Regulation of superoxide dismutase expression by copper availability

The most abundant copper proteins in green tissues are plastocyanin (PC) in thylakoids and copper/zinc superoxide dismutase (Cu/ZnSOD) of which the major isoforms are found in the cytosol and in the chloroplast stroma. An iron superoxide dismutase (FeSOD) can also be found in the stroma. The expression of superoxide dismutases (SODs) has been studied mainly in the context of abiotic stress. However, the availability of metal cofactors may also determine SOD expression patterns. Indeed, in Arabidopsis thaliana, Cu/ZnSOD enzymes were only expressed when copper was sufficient. This observation was made for plants grown on sucrose-containing tissue culture media and regulation of SOD expression by copper has not been tested for other species. To investigate the effect of copper on SOD expression, we used a hydroponic set-up in which plants grew without any evident stress symptoms. We observed that A. thaliana, Brassica juncea, Lycopersicum lycopersicum, Zea mays and Oryza sativa, downregulated Cu/ZnSOD in response to copper limitation. Under this condition, FeSOD expression was upregulated to replace Cu/ZnSOD in the stroma in all plants except Z. mays, in which FeSOD was not detectable. Copper limitation did not affect PC accumulation in any of the plants except Z. mays. Comparisons of leaf copper contents and SOD expression suggest that Cu/ZnSOD and FeSOD expression levels are good indicators of impending copper deficiency. Plants that downregulate Cu/ZnSOD and upregulate FeSOD under copper limitation can maintain superoxide scavenging and save copper for use in PC, which is essential for photosynthesis.