Synergistic effect of Chryseobacterium gleum sp. SUK with ACC deaminase activity in alleviation of salt stress and plant growth promotion in Triticum aestivum L

Tolerance Mechanisms of Olive Tree (Olea europaea) under Saline Conditions

The olive tree (Olea europaea L.) is an evergreen tree that occupies 19% of the woody crop area and is cultivated in 67 countries on five continents. The largest olive production region is concentrated in the Mediterranean basin, where the olive tree has had an enormous economic, cultural, and environmental impact since the 7th century BC. In the Mediterranean region, salinity stands out as one of the main abiotic stress factors significantly affecting agricultural production. Moreover, climate change is expected to lead to increased salinization in this region, threatening olive productivity. Salt stress causes combined damage by osmotic stress and ionic toxicity, restricting olive growth and interfering with multiple metabolic processes. A large variability in salinity tolerance among olive cultivars has been described. This paper aims to synthesize information from the published literature on olive adaptations to salt stress and its importance in salinity tolerance. The morphological, physiological, biochemical, and molecular mechanisms of olive tolerance to salt stress are reviewed.

Unveiling genomic features linked to traits of plant growth-promoting bacterial communities from sugarcane

Microorganisms are ubiquitous, and those inhabiting plants have been the subject of several studies. Plant-associated bacteria exhibit various biological mechanisms that enable them to colonize host plants and, in some cases, enhance their fitness. In this study, we describe the genomic features predicted to be associated with plant growth-promoting traits in six bacterial communities isolated from sugarcane. The use of highly accurate single-molecule real-time sequencing technology for metagenomic samples from these bacterial communities allowed us to recover 17 genomes. The taxonomic assignments for the binned genomes were performed, revealing taxa distributed across three main phyla: Bacillota, Bacteroidota, and Pseudomonadota, with the latter being the most representative. Subsequently, we functionally annotated the metagenome-assembled genomes (MAGs) to characterize their metabolic pathways related to plant growth-promoting traits. Our study successfully identified the enrichment of important functions related to phosphate and potassium acquisition, modulation of phytohormones, and mechanisms for coping with abiotic stress. These findings could be linked to the robust colonization of these sugarcane endophytes.

Comparative genomic analysis of Chryseobacterium species: deep insights into plant-growth-promoting and halotolerant capacities

Members of the genus Chryseobacterium have attracted great interest as beneficial bacteria that can promote plant growth and biocontrol. Given the recent risks of climate change, it is important to develop tolerance strategies for efficient applications of plant-beneficial bacteria in saline environments. However, the genetic determinants of plant-growth-promoting and halotolerance effects in Chryseobacterium have not yet been investigated at the genomic level. Here, a comparative genomic analysis was conducted with seven Chryseobacterium species. Phylogenetic and phylogenomic analyses revealed niche-specific evolutionary distances between soil and freshwater Chryseobacterium species, consistent with differences in genomic statistics, indicating that the freshwater bacteria have smaller genome sizes and fewer genes than the soil bacteria. Phosphorus- and zinc-cycling genes (required for nutrient acquisition in plants) were universally present in all species, whereas nitrification and sulphite reduction genes (required for nitrogen- and sulphur-cycling, respectively) were distributed only in soil bacteria. A pan-genome containing 6842 gene clusters was constructed, which reflected the general features of the core, accessory and unique genomes. Halotolerant species with an accessory genome shared a Kdp potassium transporter and biosynthetic pathways for branched-chain amino acids and the carotenoid lycopene, which are associated with countermeasures against salt stress. Protein-protein interaction network analysis was used to define the genetic determinants of Chryseobacterium salivictor NBC122 that reduce salt damage in bacteria and plants. Sixteen hub genes comprised the aromatic compound degradation and Por secretion systems, which are required to cope with complex stresses associated with saline environments. Horizontal gene transfer and CRISPR-Cas analyses indicated that C. salivictor NBC122 underwent more evolutionary events when interacting with different environments. These findings provide deep insights into genomic adaptation to dynamic interactions between plant-growth-promoting Chryseobacterium and salt stress.

Auxin-producing bacteria promote barley rhizosheath formation

The rhizosheath, or the layer of soil closely adhering to roots, can help plants to tolerate drought under moderate soil drying conditions. Rhizosheath formation is the result of poorly understood interactions between root exudates, microbes, and soil conditions. Here, we study the roles played by the soil microbiota in rhizosheath formation in barley (a dry crop). We show that barley rhizosheath formation is greater in acid soil than in alkaline soil, and inoculation with microbiota from acid soil enhances rhizosheath formation in alkaline soil. The rhizosheath-promoting activity is associated with the presence of Flavobacteriaceae and Paenibacillaceae bacteria that express genes for biosynthesis of indole-3-acetic acid (IAA, a common auxin), as determined by metagenomics and metatranscriptomics. Two bacterial strains isolated from rhizosheath (Chryseobacterium culicis and Paenibacillus polymyxa) produce IAA and enhance barley rhizosheath formation, while their IAA-defective mutants are unable to promote rhizosheath formation. Co-inoculation with the IAA-producing strains enhances barley grain yield in field experiments through an increase in spike number. Our findings contribute to our understanding of barley rhizosheath formation, and suggest potential strategies for crop improvement.

