Role of Nitric Oxide and Calcium Signaling in Abiotic Stress Tolerance in Plants, in: Protective Chemical Agents in the Amelioration of Plant Abiotic Stress.

Nitric oxide (NO) is a free radical molecule which plays a mediating role in resistance to salinity, drought, heavy metal stresses and temperature extremes in different crops. It regulates plant growth and development, such as seed germination and root formation, and promotes different biochemical and physical processes. By modulating antioxidant defense mechanisms, nitric oxide protects the plant during heat, cold, drought, salinity, excess light, herbicides and ozone toxicity, and reduces the deleterious effects of toxic metals. It can also provide resistance in cells in response to different stressors against UV-B, ABA (abscisic acid)-induced stomatal closure through regulation of Ca2+ fluxes and closely cooperates with JA (jasmonic acid), SA (salicylic acid), and ethylene. Nitric oxide improves the viability of rice seedling, root elongation in maize, and the germination rate of wheat seeds, and alleviates Cu, Al and Cd stress in soybean. Calcium is a ubiquitous second messenger, and it plays a vital role against oxidative stress, circadian rhythms, pathogen infection, and ecological and oxidative stress in plants. It is an essential plant nutrient that regulates different physiological processes and enhances the plant tolerance against biotic and abiotic (salinity, drought, cold, light, and heavy metal) stresses. Application of Ca+2 in Brassica juncea and Arabidopsis improves the growth of plants and alleviates the toxic effects of Cd. Calcium has a multifunctional role to overcome the consequences of drought and slat stress, and in the maintenance of membrane integrity and cell wall structure during unfavorable conditions.