SUPEROXIDE-DISMUTASE IN PLANTS

Superoxide dismutases (SODs) are metal-containing enzymes that catalyz e the dismutation of superoxide radicals to oxygen and hydrogen peroxi de. The enzyme has been found in all aerobic organisms examined where it plays a major role in the defense against toxic-reduced oxygen spec ies, which are generated as byproducts of many biological oxidations. The generation of oxygen radicals can be further exacerbated during en vironmental adversity and consequently SOD has been proposed to be imp ortant for plant stress tolerance. In plants, three forms of the enzym e exist, as classified by their active site metal ion: copper/zinc, ma nganese, and iron forms. The distribution of these enzymes has been st udied both at the subcellular level and at the phylogenic level. It is only in plants that all three different types of SOD coexist. Their o ccurrence in the different subcellular compartments of plant cells all ows a study of their molecular evolution and the possibility of unders tanding why three functionally equivalent but structurally different t ypes of SOD have been maintained. Several cDNA sequences that encode t he different SODs have recently become available, and the use of molec ular techniques have greatly increased our knowledge about this enzyme system and about oxidative stress in plants in general, such that now is an appropriate time to review our current knowledge.