Possible involvement of MAP kinase pathways in acquired metal-tolerance induced by heat in plants

Cross tolerance is a phenomenon that occurs when a plant, in resisting one form of stress, develops a tolerance to another form. Pretreatment with nonlethal heat shock has been known to protect cells from metal stress. In this study, we found that the treatment of rice roots with more than 25 μM of Cu²⁺ caused cell death. However, heat shock pretreatment attenuated Cu²⁺-induced cell death. The mechanisms of the cross tolerance phenomenon between heat shock and Cu²⁺ stress were investigated by pretreated rice roots with the protein synthesis inhibitor cycloheximide (CHX). CHX effectively block heat shock protection, suggesting that protection of Cu²⁺-induced cell death by heat shock was dependent on de novo protein synthesis. In addition, heat pretreatment downregulated ROS production and mitogen-activated protein kinase (MAPK) activities, both of which can be greatly elicited by Cu²⁺ stress in rice roots. Moreover, the addition of purified recombinant GST-OsHSP70 fusion proteins inhibited Cu ²⁺-enhanced MAPK activities in an in vitro kinase assay. Furthermore, loss of heat shock protection was observed in Arabidopsis mkk2 and mpk6 but not in mpk3 mutants under Cu²⁺ stress. Taken together, these results suggest that the interaction of OsHSP70 with MAPKs may contribute to the cellular protection in rice roots from excessive Cu²⁺ toxicity.