Nitric oxide (NO) is a colorless and toxic gas. It is a free radical. NO was named 'Molecule of the Year' by Science in 1992. NO readily diffuses through cytoplasm as well as lipid membranes of cell. It is a modulator of neurotransmission and is an elicitor of immune responses. NO act as a signal molecule in plants. The qualities such as high reactivity and diffusibility across membranes make NO an ideal intercellular as well as intracellular signal molecule. NO is gaseous signaling molecules and an important vertebrate biological messenger. It plays important role in number of biological processes. Vessel dilatation, neurotransmission, modulation of the hair cycle, and penile erections are some of the effects of NO. However, NO has been also involved in number of cellular processes such as growth and development, various metabolic processes senescence and biotic and a biotic stressors. NO was classified as a phytohormone as well as a nontraditional plant growth regulator. NO is applied to plants in the form of solutions such as SNP, S-Nitrosoglutathione, and S-nitroso-N-acetylpenicillinamin. There are two pathways for NO production in chloroplasts of soybean. High NO concentration generates reactive nitrogen species which may lead to destruction of the photosynthetic apparatus. No can affect photosynthesis as well as photorespiration in different plants. NO can inhibit chloroplast electron transport in a reversible manner. There are various sources of NO in plants depending upon the species, the cells/tissues, the conditions under which the plants are grown, and, of course, the signalling pathways active under those specific conditions. There are several potential sources of NO in plants such as nitric oxide synthase (NOS), nitrate reductase (NR), xanthine oxidoreductase or nonenzymatic sources. NO can exist in three forms viz NO, NO+ and NO-.
Various kinds of Abiotic stress such as moisture stress, salt stress, high and low temperature stress results in the formation of reactive oxygen species (ROS). NO interacts with ROS in number of ways and might function as an antioxidant during stresses. NO regulates superoxide formation and inhibition of lipid peroxidation. NO plays a role of antioxidant but excess of it can be harmful. So a fovourable balance of ROS and NO is required. Further, NO plays an important role in plant defence signaling. NO plays a key role in the activation of Hypersensitive Response (HR) which is a defence process activated in plants in response to pathogen attack.
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