Bioprospecting acid- and arsenic-tolerant plant growth-promoting rhizobacteria for mitigation of arsenic toxicity in acidic agricultural soils

Widespread use of chemical fertilizers and falling productivity in traditional agricultural practices has led to the biodiversity hotspot of North-Eastern region of India to face imminent threat to soil nutrients and biodiversity. The present work aimed to isolate rhizobacteria from Oryza sativa L. to evaluate their plant growth-promoting traits like indole, ammonia, siderophore production, and phosphate solubilization followed by in vitro plant growth promotion and anti-fungal assessment against Curvularia oryzae. Moreover, presence of heavy metals such as arsenic in chemical fertilizers and in groundwater contributes to arsenic contamination of agricultural soil. Taking this into consideration for the present study, the background metal content of the bulk soil, roots and grains of rice was measured. Arsenic tolerance of the rhizobacterial isolates was assessed using different concentrations of arsenite- and arsenate-supplemented media. 16S rRNA gene sequencing and phylogenetic tree analysis identified the isolates as Bacillus paramycoides, B. albus, B. altitudinis, B. koreensis, B. megaterium, B. wiedmannii, B. paramycoides, Chryseobacterium gleum, Stenotrophomonas maltophilia and Pseudomonas shirazica. Considering the acidic nature of the paddy growing soil, the growth kinetics of the isolates were monitored in acid and arsenic-supplemented conditions for 48 h of growth. Few isolates showed potent anti-fungal activity against the late blight phytopathogen, Curvularia oryzae MTCC 2605, apart from being potential growth promoters. The findings open vistas for the mass production of the characterized PGP rhizobacteria for their application in rehabilitation of the degrading arsenic contaminated paddy fields.

Synergism: biocontrol agents and biostimulants in reducing abiotic and biotic stresses in crop

In today's fast-shifting climate change scenario, crops are exposed to environmental pressures, abiotic and biotic stress. Hence, these will affect the production of agricultural products and give rise to a worldwide economic crisis. The increase in world population has exacerbated the situation with increasing food demand. The use of chemical agents is no longer recommended due to adverse effects towards the environment and health. Biocontrol agents (BCAs) and biostimulants, are feasible options for dealing with yield losses induced by plant stresses, which are becoming more intense due to climate change. BCAs and biostimulants have been recommended due to their dual action in reducing both stresses simultaneously. Although protection against biotic stresses falls outside the generally accepted definition of biostimulant, some microbial and non-microbial biostimulants possess the biocontrol function, which helps reduce biotic pressure on crops. The application of synergisms using BCAs and biostimulants to control crop stresses is rarely explored. Currently, a combined application using both agents offer a great alternative to increase the yield and growth of crops while managing stresses. This article provides an overview of crop stresses and plant stress responses, a general knowledge on synergism, mathematical modelling used for synergy evaluation and type of in vitro and in vivo synergy testing, as well as the application of synergism using BCAs and biostimulants in reducing crop stresses. This review will facilitate an understanding of the combined effect of both agents on improving crop yield and growth and reducing stress while also providing an eco-friendly alternative to agroecosystems.

Salinity alleviator bacteria in rice (Oryza sativa L.), their colonization efficacy, and synergism with melatonin

In this study, rhizospheric and endophytic bacteria were tested for the alleviation of salinity stress in rice. Endophytic isolates were taken from previous studies based on their salt stress-alleviating traits. The rhizospheric bacteria were isolated from rice and screened based on salt tolerance and plant growth-promoting traits. Molecular identification indicated the presence of class Gammaproteobacteria, Bacillota, and Actinomycetia. Two-two most potential isolates each from rhizospheric and endophytic bacteria were selected for in planta trials. Results showed that microbial inoculation significantly improved germination and seedling vigor under elevated salinity. The confocal scanning laser microscopy showed higher bacterial colonization in inoculated rice roots than in control. Based on this experiment, rhizospheric bacteria Brevibacterium frigoritolerans W19 and endophytic Bacillus safensis BTL5 were selected for pot trial along with a growth-inducing compound melatonin 20 ppm. Inoculation of these two bacteria improved the levels of chlorophyll, proline, phenylalanine ammonia-lyase, catalase, superoxide dismutase, polyphenol oxidase, root-shoot length, and dry weight under elevated salt concentration. The gene expression studies showed modulation of SOD1, CATa, NHX1, and PAL1 genes by the bacterial strains and melatonin application. The inoculation was found to have additive effects with 20 ppm melatonin. This enhancement in dry matter accumulation, compatible solute production, and oxidative stress regulation could help plants in mitigating the ill effects of high salinity. Exploring such a combination of microbes and inducer molecules could be potentially useful in developing stress-alleviating bioformulations.

