Bacillus megaterium strain A12 ameliorates salinity stress in tomato plants through multiple mechanisms

Halo-tolerant plant growth-promoting bacteria-mediated plant salt resistance and microbiome-based solutions for sustainable agriculture in saline soils

Soil salinization has been the major form of soil degradation under the dual influence of climate change and high-intensity human activities, threatening global agricultural sustainability and food security. High salt concentrations induce osmotic imbalance, ion stress, oxidative damage, and other hazards to plants, resulting in retarded growth, reduced biomass, and even total crop failure. Halo-tolerant plant growth promoting rhizobacteria (HT-PGPR), as a widely distributed group of beneficial soil microorganisms, are emerging as a valuable biological tool for mitigating the toxic effects of high salt concentrations and improve plant growth while remediating degraded saline soil. Here, the current status, harm, and treatment measures of global soil salinization are summarized. The mechanism of salt tolerance and growth promotion induced by HT-PGPR are reviewed. We highlight that advances in multiomics technologies are helpful for exploring the genetic and molecular mechanisms of microbiota centered on HT-PGPR to address the issue of plant losses in saline soil. Future research is urgently needed to comprehensively and robustly determine the interaction mechanism between the root microbiome centered on HT-PGPR and salt-stressed plants via advanced means to maximize the efficacy of HT-PGPR as a microbial agent.

Genetic diversity in F3 segregating populations of rice (Oryza sativa L.) genotypes under salt stress

Rice is an important cereal crop rich in starch and carbohydrates grown around the globe. Despite its significance, rice exhibits substantial genetic variation, particularly under environmental stresses such as salinity. This study investigates the genetic diversity of F3 segregating populations of rice under normal and salt stress. Various segregating genotypes were evaluated, demonstrating statistically significant differences (p<0.01 and p<0.05, ANOVA) in morphological and physiological parameters. The genotypes Kharagnjia and L-12 performed well in normal soils, while Shua-92 and L-20 showed better performance in tiller plant-1 and panicle length. The cluster analysis grouped rice genotypes into four major clusters based on genetic similarity. Principal Component Analysis (PCA) identified tillers per plant, panicle length, grain yield per plant, and leaf area as key contributors to genetic variation. The highest variability was observed in PC-XII (100%) and PC-XI (98.3%). These findings provide valuable insights for breeding programs aimed at enhancing salt tolerance in rice.

Microencapsulation of Bacillus megaterium in cationic starch/PVA-based matrices

The increasing demand for sustainable and effective agricultural bio-based products is driving innovation in encapsulation technologies for beneficial microorganisms, such as Bacillus strains, known for promoting plant growth and controlling pathogens. This study proposes an efficient methodology for encapsulating Bacillus megaterium within microparticles using a simple cross-linking/emulsion process based on Cationic Starch (CS) and Polyvinyl Alcohol (PVA). Citric acid (CA) and Sodium Trimetaphosphate (STMP) were employed as cross-linking agents, while bentonite (Bent) was added to modify the materials. The resulting microparticles exhibited a range of properties influenced by the cross-linking agents and bentonite. Encapsulation matrices with STMP and STMP-Bent demonstrated superior performance in providing controlled bacterial release and enhanced protection under adverse conditions. These matrices maintained up to 95 % of the initial cell viability after exposure to heat (55 °C for 24 h), and up to 98 % viability after exposure to UV light (180 min) or pesticides (30 °C for 24 h). In contrast, matrices containing CA or CA-Bent, as well as free bacteria, showed significantly lower bacterial survival rates in the same tests. In a high-salinity soil test (200 mM), STMP-based matrices provided the best protection for encapsulated bacteria, leading to a 10 % increase in B. megaterium growth. Furthermore, these matrices demonstrated an estimated shelf-life of over 40 months when stored at temperatures of 15 °C, 30 °C, and 45 °C. These findings suggest that CS/PVA matrices cross-linked with STMP offer a promising approach for developing more resilient and effective bio-based agricultural products.

