The biogenesis and physiological function of chloroplast superoxide dismutases

Iron-superoxide dismutase (FeSOD) and copper/zinc-superoxide dismutase (Cu/ZnSOD) are evolutionarily conserved proteins in higher plant chloroplasts. These enzymes are responsible for the efficient removal of the superoxide formed during photosynthetic electron transport and function in reactive oxygen species metabolism. The availability of copper is a major determinant of Cu/ZnSOD and FeSOD expression. Analysis of the phenotypes of plants that over-express superoxide dismutases in chloroplasts has given support for the proposed roles of these enzymes in reactive oxygen species scavenging. However, over-production of chloroplast superoxide dismutase gives only limited protection to environmental stress and does not result in greatly improved whole plant performance. Surprisingly, plant lines that lack the most abundant Cu/ZnSOD or FeSOD activities perform as well as the wild-type under most conditions tested, indicating that these superoxide dismutases are not limiting to photoprotection or the prevention of oxidative damage. In contrast, a strong defect in chloroplast gene expression and development was seen in plants that lack the two minor FeSOD isoforms, which are expressed predominantly in seedlings and that associate closely with the chloroplast genome. These findings implicate reactive oxygen species metabolism in signaling and emphasize the critical role of sub-cellular superoxide dismutase location. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.