Community succession and functional prediction of microbial consortium with straw degradation during subculture at low temperature

To systematically explore and analyze the microbial composition and function of microbial consortium M44 with straw degradation in the process of subculture at low temperature. In this study, straw degradation characteristics of samples in different culture stages were determined. MiSeq high-throughput sequencing technology was used to analyze the evolution of community structure and its relationship with degradation characteristics of microbial consortium in different culture periods, and the PICRUSt function prediction analysis was performed. The results showed that straw degradation rate, endoglucanase activity, and filter paper enzyme activity of M44 generally decreased with increasing culture algebra. The activities of xylanase, laccase, and lignin peroxidase, as well as VFA content, showing a single-peak curve change with first an increase and then decrease. In the process of subculture, Proteobacteria, Bacteroidetes, and Firmicutes were dominant in different culture stages. Pseudomonas, Flavobacterium, Devosia, Brevundimonas, Trichococcus, Acinetobacter, Dysgonomonas, and Rhizobium were functional bacteria in different culture stages. It was found by PICRUSt function prediction that the functions were concentrated in amino acid transport and metabolism, carbohydrate transship and metabolism related genes, which may contain a large number of fibers and lignin degrading enzyme genes. In this study, the microbial community succession and the gene function in different culture periods were clarified and provide a theoretical basis for screening and rational utilization of microbial consortia.

Cadmium-resistant Chryseobacterium sp. DEMBc1 strain: characteristics and potential to assist phytoremediation and promote plant growth

The effectiveness of phytoremediation is closely related to the various interactions between pollutants, soil particles, rhizosphere microorganisms, and plants. Therefore, the object of current study was a cadmium-tolerant bacterium isolated from the rye rhizosphere, with a high degree of genetic similarity to the genus Chryseobacterium. Chryseobacterium sp. DEMBc1 was able to grow with 36 different BiologGN2 carbon sources and show the adaptation to stress factors such as Cd (100 μg ml^-1), low temperature (8 °C), and salinity (2% NaCl). Furthermore, it was shown that DEMBc1 had the characteristics of plant growth-promoting microorganisms: it was able to produce ammonia, indole acetic acid, 1-aminocyclopropane-1-carboxylic acid deaminase, and siderophores, as well as solubilize Ca₃(PO₄)₃. After inoculation with DEMBc1, a significant decrease in the concentration of Cd was observed in the roots of Festuca ovina grown in Cd-polluted soil, compared to the non-inoculated Cd-polluted soil. It was also noticed that DEMBc1 produced a large amount of extracellular polymeric substances that were significantly higher than the cellular biomass. These polymers can form a barrier to reduce the translocation of Cd from the growth medium to the plant roots. According to the current study, DEMBc1 has a stabilizing potential and can decrease the mobility of Cd in the F. ovina rhizosphere, bioaccumulate metals in plant tissues, and effectively improve the bioavailability of nutrients, especially Fe, N, and P in a polluted environments.

Salinity of irrigation water selects distinct bacterial communities associated with date palm (Phoenix dactylifera L.) root

Saline water irrigation has been used in date palm (Phoenix dactylifera L.) agriculture as an alternative to non-saline water due to water scarcity in hyper-arid environments. However, the knowledge pertaining to saline water irrigation impact on the root-associated bacterial communities of arid agroecosystems is scarce. In this study, we investigated the effect of irrigation sources (non-saline freshwater vs saline groundwater) on date palm root-associated bacterial communities using 16S rDNA metabarcoding. The bacterial richness, Shannon diversity and evenness didn't differ significantly between the irrigation sources. Soil electrical conductivity (EC) and irrigation water pH were negatively related to Shannon diversity and evenness respectively, while soil organic matter displayed a positive correlation with Shannon diversity. 40.5% of total Operational Taxonomic Units were unique to non-saline freshwater irrigation, while 26% were unique to saline groundwater irrigation. The multivariate analyses displayed strong structuring of bacterial communities according to irrigation sources, and both soil EC and irrigation water pH were the major factors affecting bacterial communities. The genera Bacillus, Micromonospora and Mycobacterium were dominated while saline water irrigation whereas contrasting pattern was observed for Rhizobium, Streptomyces and Acidibacter. Taken together, we suggest that date-palm roots select specific bacterial taxa under saline groundwater irrigation, which possibly help in alleviating salinity stress and promote growth of the host plant.

