In a previous study, we found copper (Cu) accumulated to a higher level in the aerial parts of soil-grown plants of the SUMO E3 ligase siz1 mutant than in those of the wild type. Here, we found that all superoxide dismutase (SOD) isoforms, such as FeSOD, MnSOD and different types of Cu/ZnSOD, were more active in the siz1 mutant than in the wild type under normal growth conditions. We further examined the expression and enzymatic activity of Cu/ZnSOD1 (CSD1) in shoots of the siz1 mutant under excess Cu. Shoot CSD1 protein level and activity were reduced in siz1 with excess Cu but induced in the wild type. SIZ1-dependent SUMOylation may be involved in maintaining CSD1 protein stability or repelling a feedback regulation under Cu stress.