Development of Salt Tolerance in Crops Employing Halotolerant Plant Growth–Promoting Rhizobacteria Associated with Halophytic Rhizosphere Soils

Abstract

Increasing levels of salinity in agricultural lands is one of the most serious environmental concerns that pose a risk to the food security of the growing human population of the world. According to the United Nations Environment Program, the total areas of salt-stressed agricultural lands and croplands have increased by approximately 20% and 50%, respectively, worldwide. The total land area that cannot be used as agricultural land is increasing by 1–2% every year as a result of soil salinization, mostly in dry areas. Increasing soil salinity is becoming the prime reason for substantial decreases in agricultural yield due to inhibitory effects of salinity on growth, photosynthesis, protein synthesis, lipid metabolism, and many other metabolic processes of plants. Production of salt-tolerant crop varieties is a prerequisite for meeting increasing food demands and creating sustainable agriculture practices. The halophytic rhizosphere is a reservoir of plant growth–promoting rhizobacteria (PGPRs), which can enhance plant adaptation and growth under high salinity. Among free-living soil bacteria, PGPRs play an essential role in promoting plant growth even in stress conditions. PGPRs have both direct and indirect effects on plant growth. The direct mechanisms involve biosynthesis of phytohormones, enhanced nitrogen fixation, and higher levels of phosphate solubilization. The indirect mechanisms involve inhibition of phytopathogens that reduce plant growth. Various studies have illustrated that salinity-tolerant PGPRs obtained from rhizosphere soils of various halophytic species have potential for use in development of glycophytic salt-tolerant crops in salt-dominated agricultural lands through their use as bioinoculants. To accomplish this goal, PGPRs adapt various mechanisms such as modulation of phytohormones, gene expression, protein function, and metabolite synthesis. PGPRs modulate synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase along with indoleacetic acid (IAA), which function in stress signaling and induce various stress-responsive pathways. Implementation of PGPR inoculation in the advancement of agriculture to increase global food security is desirable. This chapter focuses on the salinity tolerance mechanisms of PGPRs and the roles of PGPRs in developing salt tolerance in various glycophytic crop species.