Community succession and straw degradation characteristics using a microbial decomposer at low temperature

This study explored changes in the microbial community structure during straw degradation by a microbial decomposer, M44. The microbial community succession at different degradation periods was analyzed using MiSeq high-throughput sequencing. The results showed that 14 days after inoculation, the filter paper enzyme and endoglucanase activities increased to 2.55 U·mL^-1 and 2.34 U·mL^-1. The xylanase, laccase, and lignin peroxidase activities rose to 9.86 U·mL^-1, 132.16 U·L^-1, and 85.43 U·L^-1 after 28 d, which was consistent with changes in the straw degradation rate. The degradation rates of straw, lignin, cellulose, and hemicellulose were 31.43%, 13.67%, 25.04%, and 21.69%, respectively, after 28 d of fermentation at 15°C. Proteobacteria, Firmicutes, and Bacteroidetes were the main bacterial species in samples at different degradation stages. The dominant genera included Pseudomonas, Delftia, and Paenibacillus during the initial stage (1 d, 7 d) and the mid-term stage (14 d). The key functional microbes during the late stage (21 d, 28 d) were Rhizobium, Chryseobacterium, Sphingobacterium, Brevundimonas, and Devosia. Changes in the bacterial consortium structure and straw degradation characteristics during different degradation periods were clarified to provide a theoretical basis for the rational utilization of microbial decomposer M44.

Halotolerant biofilm-producing rhizobacteria mitigate seawater-induced salt stress and promote growth of tomato

Biofilm-producing rhizobacteria (BPR) enhance productivity and mitigate abiotic stresses in plants. This study showed that 21 out of 65 halotolerant rhizobacteria could build biofilms. The components of the biofilm matrices i.e., extracellular polymeric substances (EPS) are proteins, curli, nanocelloluse, nucleic acids, lipids, and peptidoglycans. Various functional groups including carbonyl, carboxyl, amino, hydroxyl, and phosphate were identified. Positions of these groups were shifted by application of 5% NaCl, suggesting Na⁺ biosorption. By sequencing, Glutamicibacter arilaitensis (ESK1, ESM4 and ESM7), G. nicotianae (ESK19, ESM8 and ESM16), Enterobacter ludwigii (ESK15, ESK17, ESM2 and ESM17), E. cloacae (ESM5 and ESM12), Exiguobacterium acetylicum (ESM24 and ESM25), Staphylococcus saprophyticus ESK6, Leclercia adecarboxylata ESK12, Pseudomonas poae ESK16, Bacillus subtilis ESM14, and P. putida ESM17 were identified. These rhizobacteria exhibited numerous plant growth-promoting (PGP) activities including producing IAA, ACC deaminase, and siderophores, and solubilizing phosphate. Under non-stress, bacterized plants increased biomass accumulation (8–23.2% roots and 23–49.4% shoots), while under seawater-induced salt stress only ESK12, ESM4, ESM12, and ESM14 enhanced biomass production (5.8–52.9% roots and 8.8–33.4% shoots). Bacterized plants induced antioxidant defense system (19.5–142% catalase and 12.3–24.2% DPPH radical scavenging activity), retained a greater relative water content (17–124%), showed lesser membrane injuries (19.9–26.5%), and a reduced Na⁺ (6–24% in roots) and increased K⁺/Na⁺ ratio (78.8 and 103% in roots by ESK12 and ESM24, respectively) than the non-bacterized plants in saline conditions. Thus, native halotolerant BPR can be utilized as ameliorators of salt stress.

Screening of Carbofuran-Degrading Bacteria Chryseobacterium sp. BSC2-3 and Unveiling the Change in Metabolome during Carbofuran Degradation

Carbofuran is one of the most commonly used N-methylcarbamate-based pesticides and is excellent for controlling pests; however, carbofuran also causes soil and water pollution. Although various studies have been conducted on the bioremediation of pesticide-contaminated soil, the changes occurring in the metabolome during the bioremediation of carbofuran are not fully understood. In this study, the intracellular and extracellular metabolites of the Chryseobacterium sp. BSC2-3 strain were analysed during carbofuran degradation by using a liquid chromatography–mass spectrometry-based metabolomics approach. We found that the BSC2-3 strain extracellularly transformed carbofuran into 3-hydroxycarbofuran. Intracellular metabolite analysis revealed that carbofuran mainly affected aminobenzoate degradation, ubiquinone and terpenoid-quinone biosynthesis, and arginine and proline metabolism. Carbofuran especially affected the metabolic pathway for the degradation of naphthalene and aminobenzoate. Metabolomics additionally revealed that the strain produces disease resistance inducers and plant growth regulators. We also identified the genes involved in the production of indole-3-acetic acid, which is one of the most active auxins. Overall, we identified the metabolic changes induced in carbofuran-degrading bacteria and the genes predicted to be responsible for the degradation of carbofuran.

