straw (r = 0.77) and grains (r = 0.75). In addition, urease activities also exhibited positive correlation with N contents in maize straw (r = 0.92) and grains (r = 0.91). Moreover, inoculation of Acinetobacter johnsonii caused a significant decline in catalase (39%), superoxide dismutase (26%) activities, and malondialdehyde contents (27%). The PGPR inoculation improved the soil's biological health and increased the uptake of essential nutrients and conferred salinity tolerance in maize. We conclude that the inoculation of salt-tolerant PGPR improves soil enzyme activities and soil biological health, overcomes nutritional imbalance, and thereby improves nutrient acquisition by the plant under salt stress.

Bacillus atrophaeus WZYH01 and Planococcus soli WZYH02 Improve Salt Tolerance of Maize (Zea mays L.) in Saline Soil

With the increasing shortage of land resources and people's attention to the ecological environment, the application of microbial fertilizer with natural soil microorganisms as the main component has attracted increasing attention in saline agriculture. In this study, two salt-tolerant strains, YL07 (Bacillus atrophaeus) and YL10 (Planococcus soli), were isolated from maize (Zea mays L.) rhizosphere soil with a saturated conductivity (EC_e) of 6.13 dS/m and pH of 8.32 (Xinjiang, China). The effects of B. atrophaeus WZYH01 (YL07) and Planococcus soli WZYH02 (YL10) on the growth and development of maize (Zea mays L.) under salt stress (EC_e = 5.9 dS/m) were further studied. The results showed that compared with uninoculation, inoculation with B. atrophaeus WZYH01 and Planococcus soli WZYH02 significantly improved maize growth performance, biomass yield, and antioxidant levels under salt stress, and the effect of Planococcus soli WZYH02 was more prominent than the effect of B. atrophaeus WZYH01. Moreover, inoculation with B. atrophaeus WZYH01 and Planococcus soli WZYH02 protected maize from salt stress by regulating plant hormone [IAA and abscisic acid (ABA)] levels and increasing nutrient acquisition. In addition, the tested strains were most efficient for maize growth and health, increasing the content of K⁺ accompanied by an effective decrease in Na⁺ in maize tissues. The transcription levels of salt tolerance genes (ZMNHX1, ZMNHX2, ZMHKT, ZMWRKY58, and ZMDREB2A) in inoculated maize were also dramatically higher than the transcription levels of the specified salt tolerance genes in uninoculated maize. In conclusion, B. atrophaeus WZYH01 and Planococcus soli WZYH02 can alleviate the harmful effects of salt stress on crop growth, thereby promoting sustainable agricultural development.

Encapsulation of Bacillus pumilus G5 from polyvinyl alcohol‑sodium alginate (PVA-SA) and its implications in improving plant growth and soil fertility under drought and salt soil conditions

Drought and salt stresses adversely affect the growth and yield of plants in agricultural production. Bacillus pumilus, an important plant growth-promoting bacterium, play a significant role in improving plant tolerance to abiotic stresses. In this study, B. pumilus G5 were immobilized in polyvinyl alcohol‑sodium alginate (PVA-SA) microbeads and then applied on the Pharbitis nil under drought and salt stresses by pot experiment. Orthogonal array experiments showed that the optimal immobilization conditions of PVA-SA immobilized G5 microbeads were adsorbent 6.0%, PVA: SA 1:1 (3.0%), CaCl₂ 4.0%, and bacterium: embedding agent (PVA-SA) 3:4; And the G5 microbeads produced at the optimal condition exhibited better cultivable bacteria count, encapsulation rate, expansion rate and mechanical strength. Pot experiment showed that G5 microbeads significantly increased the length and diameter of root and stem, and dry weight of P. nil during experimental stage under drought and salt stress. G5 microbeads also increased the total cultivable bacteria population, the activities of invertase (INV), urease (URE), phosphatase (PHO) and catalase (CAT), and the contents of available nitrogen (AN) and available phosphorus (AP) in the rhizosphere soil of P. nil. Therefore, our study obtained the optimal process of G5 microbeads, and confirmed its effect on improved plant growth and soil chemical and biological properties of P. nil. Thus it can be used as sustainable tool for eco-friendly bio-inoculants at salinity soil within arid and semi-arid areas.

