A review on drought stress in plants: Implications, mitigation and the role of plant growth promoting rhizobacteria

Increasing population and subsequent food requirement puts a lot of onus on agriculture worldwide. However, agricultural output is constantly influenced by several factors such as small land holdings, lack of proper mechanization, and occurrence of several abiotic and biotic stresses. Drought stress is one such abiotic stress which causes major setbacks to agricultural productivity every year. Water scarcity due to decline in rainfall and increased frequency of dry spells give rise to drought conditions. Often drought is accompanied by other detrimental effects like salinity, heat and attack of pathogens. In response plants undergo several physiological and morphological modifications like reduced transpiration and photosynthesis rate, osmotic adjustments, repressed root and shoot growth, overproduction of reactive oxygen species (ROS), modified stress signalling pathways, and senescence. These modifications can cause permanent injury to the plant and hence there is a need to develop mitigation strategies. Employment of nanoparticles, film farming, use of drought resistant plant varieties, super-absorbent hydrogels, and biochar are some techniques used to ameliorate drought stress. Though, most of these practices are labour-intensive and expensive with limited benefits. Application of plant-growth-promoting rhizobacteria (PGPR), however, is proving to be a preferential strategy that encompasses many direct and indirect benefits. PGPRs impart induced systemic tolerance (IST) to plants towards drought stress through a variety of mechanisms like improvement of antioxidant system, production of ACC-deaminase and phytohormones, nitrogen fixation, phosphate solubilization, siderophore and exopolysaccharides production, enhanced root and shoot system, amplified photosynthesis rates and carotenoid production. The commercial applicability of PGPRs as biostimulants will still be dependent on the selection and performance of robust strains in various environmental conditions.