Combined application of H2S and a plant growth promoting strain JIL321 regulates photosynthetic efficacy, soil enzyme activity and growth-promotion in rice under salt stress

Salinity stress is one of the most harmful abiotic stresses that inhibit crop growth and grain yield. In this study, a salt-tolerant bacterium was isolated from the soil of the rice rhizosphere and named Myroides sp. JIL321, based on the results of the phylogenetic tree analysis. The strain JIL321 tolerated up to 1, 283.37 mM of NaCl and exhibited positive plant growth-promoting traits, such as the production of indole acetic acid (IAA) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Therefore, the effects of JIL321 on rice (Oryza sativa L.) under salinity stress were determined. The inoculation of strain JIL321 significantly increased the chlorophyll content and the accumulation of osmotic adjustment substances, such as proline and soluble sugars, in rice expose to salt stress. Additionally, strain JIL321 inoculation significantly enhanced the activities of some enzymes commonly found in soil, such as urease, invertase and catalase. Moreover, the production of hydrogen sulfide (H₂S), a pivotal signaling molecule, was also induced in rice by salt stress. Treatment with sodium hydrogen sulfide (NaHS, H₂S donor) improved salt stress tolerance of the rice, while treatment with hypotaurine (HT, H₂S scavenger) significantly suppressed it. Interestingly, NaHS treatment also improved the production of IAA and ACC deaminase in strain JIL321 under 0 mM and 150 mM salt concentrations. The combined treatment of JIL321 and NaHS could further improve the growth of salt-stressed rice seedlings, most likely due to the interaction effect between H₂S and strain JIL321. To our knowledge, this study is the first to demonstrate that the combined use of H₂S and plant growth-promoting bacteria could alleviate the adverse effects of salt stress on rice plants, and further verifies the novel role of H₂S as a signaling molecule that enhance the tolerance of plant to abiotic stresses.

Salinity Stress: Toward Sustainable Plant Strategies and Using Plant Growth-Promoting Rhizobacteria Encapsulation for Reducing It

Salinity is one of the most important abiotic stresses that influences plant growth and productivity worldwide. Salinity affects plant growth by ionic toxicity, osmotic stress, hormonal imbalance, nutrient mobilization reduction, and reactive oxygen species (ROS). To survive in saline soils, plants have developed various physiological and biochemical strategies such as ion exchange, activation of antioxidant enzymes, and hormonal stimulation. In addition to plant adaption mechanisms, plant growth-promoting rhizobacteria (PGPR) can enhance salt tolerance in plants via ion homeostasis, production of antioxidants, ACC deaminase, phytohormones, extracellular polymeric substance (EPS), volatile organic compounds, accumulation of osmolytes, activation of plant antioxidative enzymes, and improvement of nutrients uptake. One of the important issues in microbial biotechnology is establishing a link between the beneficial strains screened in the laboratory with industry and the consumer. Therefore, in the development of biocontrol agents, it is necessary to study the optimization of conditions for mass reproduction and the selection of a suitable carrier for their final formulation. Toward sustainable agriculture, the use of appropriate formulations of bacterial agents as high-performance biofertilizers, including microbial biocapsules, is necessary to improve salt tolerance and crop productivity.

Inter-Genera Colonization of Ocimum tenuiflorum Endophytes in Tomato and Their Complementary Effects on Na+/K+ Balance, Oxidative Stress Regulation, and Root Architecture Under Elevated Soil Salinity