Recent Advances in Bacterial Amelioration of Plant Drought and Salt Stress

Simple Summary

Salt and drought stress cause enormous crop losses worldwide. Several different approaches may be taken to address this problem, including increased use of irrigation, use of both traditional breeding and genetic engineering to develop salt-tolerant and drought-resistant crop plants, and the directed use of naturally occurring plant growth-promoting bacteria. Here, the mechanisms used by these plant growth-promoting bacteria are summarized and discussed. Moreover, recently reported studies of the effects that these organisms have on the growth of plants in the laboratory, the greenhouse, and the field under high salt and/or drought conditions is discussed in some detail. It is hoped that by understanding the mechanisms that these naturally occurring plant growth-promoting bacteria utilize to overcome damaging environmental stresses, it may be possible to employ these organisms to increase future agricultural productivity.

Abstract

The recent literature indicates that plant growth-promoting bacteria (PGPB) employ a range of mechanisms to augment a plant’s ability to ameliorate salt and drought stress. These mechanisms include synthesis of auxins, especially indoleacetic acid, which directly promotes plant growth; synthesis of antioxidant enzymes such as catalase, superoxide dismutase and peroxidase, which prevents the deleterious effects of reactive oxygen species; synthesis of small molecule osmolytes, e.g., trehalose and proline, which structures the water content within plant and bacterial cells and reduces plant turgor pressure; nitrogen fixation, which directly improves plant growth; synthesis of exopolysaccharides, which protects plant cells from water loss and stabilizes soil aggregates; synthesis of antibiotics, which protects stress-debilitated plants from soil pathogens; and synthesis of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which lowers the level of ACC and ethylene in plants, thereby decreasing stress-induced plant senescence. Many of the reports of overcoming these plant stresses indicate that the most successful PGPB possess several of these mechanisms; however, the involvement of any particular mechanism in plant protection is nearly always inferred and not proven.

Gibberellic acid interacts with salt stress on germination, growth and polyamine gene expression in fennel (Foeniculum vulgare Mill.) seedlings

This study aimed to rigorously investigate and integrate the underlying hypothesis that an enhancing effect of gibberellic acid (GA3, 3 µM) with increased growth actually leads to a modification of the physiological role of polyamines during salinity stress (NaCl, 100 mM) in fennel. These analyses concern both reserve tissues (cotyledons) and embryonic axes in growth. Physiological results indicate a restriction of germination, growth, mineral nutrition and damages to membranes of salt-treated seedlings. This was partially attenuated in seedlings treated with an interaction effect of GA3 and NaCl. Peroxidase and catalase activities showed a reduction or an augmentation according to the treatments and organs. The three main polyamines (PA): putrescine, spermidine and spermine were elevated in the salt-treated seedlings. Meanwhile, GA3 seed priming was extremely efficient in reducing PA levels in salt-stressed seedlings compared to the control. Response of PA genes to salinity was variable. Up-regulation was noted for SPMS1, ODC1, and ADC1 in hypocotyls and cotyledons (H + C) and down-regulation for SAMDC1 in the radicle. Interaction of salt/GA3 treatment showed different responses, only ODC1 in (H + C) and ADC1 in both radicle and (H + C) were overexpressed. Concerning other genes, no change in mRNA abundance was observed in both organs compared to the salt-treated seedlings. From these results, it could be inferred that the fennel seedlings were NaCl sensitive. This sensitivity was mitigated when GA3 applied for seed priming and applied in combination with NaCl, which resulted in a reduction of the PA content.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01140-4.

Potential Role of Foliar Application of Azotobacter on Growth, Nutritional Value and Quality of Lettuce under Different Nitrogen Levels

Vegetables can be treated with biofertilizers as an alternative to chemical fertilizers because of their low toxicity. We investigated the effects of foliar spraying of Azotobacter under different levels of nitrogen (100, 150 and 200 mg/L in nutrient solution) on the growth, nutritional value, nitrate accumulation and antioxidant enzyme activities of hydroponically grown lettuce. The experiment was laid out in a completely randomized design with four replicates in a factorial combination. Plants treated with Azotobacter and 200 mg/L nitrogen had greater leaf area and photosynthetic pigments than plants treated with 200 mg/L nitrogen without spraying with Azotobacter. Increasing nitrogen levels increased leaf number, fresh and dry weights, leaf area and nitrate accumulation in lettuce plants. Peroxidase (POD) activity increased by 95.4% at a nitrogen level of 200 mg/L compared to a nitrogen level of 100 mg/L. Ascorbate peroxidase (APX) activity and leaf phosphorous (P) and potassium (K) concentrations were the highest in plants treated with a nitrogen source of 100 mg/L without foliar application of Azotobacter. As nitrogen levels increased in all treatments, nitrate reductase (NR) activity decreased and reached a minimum at the 200 mg/L nitrogen level. In general, foliar application of Azotobacter sp. can be used to promote plant growth and reduce nitrate accumulation in lettuce.

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.

