Heat shock-mediated H2O2 accumulation and protection against Cd toxicity in rice seedlings

Rice (Oryza sativa L.) seedlings stressed with CdCl2 (0.5 mM or 50 mu M) showed typical Cd toxicity (leaf chlorosis, decrease in chlorophyll content, or increase in H2O2 and malondialdehyde contents). Rice seedlings pretreated with heat shock at 45 degrees C (HS) for 2 or 3 h were protected against subsequent Cd stress. Rice seedlings pretreated with HS had similar Cd concentration in leaves caused by CdCl2 as those non-HS. The content of H2O2 increased in leaves 1 h after HS exposure. However, APX and GR activities were higher in HS-treated leaves than their respective control, and it occurred after 2 h of HS treatment. Pretreatment of rice seedlings with H2O2 under non-HS conditions resulted in an increase in APX, GR, and CAT activities and protected rice seedlings from subsequent Cd stress. HS-induced H2O2 production and protection against subsequent Cd stress can be counteracted by imidazole, an inhibitor of NADPH oxidase complex. Results of the present study suggest that early accumulation of H2O2 during HS signals the increase in APX and GR activities, which in turn prevents rice seedlings from Cd-caused oxidative damage.