Plant Growth Promoting Rhizobacteria in Amelioration of Salinity Stress: A Systems Biology Perspective.

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.

A meeting on the molecular basis of ionic homeostasis and salt tolerance in plants took place in Madrid, Spain, October 22–24, 2001. This meeting was organized by Eduardo Blumwald (Davis, CA) and Alonso Rodriguez‐Navarro (Madrid, Spain) at the Centre for International Meetings on Biology (‘Instituto Juan March de Estudios e Investigaciones’). ![][1] Ionic homeostasis is a fundamental cellular phenomenon. All living cells maintain an intracellular ionic composition compatible with their constituent molecules, and this requires the regulation of multiple membrane transporters and signal transduction pathways. Other biophysical parameters such as turgor and electrical potential are also part of this essential regulation. How ionic homeostasis is achieved, however, is not completely understood. Although most transporters have already been identified, their physiological function is only starting to be demonstrated and the receptors and most components of the regulatory pathways that effect ionic homeostasis remain unknown. In the case of plants, this problem is related to mineral nutrition and salinity tolerance, both of which have great relevance for agriculture. In fact, as demonstrated by this meeting, salinity stress has been one of the keys to opening the black box of ionic homeostasis in general. Another has been the novel molecular genetics of the plant Arabidopsis thaliana . Of course, other approaches have also contributed to our present understanding of ion homeostasis in plants and were represented at the meeting. For further details, see Blumwald (2000), Hasegawa et al . (2000), Bohnert et al . (2001), Serrano and Rodriguez‐Navarro (2001) and Zhu (2001). ### Some physiology of salt tolerance Salt stress is an important threat to the future of agriculture in many productive areas of the planet. In countries such as Australia and Pakistan, salinity is already a national concern, as it was in the past in ancient Mesopotamia. Areas of California and the Mediterranean region are also threatened. … [1]: /embed/graphic-1.gif

Salinity stress is a major deterrent to crop growth worldwide. A high concentration of salt in soil and irrigation water reduces water uptake by plants, which triggers a range of cellular and metabolic processes ultimately resulting in lowered crop yield. Genetics of salinity tolerance in plants is well understood and has been successfully utilized to breed improved crop varieties; however, such strategies are long drawn and cost intensive. Microorganisms due to their immense metabolic diversity can be very useful to devise low-cost strategies to ameliorate salinity stress in crop plants. An intricate and highly complex interaction between plant and microbe results in alleviation of salt stress. Although the cross talk between the plant and microbe is not clearly understood, regulation of osmotic balance and ion homeostasis by microorganisms are mostly mediated through modulation of phytohormone production, alteration in gene expression, protein function, and metabolite synthesis in plants. Consequently, improved antioxidant activity, accumulation of compatible solutes, proton extrusion mechanism, salt compartmentalization, and improved nutrient status in plants can reduce the osmotic shock and ionic toxicity. Microbial management of salt stress offers an eco-friendly, cost-effective approach which however requires a rigorous selection, testing, and validation of the microbial strain(s) besides understanding the cross talk between the plant and microbe. Therefore, the aim of this chapter is to discuss the salinity response in plants and also to understand the mechanisms of microbe-mediated salinity stress alleviation on the molecular basis.

New Molecular Approaches to Improving Salt Tolerance in Crop Plants

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

Understanding the primary mechanisms for plant promotion under salt stress with plant growth promoting rhizobacteria (PGPR) inoculation of different salt-tolerant plant groups would be conducive to using PGPR efficiently. We conducted a meta-analysis to evaluate plant growth promotion and uncover its underlying mechanisms in salt-sensitive plants (SSP) and salt-tolerant plants (STP) with PGPR inoculation under salt stress. PGPR inoculation decreased proline, sodium ion (Na+) and malondialdehyde but increased plant biomass, nutrient acquisition (nitrogen, phosphorus, potassium ion (K+), calcium ion (Ca2+), and magnesium ion (Mg2+)), ion homeostasis (K+/Na+ ratio, Ca2+/Na+ ratio, and Mg2+/Na+ ratio), osmolytes accumulation (soluble sugar and soluble protein), antioxidants (superoxide dismutase), and photosynthesis (chlorophyll, carotenoid, and photosynthetic rate) in both SSP and STP. The effect size of total biomass positively correlated with the effect sizes of nutrient acquisition and the homeostasis of K+/Na+, and negatively correlated with the effect size of malondialdehyde in both SSP and STP. The effect size of total biomass also positively correlated with the effect sizes of carotenoid and the homeostasis in Ca2+/Na+ and Mg2+/Na+ and negatively correlated with the effect size of Na+ in SSP, but it only negatively correlated with the effect size of Ca2+ in STP. Our results suggest that the plant growth improvement depends on the nutrient acquisition enhancement in both SSP and STP, while ion homeostasis plays an important role and carotenoid may promote plant growth through protecting photosynthesis, reducing oxidative damage and promoting nutrient acquisition only in SSP after PGPR inoculation under salt stress.

