Induction of rice cytosolic ascorbate peroxidase mRNA by oxidative stress; the involvement of hydrogen peroxide in oxidative stress signalling

The oxidative stress response of rice cytosolic ascorbate peroxidase (APX) was examined. The transcript level of cytosolic APX was significantly increased when suspension cultures of germinating rice embryos were treated with paraquat (7.9-fold) or H2O2 (6.1-fold). Induction by paraquat reached a maximum at 8 h. Induction by H2O2 peaked earlier at 4 h of treatment. This result suggests that the induction by paraquat might be caused by the H2O2 generated from superoxide. Treatment with a superoxide dismutase inhibitor, diethyldithiocarbamate, which is supposed to decrease the cellular H2O2 level, reduced the paraquat induction of cytosolic APX. In contrast, when APX and catalase were inhibited by hydroxyurea or aminotriazole, cellular H2O2 content was elevated and cytosolic APX mRNA was markedly increased without paraquat or H2O2 treatment. This suggests that cytosolic APX is regulated by the H2O2 level within cells. An increase in H2O2 content was observed in the paraquat treated embryos, suggesting that paraquat induction of cytosolic APX was caused by H2O2 generated through superoxide dismutation. These results indicate that H2O2 is involved in oxidative stress signalling, leading to the induction of cytosolic APX.