PGPR: Key to Enhancing Crop Productivity and Achieving Sustainable Agriculture

Due to the burgeoning global population and the advancement of economies, coupled with human activities leading to the degradation of soil ecosystems and the depletion of non-renewable resources, concerns have arisen regarding food security and human survival. In order to address these adverse impacts, the spotlight has been cast upon plant growth-promoting rhizobacteria (PGPR), driven by a strong environmental consciousness. PGPR possesses the capability to foster plant growth and amplify crop yield through both direct and indirect mechanisms. By expediting plant growth, augmenting nutrient assimilation, heightening crop yield and caliber, and fortifying stress resilience, the application of PGPR can mitigate reliance on chemical fertilizers and pesticides while diminishing ecological perils. This exposition delves into the function of PGPR in modulating plant hormones, fostering nutrient solubilization, and fortifying plant resistance against biotic and abiotic stressors. This review offers valuable insights into the intricate interplay between PGPR and plants, elucidating uncertainties ripe for further investigation. Profound comprehension and judicious utilization of PGPR are indispensable for attaining sustainable agricultural progression, making substantial contributions to resolving the conundrums of global food security and environmental conservation.

Metabolomics Reveals Lysinibacillus capsici TT41-Induced Metabolic Shifts Enhancing Drought Stress Tolerance in Kimchi Cabbage (Brassica rapa L. subsp. pekinensis)

Climate change has increased variable weather patterns that affect plants. To address these issues, we developed a microbial biocontrol agent against drought stress in kimchi cabbage (Brassica rapa L. subsp. pekinensis). We selected three bacterial strains (Leifsonia sp. CS9, Bacillus toyonensis TSJ7, and Lysinibacillus capsici TT41) because they showed a survival rate of up to 50% and good growth rate when treated with 30% PEG 6000. The three strains were treated with kimchi cabbage to confirm their enhanced drought stress resistance under non-watering conditions. Among the three strains, the TT41 treated group showed a significant increase in various plant parameters compared with the negative control on the 7th day. We performed extensive profiling of primary and secondary metabolites from kimchi cabbage and the TT41 strain. Multivariate and pathway analyses revealed that only the TT41 group clustered with the well-watered group and showed almost the same metabolome on the 7th day. When treated with TT41, lactic acid was identified as an indicator metabolite that significantly improved drought stress tolerance. Furthermore, lactic acid treatment effectively induced drought stress tolerance in kimchi cabbage, similar to that achieved with the TT41 strain.

Stigmas of holoparasitic Phelipanche arenaria (Orobanchaceae) - a suitable ephemeric flower habitat for development unique microbiome

BACKGROUND

Microbial communities have occasionally been observed in part of the ephemeric reproductive structure of floral stigmas, but their prevalence, phylogenetic diversity and ecological roles are understudied. This report describes the first study of bacterial and fungal communities in immature and mature stigma tissue of the endangered holoparasitic plant Phelipanche arenaria. Culture-dependent methods coupled with next-generation sequencing indicated that a small surface of the flower stigma was an unexpectedly rich and diverse microhabitat for colonization of microbial. We also compared the enzymatic activity of the bacterial communities between immature and mature stigmas samples.

RESULTS

Using high-throughput sequencing methods, we identified and classified 39 to over 51 OTUs per sample for bacterial OTUs represented by Pantoea agglomerans and P. ananatis, comprising 50.6%, followed by Pseudomonas, Luteibacter spp., Sphingomonas spp. with 17% of total frequency. The bacterial profile of immature stigmas of P. arenaria contained unique microorganisms (21 of the most numerous OTUs) that were not confirmed in mature stigmas. However, the enzymatic activity of bacteria in mature stigmas of P. arenaria showed more activity than observed in immature stigmas. In the fungal profile, we recorded even 80 OTUs in mature stigmas, consisting of Capnodiales 45.03% of the total abundance with 28.27% of frequency was created by Alternaria eichhorniae (10.55%), Mycosphaerella tassiana (9.69%), and Aureobasidium pullulans (8.03%). Additionally, numerous putative plant growth-promoting bacteria, fungal pathogens and pathogen-antagonistic yeasts were also detected.

CONCLUSIONS

Our study uncovered that P. arenaria stigmas host diverse bacterial and fungal communities. These microorganisms are well known and have been described as beneficial for biotechnological and environmental applications (e.g., production of different enzymes and antimicrobial compounds). This research provided valuable insight into the parasitic plant-microbe interactions.

