The Role of Plant Growth-Promoting Rhizobacteria in Alleviating the Adverse Effects of Drought, Heat and Salinity Stresses in Crop Plants

Climate change and various abiotic stresses, especially, drought, salinity and heat stress on the one hand, and the issue of providing adequate quality food for the growing population of the world on the other hand, have made it necessary to revise the methods of crop production. In these conditions, the use of plant growth-promoting rhizobacteria (PGPR) in the agroecosystems is useful and profitable due to the efficiency of these organisms in regulating plant growth and strengthening the plant's natural defense system.  In this paper, considering the importance of modulating the negative effects of abiotic stresses using an environmentally friendly approach to achieve a healthy and sustainable crop system, the mechanisms of modulating abiotic stresses by growth-promoting rhizobacteria have been reviewed and the results of several studies have been presented. Growth promoting rhizobacteria through modification of plant hormone levels, restricting ethylene production, maintaining ion balance, osmolytes accumulation during stress, induction of antioxidant enzymes synthesis and removal of reactive oxygen species, production of extracellular polymeric substances, and improving the uptake of mineral elements and water, moderate the adverse effects of abiotic stresses on plant growth. By identifying useful and native rhizobacteria to each region and investigating the ability of these bacteria in producing the 1-aminocyclopropane-1-carboxylate (ACC) deaminase, balancing the plant hormones levels, producing siderophores, and enhancing solubility of mineral, the field will be provided for the successful use of these microorganisms.