In mammals, nitric oxide (NO) is a reactive free radical involved in diverse physiological functions. NO and its redox-related forms NO + and NO − react with di(oxygen) and its derivatives, with metalloproteins and thiol-containing proteins. NO-mediated nitrosation of proteins represents an important cellular regulatory mechanism. Biosynthesis of NOis catalysed by nitric oxide synthase (NOS). Three isoenzymes representing distinct gene products have been identified: the inducible NOS isoform, the constitutive neuronal and endothelial isoforms. Inducible and constitutive NOSs have the same structural features, but their activities differ in their dependence to calcium and the rate of NO produced. The principal NO-mediated functions in mammals are endothelium-dependent relaxation, neurotransmission and immune response. The role of NO in the antitumor immune response comprises both regulatory and effector functions at the intra- or inter-cellular level. The first function includes inhibition of lymphocyte proliferation or participation in different transduction pathways. The second function includes pro- or anti-tumoral effects and NO-mediated cell toxicity or cell resistance to apoptosis. In plants, NO is produced via non-enzymatic reactions, by the activity of a constitutive nitrate reductase, and by a mammalian-type NOS which corresponding gene has not yet been cloned. NO was found to regulate plantgrowth, to act as antioxidant, and to function as a signaling molecule in plants challenged by pathogens. Interestingly, key proteins involved in animal NO signaling appear to be modulated by NO in plants. This finding complements the list of evidence suggesting that portions of transduction mechanisms are shared between plants and animals.