A non-K+-solubilizing PGPB (Bacillus megaterium) increased K+ deprivation tolerance in Oryza sativa seedlings by up-regulating root K+ transporters

Potassium is one of the principal macronutrients required by all plants, but its mobility is restricted between soil compartments. Numerous studies have shown that Plant Growth Promoting Bacteria (PGPB) can facilitate nutrient uptake. The present work examined the effects of the PGPB (Bacillus megaterium) on rice plants subjected to potassium deprivation. To study only direct effects of B. megaterium, we first checked its lack of capacity to solubilize soil K. Rice plants were provided with 1.5 mM K (100%) or 0.015 mM K (1%) and growth related parameters, nutrient concentrations and gene expression of K⁺ transporters were determined. After two weeks, the 1% K treatment reduced growth of non-inoculated plants by about 50% compared with the 100% K treatment. However, there was no effect of reduced K nutrition on growth of inoculated plants. The reduction in growth in non-inoculated plants was accompanied by a similar reduction in K⁺ concentration in both roots and leaves and an overall 80% reduction of the plant potassium concentrations. In inoculated plants a 50% reduction occurred only in leaves. The expression of the K⁺ transporters HKT1;1, 1;2, 1;5, 2;2, 2;3 and 2;4 was up-regulated by the inoculation of B. megaterium under K deprivation conditions, explaining their higher K tissue concentrations and growth. Thus, the bacterial strain improved plant potassium nutrition without affecting K⁺ availability in the soil. The results demonstrate the potential of this bacteria for using as a biofertilizer to reduce the amount of potassium fertilizers to be applied in the field.

Tomato responses to salinity stress: From morphological traits to genetic changes

Tomato is an essential annual crop providing human food worldwide. It is estimated that by the year 2050 more than 50% of the arable land will become saline and, in this respect, in recent years, researchers have focused their attention on studying how tomato plants behave under various saline conditions. Plenty of research papers are available regarding the effects of salinity on tomato plant growth and development, that provide information on the behavior of different cultivars under various salt concentrations, or experimental protocols analyzing various parameters. This review gives a synthetic insight of the recent scientific advances relevant into the effects of salinity on the morphological, physiological, biochemical, yield, fruit quality parameters, and on gene expression of tomato plants. Notably, the works that assessed the salinity effects on tomatoes were firstly identified in Scopus, PubMed, and Web of Science databases, followed by their sifter according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline and with an emphasis on their results. The assessment of the selected studies pointed out that salinity is one of the factors significantly affecting tomato growth in all stages of plant development. Therefore, more research to find solutions to increase the tolerance of tomato plants to salinity stress is needed. Furthermore, the findings reported in this review are helpful to select, and apply appropriate cropping practices to sustain tomato market demand in a scenario of increasing salinity in arable lands due to soil water deficit, use of low-quality water in farming and intensive agronomic practices.

Effects of strontium on the morphological and photosynthetic physiological characteristics of Vicia faba seedlings

The adaptation of plants to strontium (Sr) stress requires a more systematic understanding. In the present study, the morphological and photosynthetic physiological characteristics of Vicia faba seedlings under Sr stress (⁸⁸Sr, 0-1,000 mg·L^-1) were analyzed in solution culture. The results showed that Sr treatment decreased the biomass and root activity of V. faba seedlings significantly, but fortunately, there was almost no root necrosis. In plant morphology, the taproot length, lateral root number, plant height, branching number and internodes number of V. faba were significantly inhibited, thus the apical dominance of taproot and terminal bud was more obvious. The accumulation of Sr resulted in the decrease of leaf area, dry weight, stomatal density and stomatal aperture, while the guard cell length increased, and the specific leaf weight (SLW) increased first and then decreased. These changes in stomatal morphology may be a positive regulation to reduce water loss. In addition, V. faba increased the non-photochemical quenching (NPQ) and the activities of peroxidase (POD) and ascorbate peroxidase (APX) to protect the photosynthetic structure. Low concentration of Sr (250 mg·L^-1) promoted the photochemical efficiency and electron transfer of PSII (e.g., increased F_v/F_m, Φ_PSII, qP and ETR). However, Sr (250-1,000 mg·L^-1) inhibited the net photosynthetic rate (P_n), transpiration rate (T_r) and stomatal conductance (G_s) in leaves. In general, the P_n was affected by both stomatal and non-stomatal factors. Since Sr did not cause significant damage to the PSII function, the non-stomatal factor may be the dark reaction in photosynthesis affected, but this needs to be proved by further studies.

