Drought-Tolerant Phosphorus-Solubilizing Microbes: Biodiversity and Biotechnological Applications for Alleviation of Drought Stress in Plants

Drought is one of the major abiotic stresses accepted as the main constraint for loss of the crop yield worldwide. Further, problems are created by nutrient limitations particularly low phosphorus (P). Soils though have higher concentration of total phosphorus but are actually deficient in available orthophosphate due to which modern agricultural systems are highly dependent on chemical fertilizers. These chemical fertilizers are neither eco-friendly nor economically feasible and sustainable. Biotechnology offers a number of sustainable solutions to mitigate these problems by using plant growth-promoting (PGP) microbes. The PGP microbes colonize the rhizospheric region, or they may be endophytic or epiphytic and are beneficial for plant growth and adaptation to abiotic stresses. These microbes help the crops to tolerate drought conditions by different mechanisms including the production of exopolysaccharide (EPS), various phytohormones, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, and a number of volatile compounds, enhancement of nutrient uptake, induction of the accumulation of osmolytes and antioxidants, upregulation or downregulation of the stress-responsive genes, or bringing about of alterations in root morphology. Inoculating plants with PGP microbes can increase tolerance against abiotic stresses such as drought, salinity, and metal toxicity. Systematic identification of bacterial strains providing cross-protection against multiple stressors would be highly valuable for agricultural production in changing environmental conditions. Among the PGP microbes, P-solubilizing microbes play an important role in plant growth and soil health, which belong to diverse genera such as Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia, Enterobacter, Klebsiella, Lysinibacillus, Paenibacillus, Pseudomonas, Serratia, and Streptomyces. The present chapter deals with biodiversity of P-solubilizing drought-tolerant microbes, mechanisms of plant growth promotion, and mitigation of drought stress in the plants.