Salt stress is one of the major abiotic constraints that inflicts impaired growth and reduces production potential in crop plants. Under salt stress conditions, numerous plant growth processes are affected, i.e., hormonal and nutritional imbalance, ion toxicity, physiological disorders, and susceptibility to insect and pest attack. The growing menace of salinity is predicted to intensify both in its extent and severity, posing a stern challenge for developing a resilient food production system in coming years. Various agro-biotechnological interventions are being employed to improve salt stress tolerance in plants. However, the complexity associated with plant salinity tolerance has only allowed marginal progress for breeders and genetic engineers. Use of soil microbial resources to promote plant growth by alleviating the exposed stress factor has gained much needed attention in recent times. The application of plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) based bioinoculant strategy is perceived to enhance plant growth under salt stress. Microbial inoculation evoked plant stress tolerance response and improved plant growth, which is mainly triggered by modulating nutritional and hormonal balance, producing plant growth regulators, solubilizing nutrients, and inducing disease resistance. In this chapter, we describe causes of soil salinization and discuss potential impacts of salinity stress on plants. In addition, we also discussed the action mechanisms of plant growth promotion and/or regulation exhibited by PGPR and AMF, and highlighted their intrinsic traits that can be up scaled to increase their usefulness as a value-added product for stress agriculture.

Increase in soil salinity is a serious concern as it adversely impacts the growth, development and yield of agricultural crops. The disruption in the ionic composition thereby affecting the fertility of the soil is observed in salt-affected soils warranting the development of technologies for improved crop growth under salinity conditions. Whilst generation of transgenic plants harbouring genes important for imparting tolerance to salinity stress conditions is considered a feasible biotechnological approach, the usage of transgenic crops for human consumption is not well received. Further, breeding approaches for generation of salt tolerant varieties is time consuming. In this context, the usage of halotolerant Plant Growth Promoting Rhizobacteria (PGPR) for improvement of salinity tolerance in crop plants is considered as an eco-friendly and cost-effective approach. This approach will also maintain the soil microbial diversity thereby protecting it from salinity-induced degradation. Few PGPR such as Pseudomonas and Bacillus species although have been effectively used for imparting plant growth promotion under control conditions, isolation of halotolerant PGPR, their effective utilization and optimization for agronomical application in salt-affected soils are key factors for improvement of salinity tolerance in crop plants. Further, whilst many of the isolated halotolerant PGPR show plant-growth promoting traits under laboratory conditions, their efficiency is questionable under field conditions. In the current chapter, the methodologies employed for inoculation of isolated halotolerant PGPR either singly or as a consortium to agricultural crops and the changes observed in the plants towards improvement in salinity tolerance will be discussed. The efficient delivery methods with carriers like nanoparticles, biopolymers and organic carriers along with preparation of formulations, the route of administration for improvement of salinity tolerance in crop plants and their survival in saline soil will be elaborately discussed.

The increasing salinization of soils and resulting degradation of irrigated lands have directly affected 2.6 billion hectares of dryland agriculture worldwide. This phenomenon has led to significant qualitative and quantitative losses in crop production. The absorption and accumulation of ions adversely affect plants by disrupting photosynthetic machinery, damaging tissues, disturbing the ionic balance of cells, and inducing oxidative stress. Rhizobacteria-induced salinity tolerance is a promising tool in crop plants that works by modulating the plant metabolism. Among rhizobacteria, halotolerant plant growth promoting rhizobacteria (PGPR) stand out as particularly significant because they can extend salinity tolerance in crop plants through various mechanisms, including secondary metabolite production, osmolyte accumulation, and modulation of plant metabolism via certain localized and systemic defense functions. Furthermore, the volatile organic compounds produced by PGPR play a vital role in salinity amelioration by regulating root ions uptake, promoting osmolyte related genes expression, reducing the level of oxidative stress markers such as electrolyte leakage, and maintaining endogenous hormonal levels. These novel salt-ameliorating mechanisms and their ability to improve plant fitness and enhance tolerance to salinized soils highlight halotolerant PGPR as eco-friendly and cost-effective tools for salt stress tolerance. This review focuses on elucidating the novel mechanisms used by halotolerant PGPR, their production of secondary metabolites under salinity stress, their application as bioinoculants for crop plants in salinized soils and the development of novel bioformulations for the bioremediation of agricultural soils facing salt stress-related challenges.