Endophytic bacilli of ethano-botanical plant Ocimum tenuiflorum were screened for salt stress-alleviating traits in tomato. Four promising O. tenuiflorum endophytes (Bacillus safensis BTL5, Bacillus haynesii GTR8, Bacillus paralicheniformis GTR11, and Bacillus altitudinis GTS16) were used in this study. Confocal scanning laser microscopic studies revealed the inter-genera colonization of O. tenuiflorum endophytes in tomato plants, giving insights for widening the applicability of potential endophytes to other crops. Furthermore, in a pot trial under 150 mM NaCl concentration, the inoculated endophytes contributed in reducing salt toxicity and improving recovery from salt-induced oxidative stress by different mechanisms. Reduction in reactive oxygen species (ROS) (sub-cellular H₂O₂ and superoxide) accumulation was observed besides lowering programmed cell death and increasing chlorophyll content. Endophyte inoculation supplemented the plant antioxidant enzyme system via the modulation of enzymatic antioxidants, viz., peroxidase, ascorbate peroxidase, superoxide dismutase, and catalase, apart from increasing proline and total phenolics. Antioxidants like proline have dual roles of antioxidants and osmoregulation, which might also have contributed to improved water relation under elevated salinity. Root architecture, viz., root length, projection area, surface area, average diameter, tips, forks, crossings, and the number of links, was improved upon inoculation, indicating healthy root growth and enhanced nutrient flow and water homeostasis. Regulation of Na⁺/K⁺ balance and water homeostasis in the plants were also evident from the modulation in the expression of abiotic stress-responsive genes, viz., LKT1, NHX1, SOS1, LePIP2, SlERF16, and SlWRKY39. Shoot tissues staining with light-excitable Na⁺ indicator Sodium Green^TM Tetra (tetramethylammonium) salt showed low sodium transport and accumulation in endophyte-inoculated plants. All four endophytes exhibited different mechanisms for stress alleviation and indicated complementary effects on plant growth. Furthermore, this could be harnessed in the form of a consortium for salt stress alleviation. The present study established inter-genera colonization of O. tenuiflorum endophytes in tomato and revealed its potential in maintaining Na⁺/K⁺ balance, reducing ROS, and improving root architecture under elevated salinity.

Plant Growth Promoting Rhizobacteria, Arbuscular Mycorrhizal Fungi and Their Synergistic Interactions to Counteract the Negative Effects of Saline Soil on Agriculture: Key Macromolecules and Mechanisms

Soil saltiness is a noteworthy issue as it results in loss of profitability and development of agrarian harvests and decline in soil health. Microorganisms associated with plants contribute to their growth promotion and salinity tolerance by employing a multitude of macromolecules and pathways. Plant growth promoting rhizobacteria (PGPR) have an immediate impact on improving profitability based on higher crop yield. Some PGPR produce 1-aminocyclopropane-1-carboxylic (ACC) deaminase (EC 4.1.99.4), which controls ethylene production by diverting ACC into α-ketobutyrate and ammonia. ACC deaminase enhances germination rate and growth parameters of root and shoot in different harvests with and without salt stress. Arbuscular mycorrhizal fungi (AMF) show a symbiotic relationship with plants, which helps in efficient uptake of mineral nutrients and water by the plants and also provide protection to the plants against pathogens and various abiotic stresses. The dual inoculation of PGPR and AMF enhances nutrient uptake and productivity of several crops compared to a single inoculation in both normal and stressed environments. Positively interacting PGPR + AMF combination is an efficient and cost-effective recipe for improving plant tolerance against salinity stress, which can be an extremely useful approach for sustainable agriculture.

Indigenous bacteria, an excellent reservoir of functional plant growth promoters for enhancing duckweed biomass yield on site

The advantages of aquatic biomass production using wastewater as a cost-free fertilizer have recently been highlighted. Here, we report a successful study in which duckweed, Lemna gibba, biomass production in a food factory effluent containing low nitrogen and high salts was enhanced by employing customized plant growth-promoting bacteria (PGPB). Two common PGPB strains previously obtained from natural pond water, Acinetobacter calcoaceticus P23 and Pseudomonas fulva Ps6, hardly promoted the growth of duckweed; on the contrary, they inhibited its growth in treated factory wastewater, far different water conditions. Then, we asked if some indigenous wastewater bacteria could promote the growth of duckweed. We found that Chryseobacterium strains, a group of bacteria with limited nitrogen metabolism, were dominantly selected as effective PGPB. Moreover, we demonstrated that nitrogen limitation is the crucial environmental factor that induces the plant growth-inhibiting behavior of A. calcoaceticus P23 through competition for mineral nutrients with the host duckweed. This study uncovered points to be considered in PGPB technology to achieve efficient production of duckweed biomass in a factory effluent with unbalanced content of mineral nutrients.

Roles of Plant Growth-Promoting Rhizobacteria (PGPR) in Stimulating Salinity Stress Defense in Plants: A Review

To date, soil salinity becomes a huge obstacle for food production worldwide since salt stress is one of the major factors limiting agricultural productivity. It is estimated that a significant loss of crops (20-50%) would be due to drought and salinity. To embark upon this harsh situation, numerous strategies such as plant breeding, plant genetic engineering, and a large variety of agricultural practices including the applications of plant growth-promoting rhizobacteria (PGPR) and seed biopriming technique have been developed to improve plant defense system against salt stress, resulting in higher crop yields to meet human's increasing food demand in the future. In the present review, we update and discuss the advantageous roles of beneficial PGPR as green bioinoculants in mitigating the burden of high saline conditions on morphological parameters and on physio-biochemical attributes of plant crops via diverse mechanisms. In addition, the applications of PGPR as a useful tool in seed biopriming technique are also updated and discussed since this approach exhibits promising potentials in improving seed vigor, rapid seed germination, and seedling growth uniformity. Furthermore, the controversial findings regarding the fluctuation of antioxidants and osmolytes in PGPR-treated plants are also pointed out and discussed.

