Salicylic acid-mediated hydrogen peroxide accumulation and protection against Cd toxicity in rice leaves

The role of H2O2 in salicylic acid (SA)-induced protection of rice leaves against subsequent Cd toxicity was investigated. SA pretreatment resulted in an increase in the contents of endogenous SA, as judged by the expression of OsWRKY45 (a SA responsive gene), and H2O2 in rice leaves. Diphenyleneiodonium (DPI) and imidazole (IMD), inhibitors of NADPH oxidase, prevented SA-increased H2O2 production, suggesting that NADPH oxidase is a H2O2-generating enzyme in SA-pretreated rice leaves. DPI and IMD also inhibited SA-increased activities of superoxide dismutase (SOD), ascorbate peroixdase (APX), and glutathione reductase (GR) activities, but had no effect on SA-increased catalase (CAT) activity. Moreover, SA-induced protection against subsequent Cd toxicity could also be prevented by DPI and IMD. The inhibitory effect of DPI and IMD on SA-induced protection against subsequent Cd toxicity could be reversed by exogenous H2O2. All these results suggested that SA-induced protection against subsequent Cd toxicity is mediated through H2O2. This conclusion is supported further by the observations that exogenous H2O2 application resulted in an increase in SOD, APX, and GR activities, but not CAT activity and a protection against subsequent Cd toxicity of rice leaves.