The Role of Nitric Oxide as a Bioactive Signaling Molecule in Plants under Abiotic Stress

Nitric oxide is a short-lived bioactive molecule initially described as a toxic compound but now recognized as an important signaling and effector molecule both in animal and in plant cells.NOis proposed to be one of the important second (should be second) messengers in plant cells. Various data indicate that NO is an endogenous signal in plants that mediates responses to several stimuli. Experimental evidence in support of such a signaling role of NO has been obtained through the application of NO, usually in the form of NO donors, through the measurement of endogenous NO, and through the manipulation of endogenous NO content by chemical and genetic means. In the last few years, many studies have described NO as both a cytotoxic and a cytoprotecting regulator involved in different physiological responses to abiotic stress in plants such as drought,UV-B, salinity, and high temperature. For example, application of the NOdonor, sodium nitroprusside, confers resistance to salt, drought, heavy metals, and chilling stresses. In plants, the sources of NO production have been the subject of considerable debate. A growing body of evidence indicates that NO is formed by mammalian-like NOS activity, nitrate reductase, or nonenzymatic sources. New evidence involving NO in signal transduction pathways mediated by some key molecules such as cyclic guanosine monophosphate, cyclic adenosine diphosphate ribose, and Ca2 recently have been reported in plants. There is also a compelling evidence suggesting that abscisic acid, hydrogen peroxide, and NO interact under abiotic stress in plants. In this chapter, nitric oxide functions as a bioactive signaling molecule in plant abiotic stress responses are discussed. The cross talk betweenNOand other key signaling components under abiotic stress is also reviewed.