Toxicity in leaves of rice exposed to cadmium is due to hydrogen peroxide accumulation

The production of H2O2 in detached rice leaves of Taichung Native 1 (TN1) caused by CdCl2 was investigated. CdCl2 treatment resulted in H2O2 production in detached rice leaves. Diphenyleneiodonium chloride (DPI) and imidazole (IMD), inhibitors of NADPH oxidase (NOX), prevented CdCl2-induced H2O2 production, suggesting that NOX is a H2O2-genearating enzyme in CdCl2-treated detached rice leaves. Phosphatidylinositol 3-kinase inhibitors wortmanin (WM) or LY294002 (LY) inhibited CdCl2-inducted H2O2 production in detached rice leaves. Exogenous H2O2 reversed the inhibitory effect of WM or LY, suggesting that phosphatidylinositol 3-phosphate is required for Cd-induced H2O2 production in detached rice leaves. Nitric oxide donor sodium nitroprusside (SNP) was also effective in reducing CdCl2-inducing accumulation of H2O2 in detached rice leaves. Cd toxicity was judged by the decrease in chlorophyll content. The results indicated that DPI, IMD, WM, LY, and SNP were able to reduce Cd-induced toxicity of detached rice leaves. Twelve-day-old TN1 and Tainung 67 (TNG67) rice seedlings were treated with or without CdCl2. In terms of Cd toxicity (leaf chlorosis), it was observed that rice seedlings of cultivar TN1 are Cd-sensitive and those of cultivar TNG67 are Cd-tolerant. On treatment with CdCl2, H2O2 accumulated in the leaves of TN1 seedlings but not in the leaves of TNG67. Prior exposure of TN1 seedlings to 45 degrees C for 3 h resulted in a reduction of H2O2 accumulation, as well as Cd tolerance of TN1 seedlings treated with CdCl2. The results strongly suggest that Cd toxicity of detached leaves and leaves attached to rice seedlings are due to H2O2 accumulation.