To determine the mechanism of regulation of superoxide dismutase (SOD; EC 18.104.22.168) isozymes in halophytes at the cellular level,we isolated calluses from Suaeda maritima plants native to Japan and Egypt. Calluses from Egyptian S. maritima could grow in a high salt medium (~400 mM NaCl), whereas Japanese S. maritima calluses could not. The expression characteristics of SOD
isozyme genes encoding two CuZn-SOD proteins, two Mn-SOD proteins, and one Fe-SOD protein in the two callus lines were analyzed. All SOD isozyme genes expressed in leaves were also expressed in both callus lines grown in normal medium and their expression levels were not affected by high salt. In-gel SOD activity assay after native-polyacrylamide gel electrophoresis revealed that both callus lines possessed a CuZn-SOD and an Fe-SOD as major SOD isozymes in normal medium. When calluses were grown on high salt medium, both SOD isozyme activities were decreased in Japanese calluses, while they were stable or slightly increased in Egyptian calluses. When calluses were incubated in normal medium with excess copper (Cu, ~100 μM), callus growth of both callus lines was less affected. In excess Cu, CuZn-SOD activity was greatly increased in both callus lines, while Fe-SOD activity was decreased in Japanese calluses and was stable in Egyptian calluses. Since both Fe-SOD and CuZn-SOD activities exhibited a positive
association with growth tolerance to high salt, they may play a role in salt tolerance in callus cells. In addition, our results indicate
that the Fe-SOD activity contributes to copper stress tolerance.