Ethylene: A Master Regulator of Plant-Microbe Interactions under Abiotic Stresses

The plant phytohormone ethylene regulates numerous physiological processes and contributes to plant-microbe interactions. Plants induce ethylene production to ward off pathogens after recognition of conserved microbe-associated molecular patterns (MAMPs). However, plant immune responses against pathogens are essentially not different from those triggered by neutral and beneficial microbes. Recent studies indicate that ethylene is an important factor for beneficial plant-microbial association under abiotic stress such as salt and heat stress. The association of beneficial microbes with plants under abiotic stresses modulates ethylene levels which control the expression of ethylene-responsive genes (ERF), and ERFs further regulate the plant transcriptome, epi-transcriptome, Na⁺/K⁺ homeostasis and antioxidant defense mechanisms against reactive oxygen species (ROS). Understanding ethylene-dependent plant-microbe interactions is crucial for the development of new strategies aimed at enhancing plant tolerance to harsh environmental conditions. In this review, we underline the importance of ethylene in beneficial plant-microbe interaction under abiotic stresses.

Dual Inoculation of Plant Growth-Promoting Bacillus endophyticus and Funneliformis mosseae Improves Plant Growth and Soil Properties in Ginger

Co-inoculation with beneficial microbes has been suggested as a useful practice for the enhancement of plant growth, nutrient uptake, and soil nutrients. For the first time in Uzbekistan the role of plant-growth-promoting Bacillus endophyticus IGPEB 33 and arbuscular mycorrhizal fungi (AMF) on plant growth, the physiological properties of ginger (Zingiber officinale), and soil enzymatic activities was studied. Moreover, the coinoculation of B. endophyticus IGPEB 33 and AMF treatment significantly increased the plant height by 81%, leaf number by 70%, leaf length by 82%, and leaf width by 40% compared to the control. B. endophyticus IGPEB 33 individually increased plant height significantly by 51%, leaf number by 56%, leaf length by 67%, and leaf width by 27% as compared to the control treatment. Compared to the control, B. endophyticus IGPEB 33 and AMF individually significantly increased chlorophyll a by 81-58%, chlorophyll b by 68-37%, total chlorophyll by 74-53%, and carotenoid content by 67-55%. However, combination of B. endophyticus IGPEB 33 and AMF significantly increased chlorophyll a by 86%, chlorophyll b by 72%, total chlorophyll by 82%, and carotenoid content by 83% compared to the control. Additionally, plant-growth-promoting B. endophyticus IGPEB 33 and AMF inoculation improved soil nutrients and soil enzyme activities compared to the all treatments. Co-inoculation with plant-growth-promoting B. endophyticus and AMF could be an alternative for the production of ginger that is more beneficial to soil nutrient deficiencies. We suggest that a combination of plant-growth-promoting B. endophyticus and AMF inoculation could be a more sustainable and eco-friendly approach in a nutrient-deficient soil.

Beauty and the pathogens: A leaf-less control presents a better image of Cymbidium orchids defense strategy

Biological control is a safe way of combating plant diseases using the living organisms. For the precise use of microbial biological control agents, the genetic information on the hypersensitive response (HR), and defense-related gene induction pathways of plants are necessary. Orchids are the most prominent stakeholders of floriculture industry, and owing to their long-awaited flowering pattern, disease control is imperative to allow healthy vegetative growth that spans more than 2 years in most of the orchids. We observed leaf-less flowering in three orchid species (Cymbidium ensifolium, C. goeringii and C. sinense). Using these materials as reference, we performed transcriptome profiling for healthy leaves from non-infected plants to identify genes specifically involved in plant-pathogen interaction pathway. For this pathway, a total of 253 differentially expressed genes (DEGs) were identified in C. ensifolium, 189 DEGs were identified in C. goeringii and 119 DEGs were found in C. sinense. These DEGs were mainly related to bacterial secretion systems, FLS2, CNGCs and EFR, regulating HR, stomatal closure and defense-related gene induction. FLS2 (LRR receptor-like serine/threonine kinase) contained the highest number of DEGs among three orchid species, followed by calmodulin. Highly upregulated gene sets were found in C. sinense as compared to other species. The great deal of DEGs, mainly the FLS2 and EFR families, related to defense and immunity responses can effectively direct the future of biological control of diseases for orchids.

