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 incre ased when suspension cultures of germinating rice embryos were treated with paraquat (7.9-fold) or H2O2 (6.1-fold). Induction by paraquat reached a ma ximum at 8 h. Induction by H2O2 peaked earlier at 4 h of treatment. This re sult suggests that the induction by paraquat might be caused by the H2O2 ge nerated from superoxide. Treatment with a superoxide dismutase inhibitor, d iethyldithiocarbamate, which is supposed to decrease the cellular H2O2 leve l, reduced the paraquat induction of cytosolic APX. In contrast, when APX a nd catalase were inhibited by hydroxyurea or aminotriazole, cellular H2O2 c ontent was elevated and cytosolic APX mRNA was markedly increased without p araquat or H2O2 treatment. This suggests that cytosolic APX is regulated by the H2O2 level within cells. An increase in H2O2 content was observed in t he paraquat treated embryos, suggesting that paraquat induction of cytosoli c APX was caused by H2O2 generated through superoxide dismutation. These re sults indicate that H2O2 is involved in oxidative stress signalling, leadin g to the induction of cytosolic APX.