Biofertilizers: An ecofriendly technology for nutrient recycling and environmental sustainability

Modern intensive agricultural practices face numerous challenges that pose major threats to global food security. In order to address the nutritional requirements of the ever-increasing world population, chemical fertilizers and pesticides are applied on large scale to increase crop production. However, the injudicious use of agrochemicals has resulted in environmental pollution leading to public health hazards. Moreover, agriculture soils are continuously losing their quality and physical properties as well as their chemical (imbalance of nutrients) and biological health. Plant-associated microbes with their plant growth- promoting traits have enormous potential to solve these challenges and play a crucial role in enhancing plant biomass and crop yield. The beneficial mechanisms of plant growth improvement include enhanced nutrient availability, phytohormone modulation, biocontrol of phytopathogens and amelioration of biotic and abiotic stresses. Solid-based or liquid bioinoculant formulation comprises inoculum preparation, addition of cell protectants such as glycerol, lactose, starch, a good carrier material, proper packaging and best delivery methods. Recent developments of formulation include entrapment/microencapsulation, nano-immobilization of microbial bioinoculants and biofilm-based biofertilizers. This review critically examines the current state-of-art on use of microbial strains as biofertilizers and the important roles performed by these beneficial microbes in maintaining soil fertility and enhancing crop productivity.

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.

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.

Physiological Response of Maize Plants (Zea mays L.) to the Use of the Potassium Quercetin Derivative

Plant production technologies based solely on the improvement of plants themselves face obstacles resulting from the natural limitations of the biological potential of varieties. Therefore, new substances are sought that positively influence the growth and development of plants and increase resistance to various biotic and abiotic stresses, which also translates into an increase in obtained yields. The exogenous application of various phytoprotectants shows great promise in terms of cost effectiveness compared to traditional breeding methods or transgenic approaches in relation to increasing plant tolerance to abiotic stresses. Quercetin is a strong antioxidant among phenolic compounds, and it plays a physiological and biochemical role in plants. As such, the aim of this research was to assess the effect of an aqueous solution of a quercetin derivative with potassium, applied in various concentrations (0.5%, 1.0%, 3.0% and 5.0%), on the efficiency of the photosynthetic apparatus and biochemical properties of maize. Among the tested variants, compared to the control, the most stimulating effect on the course of physiological processes (PN, gs, ci, CCI, Fv/Fm, Fv/F0, PI) in maize leaves was found in 3.0 and 5.0% aqueous solutions of the quercetin derivative. The highest total antioxidant capacity and total content of polyphenolic compounds were found for plants sprayed with 5.0% quercetin derivative solution; therefore, in this study, the optimal concentration could not be clearly selected.

Nutrient sequestration potential of water primrose Ludwigia stolinefera (Guill. & Perr.) P.H. Raven: A strategy for restoring wetland eutrophication

The current work investigates the capacity of the water primrose (Ludwigia stolinefera) to sequester inorganic and organic nutrients in its biomass to restore eutrophic wetlands, besides its nutritive quality as fodder for animals. The nutrient elements and nutritive value of the water primrose were assessed seasonally in polluted and unpolluted watercourses. The water primrose plants’ highest biomass was attained during summer; then, it was significantly reduced till it reached its lowest value during winter. In the polluted canal, the plant root and shoot accumulated higher contents of all nutrient elements (except Na and Mg) rather than in the unpolluted Nile. They accumulated most investigated nutrients in the growing season during summer. The shoots accumulated higher contents of N, P, Ca, and Mg than the root, which accumulated higher concentrations of Na and K. Therefore, summer season is the ideal time to harvest water primrose for removing the maximum nutrients for restoring eutrophic watercourses. The aboveground tissues had the highest values of ether extract (EE) during spring and the highest crude fibers (CF) and total proteins (TP) during summer. In contrast, the belowground tissues had the lowest EE, CF, and TP during winter. In spring, autumn, and winter seasons, the protein content in the grazeable parts (shoots) of the water primrose was within the range, while in summer, it was higher than the minimum requirement for the maintenance of animals. There was a decrease in crude fibers and total proteins, while an increase in soluble carbohydrates content in the below- and above-ground tissues of water primrose under pollution stress. The total protein, lipids, and crude fibers of the aboveground parts of water primrose support this plant as a rough forage.