Plant Growth-Promoting Rhizobacteria Eliminate the Effect of Drought Stress in Plants: A Review

Plants evolve diverse mechanisms to eliminate the drastic effect of biotic and abiotic stresses. Drought is the most hazardous abiotic stress causing huge losses to crop yield worldwide. Osmotic stress decreases relative water and chlorophyll content and increases the accumulation of osmolytes, epicuticular wax content, antioxidant enzymatic activities, reactive oxygen species, secondary metabolites, membrane lipid peroxidation, and abscisic acid. Plant growth-promoting rhizobacteria (PGPR) eliminate the effect of drought stress by altering root morphology, regulating the stress-responsive genes, producing phytohormones, osmolytes, siderophores, volatile organic compounds, and exopolysaccharides, and improving the 1-aminocyclopropane-1-carboxylate deaminase activities. The use of PGPR is an alternative approach to traditional breeding and biotechnology for enhancing crop productivity. Hence, that can promote drought tolerance in important agricultural crops and could be used to minimize crop losses under limited water conditions. This review deals with recent progress on the use of PGPR to eliminate the harmful effects of drought stress in traditional agriculture crops.

Optimization of eco-friendly amendments as sustainable asset for salt-tolerant plant growth-promoting bacteria mediated maize (Zea Mays L.) plant growth, Na uptake reduction and saline soil restoration

Soil salinity is progressively affecting global agriculture area, and act as a brutal environmental factor for the productivity of plants, therefore, sustainable remediation of the saline soil is urgently required. In this study, we tested the effectiveness of PM (poultry manure), SMS (spent mushroom substrate), and CD (cow dung) for the recovery of salt soil and the optimization of the productivity of the maize plant. PM and SMS showed the valuable source of OC, N, P, K as the CD. The HCA analysis showed that 47% of the bacterial population from PM, SMS, and CD survived at 6% NaCl (w/v), which had PGP attributes such as IAA, P-solubilizers, and siderophore activity. The results from pot experiments of plant growth and PCA analysis of bacterial PGP attributes reveled re formulation of PM, SMS, and CD, which were further optimized at the saline field level. T-2 treated plant increased their shoot length, chlorophyll content, reducing sugar, nitrogen, phosphorus, and potassium levels significantly after 30 and 60 days, followed by T-4 and T-3 as the control. A significant (P < 0.01) increase in particle density and decrease in bulk density was observed for all combinations treated (T-2 to T-7). A two-year field study revealed that the T-2 combination increased 43% OC, 57% N, 66% P, 48% K, 32% DHA, 76% PPO in the soil than the control after 60 days. T2-combination decreased ≈50% of Na content in root and shoot, and increased 27% of maize crop yield. The dose of 10% PM + 10% SMS can significantly induce the growth of maize plants and the restoration of saline soil health.

Proline confers acid stress tolerance to Bacillus megaterium G18

Proline plays a multifunctional role in several organisms including bacteria in conferring protection under stress conditions. In this paper we report the role of proline in conferring acid tolerance to Bacillus megaterium G18. An acid susceptible mutant of B. megaterium G18 which required proline for its growth under acid stress condition was generated through Tn5 mutagenesis. Further, targeted inactivation of proC involved in osmo-adaptive proline synthesis in B. megaterium G18 resulted in the loss of ability of the bacterium to grow at low pH (pH 4.5). Exogenous supply of proline (1 mM) to the growth medium restored the ability of the mutant cells to grow at pH 4.5 which was not the same in case of other osmoprotectants tested. Proline was produced and secreted to extracellular medium by B. megaterium G18 when growing in low pH condition as evidenced by the use of Escherichia coli proline auxotrophs and HPLC analysis. Further, pHT01 vector based expression of full length proC gene in the ∆proC mutant cells restored the survival capacity of the mutant cells in acidic pH, suggesting that proline production is an important strategy employed by B. megaterium G18 to survive under acid stress induced osmotic stress.

