Microbial ACC-Deaminase: Prospects and Applications for Inducing Salt Tolerance in Plants

Salinity is one of the most important stresses that hamper agricultural productivity in nearly every part of the world. Enhanced biosynthesis of ethylene in plants under salinity stress is well established. Higher ethylene concentration inhibits root growth and ultimately affects the overall plant growth. Overcoming this ethylene-induced root inhibition is a prerequisite for successful crop production. Recent studies have shown that ethylene level in plants is regulated by a key enzyme 1-aminocyclopropane-1-carboxylicacid (ACC)-deaminase. This enzyme is present in plant growth-promoting bacteria (PGPR) and lowers the ethylene level by metabolizing its precursor ACC into α-ketobutyrate and ammonia (NH₃). Inoculation of plants under salinity stress with PGPR having ACC-deaminase activity mitigates the inhibitory effects of salinity on root growth by lowering the ethylene concentration in the plant. This in turn results in prolific root growth, which is beneficial for the uptake of nutrients and maintenance of growth under stressful environment. The present review critically discusses the effects of salinity stress on plant growth with special reference to ethylene production and the effects of rhizobacteria containing ACC-deaminase on crop improvement under salinity stress. It also discusses how much progress has been made in producing transgenic lines of different crops over-expressing the gene encoding ACC-deaminase and how far such transformed lines can tolerate salinity stress.