Nitric oxide in biological systems

Against a background of a general description of the action of the free radical gas nitric oxide (NO), some of its close derivatives such as peroxynitrite, and of its formative enzyme nitric oxide synthase (NOS) in mammals, possible plant analogies are discussed. Resembling endogenous effects in mammals which may be either promotory or inhibitory, a possible physiological if not anatomical parallel pertains to NO's vasodilatory effects. The latter in mammals inter alia are encountered in extensible coronary-, blood vessel lining-, endotheliar-, pulmonary- and penile tissue while in plants such effects may be on cell wall matrix components which constrain potential turgor-promoted cell expansion. The same final outcome in both plants and mammals may be enhancement of periplasmic membrane fluidity. As with certain aspects of NO action in mammals, the nature of the response may be concentration-dependent - lower ones, promotory - higher, inhibitory. As deduced from trials on pea foliage, an apparent linkage of NO and ethylene metabolism is specific to plants. Thus, low concentrations of either endogenously-produced or exogenously-applied NO in the 10(-6) M range exert significant growth-promoting and ethylene-inhibiting effects which are reversed by higher NO concentrations or by equimolar applications of the NOS inhibitor-N-G-methyl-L-arginine or of NO releasing compounds. An overview is given of other modes of NO action in biological systems and of basic issues of NO and NOS function in plant growth regulation which await elucidation.