Cell-Free Supernatant of Bacillus Strains can Improve Seed Vigor Index of Corn (Zea mays L.) Under Salinity Stress

Currently, salinity is the second biggest challenge in the world after drought and affects all stages of plant growth. The use of environmentally friendly methods such as microorganisms and their derivatives can reduce the destructive effects of salinity stress. A growth chamber experiment was conducted to determine the effects of cell-free supernatant (CFS) from Bacillus strains on germination of corn under salinity stress. Corn seeds were subjected to three salinity levels (0, 100 and 150 mM of NaCl), cell-free supernatant of Bacillus strains (U35, U47, U48, U49, and U50) at two levels of dilution (1:50 and 1:250). Germination percentage and rate decreased with increasing salinity toward 150 mM NaCl all together leading to suppressed growth variables for corn seed seedlings including fresh and dry weight of radicle (47.71 and 52.63%, respectively), and shoot (49.52 and 49.25%, respectively), radicle and shoot lengths (39.90 and 66.07%, respectively). Seed vigor index also decreased by 63.04% at 150 mM NaCl. Contrary to salinity, the CFSs of Bacillus strains increased all the growth traits of corn seeds and reduced the negative effects of salinity, especially severe salinity. Ratios of 1:50 and 1: 250 gave best performance for CFSs from U35 and U50, respectively. In general, the highest seed vigor index was obtained by application of 1: 250 CFS from U50. Most germination traits and seed vigor index correlated significantly positive; however, mean germination time was negatively and significantly correlated with the seed vigor index of corn. The results showed that cell-free supernatant use, may as well-helped in changing the ratios of phytohormones, ROS, the activity of antioxidant enzymes and osmotic proteins, hence reduce the negative effects of salinity and improve seed vigor index which eventually increases the ability of plant seedling establishment under saline conditions.

Improvement of Phytopharmaceutical and Alkaloid Production in Periwinkle Plants by Endophyte and Abiotic Elicitors

Periwinkle plant represents a major source of immensely vital terpenoid indole alkaloids and natural antioxidants which are widely used in cancer chemotherapy. A pot experiment was done to evaluate the role of two periwinkle endophytes (Streptomyces sp. and Bacillus sp.) with or without abiotic elicitors (aluminum chloride, tryptophan, and chitosan) on plant biomass, physio-biochemical attributes, phytopharmaceutical constituents, and alkaloid production. Inoculation with endophyte microbes significantly increased plant growth, nitrogen, phosphorus, potassium, carotenoids, ascorbic acid, and alkaloid yield. It also decreased oxidative biomarkers (hydrogen peroxide and malondialdehyde) and had no significant effects on flavonoids and anthocyanin. In this regard, Streptomyces sp. was more effective than Bacillus sp. Foliar spraying with chitosan significantly increased plant growth, chlorophyll, ions, antioxidant capacity, phytopharmaceutical constituents (total soluble phenols, flavonoids, and anthocyanin), and alkaloid yield, associated with a decline in oxidative biomarkers. Conversely, aluminum chloride application generally increased oxidative biomarkers, which was associated with a decreasing effect on plant growth, chlorophyll, and ions. Application of either tryptophan or chitosan with endophyte microbes increased plant growth, chlorophyll, ions, antioxidants, and alkaloid; meanwhile, it decreased oxidative biomarkers. On the contrary, aluminum chloride with endophytes evoked oxidative damage that was associated with a reduction in plant growth, chlorophyll, ions, and phytopharmaceutical constituents. The current study provides a proof-of-concept of the use of the endophyte Streptomyces sp. with chitosan for enhancing periwinkle plant biomass, phytopharmaceuticals accumulation, and alkaloid production.

Rhizosphere Tripartite Interactions and PGPR-Mediated Metabolic Reprogramming towards ISR and Plant Priming: A Metabolomics Review

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms colonising the rhizosphere. PGPR are involved in plant growth promotion and plant priming against biotic and abiotic stresses. Plant-microbe interactions occur through chemical communications in the rhizosphere and a tripartite interaction mechanism between plants, pathogenic microbes and plant-beneficial microbes has been defined. However, comprehensive information on the rhizosphere communications between plants and microbes, the tripartite interactions and the biochemical implications of these interactions on the plant metabolome is minimal and not yet widely available nor well understood. Furthermore, the mechanistic nature of PGPR effects on induced systemic resistance (ISR) and priming in plants at the molecular and metabolic levels is yet to be fully elucidated. As such, research investigating chemical communication in the rhizosphere is currently underway. Over the past decades, metabolomics approaches have been extensively used in describing the detailed metabolome of organisms and have allowed the understanding of metabolic reprogramming in plants due to tripartite interactions. Here, we review communication systems between plants and microorganisms in the rhizosphere that lead to plant growth stimulation and priming/induced resistance and the applications of metabolomics in understanding these complex tripartite interactions.

