Sulfite Oxidation in Plants

Abstract

Sulfite oxidation in plants was a matter of controversial discussion for a long time and still is not finally understood. There is no doubt anymore about the occurrence of sulfite oxidation besides primary sulfate assimilation that takes place in the chloroplast. Sulfate is reduced via sulfite to organic sulfide which is essential for the biosynthesis of S-containing amino acids and other compounds like glutathione. However, it has also been reported that sulfite can be oxidized back to sulfate, e.g. when plants were subjected to SO2 gas. Work from our laboratory has identified sulfite oxidase as a member of molybdenumcontaining enzymes in plants, which seems to be the most important way to detoxify excess of sulfite. In this paper we show how plant cells separate the two counteracting pathways – sulfate assimilation and sulfite detoxification – into different cell organelles. We discuss how these two processes are (co-)regulated and what kind of other sulfite oxidase activities occur in the plant.