Nitric Oxide: A Ubiquitous Signal Molecule for Enhancing Plant Tolerance to Salinity Stress and Their Molecular Mechanisms

Salinity is a major constraint of agricultural productivity globally and is recognized to be severely elevated by alterations in the climatic conditions. High salinity levels cause osmotic pressure and ionic imbalance and adversely affects plant's morphological, physiological and biochemical aspects, which subsequently hampers plant growth or death of the plant. Furthermore, as alterations in soil properties lead to an acceleration in salinity levels, our concern of how plants cope with salinity stress is becoming progressively meaningful. In this context, various signaling moieties and cross-talk between several sensors and signal transduction pathways, are required to increase plant tolerance against salinity stress. To protect from salinity, plants secrete different signaling moieties that trigger several stress-adaptation responses and cause either plant acclimation or programmed cell death. Among these signaling molecules, nitric oxide (NO) is a multifaceted, small gaseous reactive moiety that regulates numerous plant developmental progressions and provides endurance to different abiotic factors, including salinity stress. NO is known to be significant for plants exposed to salinity stress. It improves plant potential to cope with salinity by boosting plant growth, photosynthetic activity, stomatal conductance, accumulation of compatible solutes, maintains ion homeostasis, and reverse oxidative damage by stimulating anti-oxidant defense apparatus. It also alters the expression of defense-associated genes, thereby influence the phenotypic response of plant genotypes. Thus, it was concluded that NO is a crucial signaling molecule which remarkably mitigate salinity-induced adverse effects in plants by regulating various developmental aspects in plants. The aim of the present review is to provide an overall update on the NO mediated salinity stress tolerance in plants including NO metabolism, signal transduction via inducing various genes and post-translational modifications (PTMs), plant growth, photosynthetic activity, mineral nutrition, anti-oxidant defense system, gene expression and its cross-talk with phytohormones and with hydrogen sulfide.