Potential Use of Beneficial Microorganisms for Soil Amelioration, Phytopathogen Biocontrol, and Sustainable Crop Production in Smallholder Agroecosystems

Smallholder agroecosystems play a key role in the world's food security providing more than 50% of the food produced globally. These unique agroecosystems face a myriad of challenges and remain largely unsupported, yet they are thought to be a critical resource for feeding the projected increasing human population in the coming years. The new challenge to increase food production through agricultural intensification in shrinking per capita arable lands, dwindling world economies, and unpredictable climate change, has led to over-dependence on agrochemical inputs that are often costly and hazardous to both human and animal health and the environment. To ensure healthy crop production approaches, the search for alternative ecofriendly strategies that best fit to the smallholder systems have been proposed. The most common and widely accepted solution that has gained a lot of interest among researchers and smallholder farmers is the use of biological agents; mainly plant growth promoting microorganisms (PGPMs) that provide essential agroecosystem services within a holistic vision of enhancing farm productivity and environmental protection. PGPMs play critical roles in agroecological cycles fundamental for soil nutrient amelioration, crop nutrient improvement, plant tolerance to biotic and abiotic stresses, biocontrol of pests and diseases, and water uptake. This review explores different research strategies involving the use of beneficial microorganisms, within the unique context of smallholder agroecosystems, to promote sustainable maintenance of plant and soil health and enhance agroecosystem resilience against unpredictable climatic perturbations.

Physiological Responses of Salinized Fenugreek (Trigonellafoenum-graecum L.) Plants to Foliar Application of Salicylic Acid

Considering the detrimental effects of salt stress on the physiological mechanisms of plants in terms of growth, development and productivity, intensive efforts are underway to improve plant tolerance to salinity. Hence, an experiment was conducted to assess the impact of the foliar application of salicylic acid (SA; 0.5 mM) on the physiological traits of fenugreek (Trigonellafoenum-graecum L.) plants grown under three salt concentrations (0, 75, and 150 mM NaCl). An increase in salt concentration generated a decrease in the chlorophyll content index (CCI); however, the foliar application of SA boosted the CCI. The malondialdehyde content increased in salt-stressed fenugreek plants, while a reduction in content was observed with SA. Likewise, SA application induced an accumulation of proline, total phenolics, and flavonoids. Moreover, further increases in total free amino acids and shikimic acid were observed with the foliar application of SA, in either control or salt-treated plants. Similar results were obtained for ascorbate peroxidase, peroxidase, polyphenol oxidase, and catalase with SA application. Hence, we concluded that the foliar application of SA ameliorates salinity, and it is a growth regulator that improves the tolerance of fenugreek plants under salt stress.

Nitrogen Fixing Azotobacter Species as Potential Soil Biological Enhancers for Crop Nutrition and Yield Stability

Biological nitrogen fixation (BNF) refers to a microbial mediated process based upon an enzymatic "Nitrogenase" conversion of atmospheric nitrogen (N₂) into ammonium readily absorbable by roots. N₂-fixing microorganisms collectively termed as "diazotrophs" are able to fix biologically N₂ in association with plant roots. Specifically, the symbiotic rhizobacteria induce structural and physiological modifications of bacterial cells and plant roots into specialized structures called nodules. Other N₂-fixing bacteria are free-living fixers that are highly diverse and globally widespread in cropland. They represent key natural source of nitrogen (N) in natural and agricultural ecosystems lacking symbiotic N fixation (SNF). In this review, the importance of Azotobacter species was highlighted as both important free-living N₂-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. In addition, we described Azotobacter beneficial plant promoting traits (e.g., nutrient use efficiency, protection against phytopathogens, phytohormone biosynthesis, etc.). We shed light also on the agronomic features of Azotobacter that are likely an effective component of integrated plant nutrition strategy, which contributes positively to sustainable agricultural production. We pointed out Azotobacter based-biofertilizers, which possess unique characteristics such as cyst formation conferring resistance to environmental stresses. Such beneficial traits can be explored profoundly for the utmost aim to research and develop specific formulations based on inoculant Azotobacter cysts. Furthermore, Azotobacter species still need to be wisely exploited in order to address specific agricultural challenges (e.g., nutrient deficiencies, biotic and abiotic constraints) taking into consideration several variables including their biological functions, synergies and multi-trophic interactions, and biogeography and abundance distribution.