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.

Revealing plant growth-promoting mechanisms of Bacillus strains in elevating rice growth and its interaction with salt stress

Soil salinity is a major environmental stress that has been negatively affecting the growth and productivity of rice. However, various salt-resistant plant growth-promoting rhizobacteria (PGPR) have been known to promote plant growth and alleviate the damaging effects of salt stress via mitigating physio-biochemical and molecular characteristics. This study was conducted to examine the salt stress potential of Bacillus strains identified from harsh environments of the Qinghai-Tibetan plateau region of China. The Bacillus strains NMTD17, GBSW22, and FZB42 were screened for their response under different salt stress conditions (1, 4, 7, 9, 11, 13, and 16%). The screening analysis revealed strains NMTD17, GBSW22, and FZB42 to be high-salt tolerant, moderate-salt tolerant, and salt-sensitive, respectively. The NMTD17 strain produced a strong biofilm, followed by GBSW22 and FZB42. The expression of salt stress-related genes in selected strains was also analyzed through qPCR in various salt concentrations. Further, the Bacillus strains were used in pot experiments to study their growth-promoting ability and antioxidant activities at various concentrations (0, 100, 150, and 200 mmol). The analysis of growth-promoting traits in rice exhibited that NMTD17 had a highly significant effect and GSBW22 had a moderately significant effect in comparison with FZB42. The highly resistant strain NMTD17 that stably promoted rice plant growth was further examined for its function in the composition of rhizobacterial communities. The inoculation of NMTD17 increased the relative abundance and richness of rhizobacterial species. These outcomes propose that NMTD17 possesses the potential of PGPR traits, antioxidants enzyme activities, and reshaping the rhizobacterial community that together mitigate the harmful effects of salinity in rice plants.

Phytostimulation and biocontrol potential of Gram-positive endospore-forming Bacilli

The spore-forming Bacillus and Paenibacillus species represent the phyla of beneficial bacteria for application as agricultural inputs in form of effective phytostimulators, biofertilizers, and biocontrol agents. The members of the genera Bacillus and Paenibacillus isolated from several ecological habitats are been thoroughly dissected for their effective application in the development of sustainable and eco-friendly agriculture. Numerous Bacillus and Paenibacillus species are reported as plant growth-promoting bacteria influencing the health and productivity of the food crops. This review narrates the mechanisms utilized by these species to enhance bioavailability and/or facilitate the acquisition of nutrients by the host plant, modulate plant hormones, stimulate host defense and stress resistance mechanisms, exert antagonistic action against soil and airborne pathogens, and alleviate the plant health. The mechanisms employed by Bacillus and Paenibacillus are seldom mutually exclusive. The comprehensive and systematic exploration of the aforementioned mechanisms in conjunction with the field investigations may assist in the exploration and selection of an effective biofertilizer and a biocontrol agent. This review aims to gather and discuss the literature citing the applications of Bacillus and Paenibacillus in the management of sustainable agriculture.

Multi-Trait Wheat Rhizobacteria from Calcareous Soil with Biocontrol Activity Promote Plant Growth and Mitigate Salinity Stress

Plant growth promoting rhizobacteria (PGPR) can be functional microbial fertilizers and/or biological control agents, contributing to an eco-spirit and safe solution for chemical replacement. Therefore, we have isolated rhizospheric arylsulfatase (ARS)-producing bacteria, belonging to Pseudomonas and Bacillus genus, from durum wheat crop grown on calcareous soil. These isolates harbouring plant growth promoting (PGP) traits were further evaluated in vitro for additional PGP traits, including indole compounds production and biocontrol activity against phytopathogens, limiting the group of multi-trait strains to eight. The selected bacterial strains were further evaluated for PGP attributes associated with biofilm formation, compatibility, salt tolerance ability and effect on plant growth. In vitro studies demonstrated that the multi-trait isolates, Bacillus (1.SG.7, 5.SG.3) and Pseudomonas (2.SG.20, 2.C.19) strains, enhanced the lateral roots abundance and shoots biomass, mitigated salinity stress, suggesting the utility of beneficial ARS-producing bacteria as potential microbial fertilizers. Furthermore, in vitro studies demonstrated that compatible combinations of multi-trait isolates, Bacillus sp. 1.SG.7 in a mixture coupled with 5.SG.3, and 2.C.19 with 5.SG.3 belonging to Bacillus and Pseudomonas, respectively, may enhance plant growth as compared to single inoculants.