Plant Growth-Promoting Bacteria: Biological Tools for the Mitigation of Salinity Stress in Plants

Salinity stress is one of the major abiotic stresses threatening sustainable crop production worldwide. The extent of salinity affected area is expected to cover about 50% of total agricultural land by 2050. Salinity stress produces various detrimental effects on plants’ physiological, biochemical, and molecular features and reduces productivity. The poor plant growth under salinity stress is due to reduced nutrient mobilization, hormonal imbalance, and formation of reactive oxygen species (ROS), ionic toxicity, and osmotic stress. Additionally, salinity also modulates physicochemical properties and reduces the microbial diversity of soil and thus decreases soil health. On the other hand, the demand for crop production is expected to increase in coming decades owing to the increasing global population. Conventional agricultural practices and improved salt-tolerant crop varieties will not be sufficient to achieve the yields desired in the near future. Plants harbor diverse microbes in their rhizosphere, and these have the potential to cope with the salinity stress. These salinity-tolerant plant growth-promoting bacteria (PGPB) assist the plants in withstanding saline conditions. These plant-associated microbes produce different compounds such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase, indole-3-acetic acid (IAA), antioxidants, extracellular polymeric substance (EPS), and volatile organic compounds (VOC). Additionally, the naturally associated microbiome of plants has the potential to protect the host through stress avoidance, tolerance, and resistance strategies. Recent developments in microbiome research have shown ways in which novel microbe-assisted technologies can enhance plant salt tolerance and enable higher crop production under saline conditions. This focused review article presents the global scenario of salinity stress and discusses research highlights regarding PGPB and the microbiome as a biological tool for mitigation of salinity stress in plants.

Thermotolerance effect of plant growth-promoting Bacillus cereus SA1 on soybean during heat stress

BACKGROUND

Incidences of heat stress due to the changing global climate can negatively affect the growth and yield of temperature-sensitive crops such as soybean variety, Pungsannamul. Increased temperatures decrease crop productivity by affecting biochemical, physiological, molecular, and morphological factors either individually or in combination with other abiotic stresses. The application of plant growth-promoting endophytic bacteria (PGPEB) offers an ecofriendly approach for improving agriculture crop production and counteracting the negative effects of heat stress.

RESULTS

We isolated, screened and identified thermotolerant B. cereus SA1 as a bacterium that could produce biologically active metabolites, such as gibberellin, indole-3-acetic acid, and organic acids. SA1 inoculation improved the biomass, chlorophyll content, and chlorophyll fluorescence of soybean plants under normal and heat stress conditions for 5 and 10 days. Heat stress increased abscisic acid (ABA) and reduced salicylic acid (SA); however, SA1 inoculation markedly reduced ABA and increased SA. Antioxidant analysis results showed that SA1 increased the ascorbic acid peroxidase, superoxide dismutase, and glutathione contents in soybean plants. In addition, heat stress markedly decreased amino acid contents; however, they were increased with SA1 inoculation. Heat stress for 5 days increased heat shock protein (HSP) expression, and a decrease in GmHSP expression was observed after 10 days; however, SA1 inoculation augmented the heat stress response and increased HSP expression. The stress-responsive GmLAX3 and GmAKT2 were overexpressed in SA1-inoculated plants and may be associated with decreased reactive oxygen species generation, altered auxin and ABA stimuli, and enhanced potassium gradients, which are critical in plants under heat stress.

CONCLUSION

The current findings suggest that B. cereus SA1 could be used as a thermotolerant bacterium for the mitigation of heat stress damage in soybean plants and could be commercialized as a biofertilizer only in case found non-pathogenic.