Increasing levels of salinity in agricultural lands is one of the most serious environmental concerns that pose a risk to the food security of the growing human population of the world. According to the United Nations Environment Program, the total areas of salt-stressed agricultural lands and croplands have increased by approximately 20% and 50%, respectively, worldwide. The total land area that cannot be used as agricultural land is increasing by 1–2% every year as a result of soil salinization, mostly in dry areas. Increasing soil salinity is becoming the prime reason for substantial decreases in agricultural yield due to inhibitory effects of salinity on growth, photosynthesis, protein synthesis, lipid metabolism, and many other metabolic processes of plants. Production of salt-tolerant crop varieties is a prerequisite for meeting increasing food demands and creating sustainable agriculture practices. The halophytic rhizosphere is a reservoir of plant growth–promoting rhizobacteria (PGPRs), which can enhance plant adaptation and growth under high salinity. Among free-living soil bacteria, PGPRs play an essential role in promoting plant growth even in stress conditions. PGPRs have both direct and indirect effects on plant growth. The direct mechanisms involve biosynthesis of phytohormones, enhanced nitrogen fixation, and higher levels of phosphate solubilization. The indirect mechanisms involve inhibition of phytopathogens that reduce plant growth. Various studies have illustrated that salinity-tolerant PGPRs obtained from rhizosphere soils of various halophytic species have potential for use in development of glycophytic salt-tolerant crops in salt-dominated agricultural lands through their use as bioinoculants. To accomplish this goal, PGPRs adapt various mechanisms such as modulation of phytohormones, gene expression, protein function, and metabolite synthesis. PGPRs modulate synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase along with indoleacetic acid (IAA), which function in stress signaling and induce various stress-responsive pathways. Implementation of PGPR inoculation in the advancement of agriculture to increase global food security is desirable. This chapter focuses on the salinity tolerance mechanisms of PGPRs and the roles of PGPRs in developing salt tolerance in various glycophytic crop species.

ABSTRACTA novel plant growth-promoting rhizobacterium (PGPR), which was designated strain BY2G20, was isolated from saline-alkaline soil in Dongying, China. Strain BY2G20 can grow at a NaCl range from 0 to 7% and a pH range from 7 to 9 and can prevent the growth of the phytopathogen Ralstonia solanacearum. Based on its phenotypic and genomic characteristics and phylogenetic analysis, strain BY2G20 represents a novel species of the genus Metabacillus, for which the name Metabacillus dongyingensis sp. nov. is proposed. Comparative genomic analysis of strain BY2G20 with its closely related species exhibited a high level of evolutionary plasticity derived by horizontal gene transfer, which facilitated adaptative evolution. Different evolutionary constraints have operated on the diverse functions of BY2G20, with the gene adapted to saline-alkaline ecosystems experiencing functional constraints. We determined the genetic properties of saline-alkaline tolerance and plant growth promotion, such as cation-proton antiporters, cation transporters, osmoprotectant synthesis and transport, H+-transporting F1F0-ATPase, indole-3-acetic acid production, and secondary metabolite synthesis. We also evaluated the effects of strain BY2G20 on the growth of Zea mays L. (maize) under salt stress. The physiological parameters of maize such as plant height, stem diameter, dry biomass, and fresh biomass were significantly higher after inoculating strain BY2G20 under salt stress, indicating that inoculation with BY2G20 enhanced the growth of maize in saline areas. This study demonstrates that M. dongyingensis sp. nov. BY2G20 is a potential candidate for organic agriculture biofertilizers in saline-alkaline areas.IMPORTANCE Plant growth and yield are adversely affected by soil salinity. PGPRs can promote plant growth and enhance plant tolerance to salt stress. In this study, a saline-alkaline tolerant PGPR strain BY2G20 was isolated from the rhizosphere of Ulmus pumila in Dongying, China. Strain BY2G20 represents a novel species within the genus Metabacillus based on phenotypic, genomic, and phylogenetic analysis. Genomic components have undergone different functional constraints, and the disparity in the evolutionary rate may be associated with the adaptation to a specific niche. Genomic analysis revealed numerous adaptive features of strain BY2G20 to a saline-alkaline environment and rhizosphere, especially genes related to salt tolerance, pH adaptability, and plant growth promotion. Our work also exhibited that inoculation of strain BY2G20 enhanced the growth of maize under salt stress. This study demonstrates that PGPRs play an important role in stimulating salt tolerance in plants and can be used as biofertilizers to enhance the growth of crops in saline-alkaline areas.