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.

Effect of Bacillus spp. and Brevibacillus sp. on the Photosynthesis and Redox Status of Solanum lycopersicum

Plant-growth-promoting bacteria (PGPB) are gaining attention as a sustainable alternative to current agrochemicals. This study evaluated the impact of three Bacillus spp. (5PB1, 1PB1, FV46) and one Brevibacillus sp. (C9F) on the important crop tomato (Solanum lycopersicum) using the model cv. ‘MicroTom’. The effects of these isolates were assessed on (a) seedlings’ growth and vigor, and (b) adult potted plants. In potted plants, several photosynthetic parameters (chlorophylls (a and b), carotenoids and anthocyanins contents, transpiration rate, stomatal conductance, net CO2 photosynthetic rate, and intercellular CO2 concentration, and on chlorophyll fluorescence yields of light- and dark-adapted leaves)), as well as soluble sugars and starch contents, were quantified. Additionally, the effects on redox status were evaluated. While the growth of seedlings was, overall, not influenced by the strains, some effects were observed on adult plants. The Bacillus safensis FV46 stimulated the content of pigments, compared to C9F. Bacillus zhangzhouensis 5PB1 increased starch levels and was positively correlated with some parameters of the photophosphorylation and the gas exchange phases. Interestingly, Bacillus megaterium 1PB1 decreased superoxide (O2−) content, and B. safensis FV46 promoted non-enzymatic antioxidant defenses, increasing total phenol content levels. These results, conducted on a model cultivar, support the theory that these isolates differently act on tomato plant physiology, and that their activity depends on the age of the plant, and may differently influence photosynthesis. It would now be interesting to analyze the influence of these bacteria using commercial cultivars.

Insights Into Microbially Induced Salt Tolerance and Endurance Mechanisms (STEM) in Plants

Salt stress threatens the achievement of sustainable global food security goals by inducing secondary stresses, such as osmotic, ionic, and oxidative stress, that are detrimental to plant growth and productivity. Various studies have reported the beneficial roles of microbes in ameliorating salt stress in plants. This review emphasizes salt tolerance and endurance mechanisms (STEM) in microbially inoculated (MI) plants that ensure plant growth and survival. Well-established STEM have been documented in MI plants and include conglomeration of osmolytes, antioxidant barricading, recuperating nutritional status, and ionic homeostasis. This is achieved via involvement of P solubilization, siderophore production, nitrogen fixation, selective ion absorption, volatile organic compound production, exopolysaccharide production, modifications to plant physiological processes (photosynthesis, transpiration, and stomatal conductance), and molecular alterations to alter various biochemical and physiological processes. Salt tolerance and endurance mechanism in MI plants ensures plant growth by improving nutrient uptake and maintaining ionic homeostasis, promoting superior water use efficiency and osmoprotection, enhancing photosynthetic efficiency, preserving cell ultrastructure, and reinforcing antioxidant metabolism. Molecular research in MI plants under salt stress conditions has found variations in the expression profiles of genes such as HKT1, NHX, and SOS1 (ion transporters), PIPs and TIPs (aquaporins), RBCS, RBCL (RuBisCo subunits), Lipoxygenase2 [jasmonic acid (JA) signaling], ABA (abscisic acid)-responsive gene, and APX, CAT, and POD (involved in antioxidant defense). Proteomic analysis in arbuscular mycorrhizal fungi-inoculated plants revealed upregulated expression of signal transduction proteins, including Ca2+ transporter ATPase, calcium-dependent protein kinase, calmodulin, and energy-related proteins (NADH dehydrogenase, iron-sulfur protein NADH dehydrogenase, cytochrome C oxidase, and ATP synthase). Future research should focus on the role of stress hormones, such as JA, salicylic acid, and brassinosteroids, in salt-stressed MI plants and how MI affects the cell wall, secondary metabolism, and signal transduction in host plants.