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

Abstract

Salinity affects plant growth and is a major abiotic stress that limits crop productivity. It is well-understood that environmental adaptations and genetic traits regulate salinity tolerance in plants, but imparting the knowledge gained towards crop improvement remain arduous. Harnessing the potential of beneficial microorganisms present in the rhizosphere is an alternative strategy for improving plant stress tolerance. This review intends to elucidate the understanding of salinity tolerance mechanisms attributed by plant growth promoting rhizobacteria (PGPR). Recent advances in molecular studies have yielded insights into the signaling networks of plant-microbe interactions that contribute to salt tolerance. The beneficial effects of PGPR involve boosting key physiological processes, including water and nutrient uptake, photosynthesis, and source-sink relationships that promote growth and development. The regulation of osmotic balance and ion homeostasis by PGPR are conducted through modulation of phytohormone status, gene expression, protein function, and metabolite synthesis in plants. As a result, improved antioxidant activity, osmolyte accumulation, proton transport machinery, salt compartmentalization, and nutrient status reduce osmotic stress and ion toxicity. Furthermore, in addition to indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid deaminase biosynthesis, other extracellular secretions of the rhizobacteria function as signaling molecules and elicit stress responsive pathways. Application of PGPR inoculants is a promising measure to combat salinity in agricultural fields, thereby increasing global food production.

Keywords: phytohormones; plant tolerance; rhizobacteria; salinity stress; signaling.

FIGURE 1

Illustration of salt tolerance mechanisms induced by plant growth promoting rhizobacteria (PGPR). Root surfaces are colonized by PGPR and extracellular polysaccharide matrix acts as a protective barrier against salt stress. Some extracellular molecules function as signaling cues that manipulate phytohormonal status in plants. Enhanced root-to-shoot communication improves water and nutritional balance, source-sink relations and stomatal conductance. Stimulating osmolyte accumulation, carbohydrate metabolism and antioxidant activity delay leaf senescence, which inturn contribute to photosynthesis. Regulation of physiological processes are indicated by black arrows and signaling pathways are indicated by purple arrows.

FIGURE 2

Plant growth promoting rhizobacteria interaction mediate cellular activity in plants to ameliorate salinity stress. Osmotic imbalance and oxidative damage are reduced by enhanced biosynthesis of compatible solutes and antioxidants. Ion homeostasis is maintained by increase in activity of K⁺ transporters (HKT) and H⁺ exchangers (NHX) that facilitate salt compartmentalization/exclusion. PGPR also upregulate the expression of stress responsive genes (phytohormone signaling) and proteins (vegetative storage, photosynthesis, and antioxidant enzymes).