Role of Metabolites Produced by Plant Growth-Promoting Bacteria in Biocontrol of Phytopathogens Under Saline Conditions

Soil salinity has continuously degraded the quality and quantity of crops and has challenged their health and defense mechanisms. Although relationship between salinity and biotic stress is poorly understood, yet due to negative impact of salts on plants, they become susceptible to diseases. Limiting the use of agro-chemicals, salt-tolerant plant growth-promoting rhizobacteria (ST-PGPR) are emerging as potential replacements for use against combinatorial stresses such as salinity and pathogens. Under combinatorial stress, i.e., salt and biotic, ST-PGPR produce an array of metabolites which directly act upon the pathogens and also induce a number of molecular and signaling pathways activating a two-way defensive response. Siderophores, antibiotics, volatile organic compounds, and biosurfactants are among the metabolites produced by PGPR for suppressing the phytopathogens. Apart from these, PGPR initiate induced systemic resistance and resultantly protect and prepare the plants against the diseases. With dual role against salinity and pathogens, PGPR produce various metabolites such as exopolysaccharides, osmoprotectants, and antioxidants and initiate nutrient chelation and biofilming activity, thereby improving the cropping system. Quorum sensing and quorum quenching are also initiated by PGPR to limit the negative impact of salinity and pathogens and promote plant growth. Therefore, ST-PGPR exhibiting combinatorial defense mechanisms can be applied as stress relievers, antagonists, and biofertilizers to saline pathogen-infested soils and improve the cropping system by maintaining agricultural sustainability.