Nitric oxide regulates plant responses to drought, salinity, and heavy metal stress

Plants are exposed to diverse abiotic stresses like drought, heat, salinity, and high-metal concentrations at different stages of their life cycle. As protection against stress, plants release signaling molecules that initiate a cascade of stress-adaptation responses leading either to programmed cell death or plant acclimation. Nitric oxide (NO) is a small but important redox signaling molecule that in plants is involved in a diverse range of physiological processes including germination, development, flowering, senescence, and abiotic stress. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulations in stress in plants. In this article, we review and discuss the biosynthesis of NO, sources and exogenous application of NO donors under drought, salt, and heavy metal stress. A review of publications indicated that, in general, application of exogenous NO alleviates the negative stress effects in plants and improves antioxidant activity in most plant species. In addition, S-nitrosylation and tyrosine nitration are two NO-mediated posttranslational modification. All these factors are important in protecting plants from diverse stresses and vary with the species. Furthermore, to determine precise mechanisms of action of NO is expected to help in efficient utilization of crop cultivation under stress conditions.