Extending thermotolerance to tomato seedlings by inoculation with SA1 isolate of Bacillus cereus and comparison with exogenous humic acid application

Heat stress is one of the major abiotic stresses that impair plant growth and crop productivity. Plant growth-promoting endophytic bacteria (PGPEB) and humic acid (HA) are used as bio-stimulants and ecofriendly approaches to improve agriculture crop production and counteract the negative effects of heat stress. Current study aimed to analyze the effect of thermotolerant SA1 an isolate of Bacillus cereus and HA on tomato seedlings. The results showed that combine application of SA1+HA significantly improved the biomass and chlorophyll fluorescence of tomato plants under normal and heat stress conditions. Heat stress increased abscisic acid (ABA) and reduced salicylic acid (SA) content; however, combined application of SA1+HA markedly reduced ABA and increased SA. Antioxidant enzymes activities revealed that SA1 and HA treated plants exhibited increased levels of ascorbate peroxidase (APX), superoxide dismutase (SOD), and reduced glutathione (GSH). In addition, heat stress markedly reduced the amino acid contents; however, the amino acids were increased with co-application of SA1+HA. Similarly, inductively-coupled plasma mass-spectrometry results showed that plants treated with SA1+HA exhibited significantly higher iron (Fe+), phosphorus (P), and potassium (K+) uptake during heat stress. Heat stress increased the relative expression of SlWRKY33b and autophagy-related (SlATG5) genes, whereas co-application of SA1+HA augmented the heat stress response and reduced SlWRKY33b and SlATG5 expression. The heat stress-responsive transcription factor (SlHsfA1a) and high-affinity potassium transporter (SlHKT1) were upregulated in SA1+HA-treated plants. In conclusion, current findings suggest that co-application with SA1+HA can be used for the mitigation of heat stress damage in tomato plants and can be commercialized as a biofertilizer.

Beneficial Plant Microorganisms Affect the Endophytic Bacterial Communities of Durum Wheat Roots as Detected by Different Molecular Approaches

This study aimed at characterising the endophytic bacterial communities living in durum wheat roots, as affected by wheat cultivar and inoculation of the Arbuscular mycorrhizal fungus Funneliformis mosseae IMA1 and the wheat root endophytic bacterium Lactobacillus plantarum B.MD.R.A2. These microorganisms were inoculated, alone or in combination, in durum wheat (cultivars Odisseo and Saragolla). Non-inoculated plants of each cultivar represented the controls. Forty-three days after sowing, roots were deprived of the epiphytic microbiota and subjected to DNA extraction. The DNA was used as template in PCR-DGGE analysis of the 16S rRNA gene (variable region V3–V5) and 16S (region V1–V3) metagenetics. Odisseo and Saragolla root endophytic bacterial biotas differed for number of OTUs and composition. In detail, Pseudomonas was higher in Odisseo than in Saragolla. The inoculation of F. mosseae and L. plantarum increased the abundance of Pseudomonas, some Actinobacteria (e.g., Streptomyces, Microbacterium, two genera including several plant growth promoting (PGP) strains) and Bacteroidetes in both cultivars. However, the endophytic bacterial biota of Saragolla roots inoculated just with lactobacilli did not differ from that of the control. The inoculation of Saragolla with F. mosseae, alone or in combination with lactobacilli, led to higher abundance of Rhodococcus, belonging to Actinobacteria and encompassing PGP strains. First, this work showed that F. mosseae and L. plantarum shape the endophytic bacterial biota of durum wheat roots. Abundance of some OTUs was affected by the microbial inoculation, depending on the cultivar. This result represents a starting point for exploitation of beneficial endophytes of wheat roots.

Enhancement of growth and salt tolerance of rice seedlings by ACC deaminase-producing Burkholderia sp. MTCC 12259

Increasing soil salinity is often associated with accelerated ethylene production in plants, leading to overall growth reduction. The salt-tolerant 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing PGPR may alleviate salt stress by reducing the production of stress ethylene. In this study, a salt-tolerant ACC deaminase-producing strain named P50 was isolated from a coastal rice field in Odisha, India, which enhanced the growth of rice seedlings under salt stress. The P50 strain was identified as Burkholderia sp. based on phenotypic characteristics, MALDI-TOF MS data for ribosomal proteins and 16S rDNA sequence-based homology. Various PGP traits of strain P50 were characterized, among which the ACC deaminase activity was optimized at different physical conditions and confirmed by enzyme assay, as well as FTIR. The IAA, EPS and proline production of this strain were estimated under increasing NaCl concentrations essential for plant growth promotion under salt stress. Finally, the P50 strain was utilized in a gnotobiotic assay using rice seedlings (cv. Swarnamasuri) under saline stress. Seedlings treated with the P50 strain showed improvement in various morphological and biochemical characteristics, ROS scavenging antioxidant enzymatic activities, and reduced amounts of stress ethylene compared to non-inoculated strains under salinity. In addition, isolation of the ACC deaminase mutant of this strain was not found to reduce stress ethylene, confirming that the P50 strain was associated with a reduction in stress ethylene. Strain P50 was also found to colonize the root surfaces of rice seedlings associated with the plant-microbe interaction process. Thus, as an effective salt-tolerant PGPR, strain P50 can be utilized in salt-affected agricultural fields to improve plant growth in a sustainable manner.