ABSTRACT
Soil salinity is a major environmental problem that negatively affects crop yield in arid and semi-arid areas of the world. Salinity stress suppresses seed germination and inhibits plant growth by causing physiological alterations such as ion toxicity, nutrient imbalance, membrane disorganization, and inhibition of water uptake. Nitric oxide (NO) is an important signaling molecule, regulating a broad spectrum of developmental and physiological processes in plants under normal as well as in environmental stress conditions. Several lines of evidence indicate NO involvement in regulation of various plant responses such as photosynthesis, antioxidant defense, osmolyte accumulation, protein modifications, and gene expression under salinity stress. Investigations on NO in plants have centered around three main topics: (i) its source of production, (ii) physiological and molecular effects of exogenous NO treatments, and (iii) elucidation of signal transduction pathways. The biological source of endogenous NO in animals is well documented and understood; however, the origin of NO production in plants is poorly understood. Nitrate reductase and nitric oxide synthase are the two widely reported sources of endogenous NO production in plants under salt-stress conditions. The interaction of NO with components of cell signaling (cGMP, cADP-ribose, mitogen-activated protein kinases (MAPK), and Ca2+) and NO-mediated modification of proteins by post-translational modifications (S-nitrosylation and tyrosine nitration) are considered as crucial mechanisms for regulating the physiological responses in plants under salinity stress. The fundamental knowledge of NO production, sensing, and transduction in plants is largely unknown or inadequately characterized. In view of this, an attempt has been made to describe recent advances in NO biosynthesis, signaling, and function in plants under salinity stress. Moreover, the impact of endogenously synthesized and exogenously supplied NO on regulating ion homeostasis, osmolyte accumulation, antioxidant defense, and photosynthesis has also been discussed.
