Differential expression of CuZn- and Fe-superoxide dismutase genes of tobacco during development, oxidative stress, and hormonal treatments.

Chloroplasts of Nicotiana tabacum have two superoxide dismutases: a Fe- and a CuZn-containing enzyme, encoded by the nuclear genes sodB and sodCp, respectively. As a first step in studying the physiological function of these two enzymes, we compared the expression of sodB and sodCp in different plant organs, in response to hormonal treatments, and upon treatment with paraquat and Norflurazon. The sodCp transcript and active enzyme were detected only in young leaves of mature plants. The sodB transcript was more abundant in young compared to old leaves, but the enzymatic activity was higher in mature and senescent leaves. sodCp and sodB exhibited a different expression pattern upon treatment with abscisic acid, indole-3-acetic acid, kinetin, gibberellin, and 1-aminocyclopropane-1-carboxylate. Paraquat treatment caused a decrease in abundance of both transcripts, although the dose dependency of this decrease differed. Norflurazon-induced photooxidation resulted in a 10-fold increase of sodCp mRNA whereas the sodB transcript level was 25% higher than the control. These differences in expression might explain why both plastid-located superoxide dismutase enzymes are needed, particularly under stress conditions.