Impact of Foliar Application of Chitosan Dissolved in Different Organic Acids on Isozymes, Protein Patterns and Physio-Biochemical Characteristics of Tomato Grown under Salinity Stress

In this study, the anti-stress capabilities of the foliar application of chitosan, dissolved in four different organic acids (acetic acid, ascorbic acid, citric acid and malic acid) have been investigated on tomato (Solanum lycopersicum L.) plants under salinity stress (100 mM NaCl). Morphological traits, photosynthetic pigments, osmolytes, secondary metabolites, oxidative stress, minerals, antioxidant enzymes activity, isozymes and protein patterns were tested for potential tolerance of tomato plants growing under salinity stress. Salinity stress was caused a reduction in growth parameters, photosynthetic pigments, soluble sugars, soluble proteins and potassium (K⁺) content. However, the contents of proline, ascorbic acid, total phenol, malondialdehyde (MDA), hydrogen peroxide (H₂O₂), sodium (Na⁺) and antioxidant enzyme activity were increased in tomato plants grown under saline conditions. Chitosan treatments in any of the non-stressed plants showed improvements in morphological traits, photosynthetic pigments, osmolytes, total phenol and antioxidant enzymes activity. Besides, the harmful impacts of salinity on tomato plants have also been reduced by lowering MDA, H₂O₂ and Na⁺ levels. Chitosan treatments in either non-stressed or stressed plants showed different responses in number and density of peroxidase (POD), polyphenol oxidase (PPO) and superoxide dismutase (SOD) isozymes. NaCl stress led to the diminishing of protein bands with different molecular weights, while they were produced again in response to chitosan foliar application. These responses were varied according to the type of solvent acid. It could be suggested that foliar application of chitosan, especially that dissolved in ascorbic or citric acid, could be commercially used for the stimulation of tomato plants grown under salinity stress.

Hydrogen sulfide priming can enhance the tolerance of artichoke seedlings to individual and combined saline-alkaline and aniline stresses

Regulatory roles of hydrogen sulfide (H₂S) under saline-alkaline and/or aniline stress have not been studied yet. In this study, we investigated the insights into saline-alkaline and/or aniline stresses-induced toxicity in artichoke plants and its alleviation by H₂S priming. Individual saline-alkaline or aniline stress and their combination reduced plant growth and photosynthetic pigments. Principal component analysis (PCA) revealed that these detrimental impacts were caused by the higher oxidative damage and disruption of osmolyte homeostasis. Interestingly, only aniline stress (25 mg L^-1) caused neither oxidative nor osmotic stress thus almost slight growth retarding effects had ensued. On the other hand, the presence of aniline in saline-alkaline conditions exacerbated stress-induced deleterious effects on plants, as evidenced by PCA and heatmap. However, H₂S priming markedly eased the stress-induced deleteriousness as evident by enhanced chlorophyll, soluble proteins, soluble carbohydrates and up-regulated water relation in H₂S-primmed plants compared with only stressed plants resulting in improved plant phenotypic features. Furthermore, H₂S priming enhanced endogenous H₂S content, phenylalanine ammonia-lyase, non-enzymatic antioxidants (ascorbic acid, flavonoids, glutathione, α-tocopherol, and anthocyanins) and enzymatic antioxidants (superoxide dismutase, catalase, and ascorbate peroxidase), whereas reduced oxidative stress markers (superoxide, hydrogen peroxide, hydroxyl radical, malondialdehyde, and methylglyoxal) compared with only stressed plants, indicating a protective function of H₂S against oxidative damage. The PCA also clarified that H₂S-mediated saline-alkaline and/or aniline stress tolerance strongly connected with the improved antioxidant system. Overall, our finding proposed that H₂S priming could be an effective technique to mitigate saline-alkaline and/or aniline stress in artichoke, and perhaps in other crop plants.

Strategy of Salt Tolerance and Interactive Impact of Azotobacter chroococcum and/or Alcaligenes faecalis Inoculation on Canola (Brassica napus L.) Plants Grown in Saline Soil

A pot experiment was designed and performed in a completely randomized block design (CRBD) to determine the main effect of two plant growth-promoting rhizobacteria (PGPR) and their co-inoculation on growth criteria and physio-biochemical attributes of canola plants (Brassica napus L.) plant grown in saline soil. The results showed that inoculation with two PGPR (Azotobacter chroococcum and/or Alcaligenes faecalis) energized the growth parameters and photosynthetic pigments of stressed plants. Moreover, soluble sugars’ and proteins’ contents were boosted due to the treatments mentioned above. Proline, malondialdehyde (MDA), and hydrogen peroxide (H₂O₂) contents were markedly declined. At the same time, antioxidant enzymes, viz. superoxide dismutase (SOD), ascorbate peroxidase (APX), and peroxidase (POD), were augmented due to the inoculation with Azotobacter chroococcum and/or Alcaligenes faecalis. Regarding minerals’ uptake, there was a decline in sodium (Na) and an increase in nitrogen (N), potassium (K), calcium (Ca), and magnesium (Mg) uptake due to the application of either individual or co-inoculation with the mentioned bacterial isolates. This study showed that co-inoculation with Azotobacter chroococcum and Alcaligenes faecalis was the most effective treatment and could be considered a premium tool used in facing environmental problems, especially saline soils.