Nitric oxide (NO) is a gaseous di-atomic radical, readily diffusible through biological membranes and readily soluble in water is having a wide array of physiological, biochemical and molecular events in animals, plants and microbes. NO reacts directly with metal complexes and other radicals and indirectly as a reactive nitrogen oxide species with DNA, proteins, and lipids. In animals, NO is well established as a signaling molecule and is synthesized by multiple forms of nitric oxide synthase (NOS) enzyme from L-arginine. NO functions in almost all tissues and interacts with multiple target compounds in neurotransmission, vascular smooth muscle relaxation, and platelet inhibition. In plants, NO is synthesized through different enzymatic and non-enzymatic pathways, and play a diverse role in maintaining the functional homeostasis in normal (unstressed) systems as well in the protection against biotic and abiotic stresses. The present knowledge on the biochemistry of evolution, localization, mode of action and signaling of NO in plants shows that NO is one of the versatile molecule which can be transported easily to any compartment in the plant cell and elucidate its impact through various signal transduction pathways. The literature available is vast. This review deals with the biosynthesis, detection and estimation methods used in plants, translocation and detoxification, and role of NO in plants. Emphasis is given to the role of NO as a signal regulating molecule in plants.
Key words: Nitric oxide (NO), nitric oxide synthase (NOS), gene regulation, senescence, synthesis, signal transduction, plant growth, development, stress, programmed cell death, localization, detoxification, movement.
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