Physiological and biochemical traits in coriander affected by plant growth-promoting rhizobacteria under salt stress

Soil salinity is one of the main factors limiting agricultural development worldwide and has an adverse effect on plant growth and yield. To date, plant growth-promoting rhizobacteria (PGPR) are considered to be one of the most promising eco-friendly strategies for improving saline soils. The bacterium Bacillus megaterium ZS-3 is an excellent PGPR strain that induces growth promotion as well as biotic stress resistance and tolerance to abiotic stress in a broad range of host plants. In this study, the potential mechanisms of protection against salinity stress by B. megaterium ZS-3 in Arabidopsis thaliana were explored. Regulation by ZS-3 improved growth in A. thaliana under severe saline conditions. The results showed that ZS-3 treatment significantly increased the biomass, chlorophyll content and carotenoid content of A. thaliana. Compared to the control, the leaf area and total fresh weight of plants inoculated with ZS-3 increased by 245% and 271%, respectively; the chlorophyll a, chlorophyll b, and carotenoid contents increased by 335%, 146%, and 372%, respectively, under salt stress. Physiological and biochemical tests showed that ZS-3 regulated the content of osmotic substances in plants under salt stress. Compared to the control, the soluble sugar content of the ZS-3-treated group was significantly increased by 288%, while the proline content was significantly reduced by 41.43%. Quantification of Na+ and K+ contents showed that ZS-3 treatment significantly reduced Na+ accumulation and increased the K+/Na+ ratio in plants. ZS-3 also isolated Na+ in vesicles by upregulating NHX1 and AVP1 expression while limiting Na+ uptake by downregulating HKT1, which protected against Na+ toxicity. Higher levels of peroxidase and catalase activity and reduced glutathione were detected in plants inoculated with ZS-3 compared to those in uninoculated plants. In addition, it was revealed that ZS-3 activates salicylic acid (NPR1 and PR1) and jasmonic acid/ethylene (AOS, LOX2, PDF1.2, and ERF1) signaling pathways to induce systemic tolerance, thereby inducing salt tolerance in plants. In conclusion, the results of this study indicate that ZS-3 has the potential to act as an environmentally friendly salt tolerance inducer that can promote plant growth in salt-stressed environments.

Soil salinity is imposing serious threats for crop production particularly in arid and semi-arid regions. Various causes for increasing soil salinity in agricultural lands around the globe include weathering of rocks, excessive irrigation, deforestation and poor drainage. Scraping, flushing and leaching are physical means by which soil salinity can be managed, but to a limited extent. Salt-tolerant crop plant varieties are developed by plant biotechnologists to overcome the salinity issues. Bacteria that exist in the rhizoplane and rhizosphere and that are endophytic have shown positive effects on the crop with respect to nutrient availability and therefore are of great importance. The current chapter encompasses the adverse effects of salinity on crop plants and direct and indirect effects of plant growth-promoting rhizobacteria (PGPR) in amelioration of salinity stress and the mechanisms involved thereby. Nitrogen fixation, phosphate solubilisation, phytohormones and the siderophores produced by PGPRs directly make the nutrients available to the plants and allow the crops to grow vigorously. The indirect mechanisms involve production of lytic enzymes, antibiotics that inhibit the pathogen. PGPRs produce osmotolerant chemicals, reactive oxygen species scavenging enzymes and the enzymes that reduce the oxidative stress on the plant system and thereby induce systemic resistance to saline conditions in the plants. In conclusion, the PGPRs can be used as alternate strategy for not just flourishing of the crop plants but also allowing them to withstand a stress condition and thus can be used so that the barren saline lands can be brought under cultivation.

Salinity is the major environmental factor limiting plant growth and productivity. Under salinity conditions, plant growth is affected by a number of factors such as hormonal and nutritional imbalance, ion toxicity, physiological disorders, susceptibility to diseases, etc. Plant growth under stress conditions may be enhanced by the application of microbial inoculation including plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi. These microbes can promote plant growth by regulating nutritional and hormonal balance, producing plant growth regulators, solubilizing nutrients and inducing resistance against plant pathogens. The present review comprehensively discusses on the effectiveness of PGPR and mycorrhizal fungi for enhancing plant growth under salinity stress. The mechanisms involved in plant salinity tolerance under stress conditions have been discussed at length in this review. Also the review discusses the role of rhizobacteria and mycorrhizae in combination in enhancing plant growth under stress conditions.