Mitogen activated protein kinase cascades mediate the regulation of antioxidant enzymes under abiotic stresses in Arabidopsis

Catalase and H2O2 play important roles in plant adaptive responses to biotic and abiotic stresses, whereas little is known about their upstream signaling cascades leading to the gene expression of catalase and H2O2 production. We here report that catalase gene family regulated by AtMKK1, an Arabidopsis MAPK kinase, responded differently to ABA, drought and salt stress. CAT1 expression was sensitive to ABA, drought and salt stresses, and at the mean time, AtMKK1 activity was also sensitively activated by all these stresses. Both CAT1 expression and AtMKK1 activity could be highly inhibited by MAPK signaling inhibitor PD98059, suggesting that AtMKK1 might be involved in the stress-induced CAT1 expression. The AtMKK1 mutant, mkk1, totally blocked the stressed-induced CAT1 expression, and interestingly, the stress-induced H2O 2 production was also blocked. Over-expression of AtMKK1 significantly promoted the stress-induced CAT1 expression, and also promoted H2O2 production. These results conclusively indicate that stress-induced CAT1 expression is mediated by AtMKK1, and further more, the triggering of H2O2 production might be involved in the process, as further proved by the observation that CAT1 expression was sensitively induced by applied H 2O2. Surprisingly, the signaling mechanisms for the stress-induced gene expression of CAT2 and CAT3 were observed to be rather different from CAT1. Except for drought stress, CAT2 and CAT3 expressions were not sensitive to ABA or salt stress, and AtMKK1 was not proved to be involved in the drought-induced CAT2, or CAT3 expressions. Further studies showed that stomatal movement was much less sensitive to ABA in mkk1, and over-expression of AtMKK1 in Arabidopsis increased the plant resistance to drought or salt stress, which further demonstrate that AtMKK1 is a crucial mediator in plant stress signal transduction.;In response to ABA treatment, CAT1 expression was remarkably induced, and moreover, the CAT1 expression and H2O 2 production were both totally arrested in mkk1. Over-expression of AtMKK1 significantly enhanced the ABA-induced CAT1 expression and H2O2 production. Further studies showed that the ABA-induced CAT1 expression and H 2O2 production also were blocked in mpk6 mutant plants, and by contrast, promoted in AtMPK6 overexpressing plants. Moreover, the AtMPK6 kinase activity was observed to be activated by ABA in an AtMKK1-dependent manner. These data strongly suggest that the ABA-induced CAT1 expression and H2O2 production are mediated by AtMKK1 via AtMPK6-coupled signaling. Further investigation showed that, compared to wild type plants, mkk1 exhibited a much less sensitivity in germination to ABA, and a decreased tolerance to drought, whereas over-expression of AtMKK1 exhibited a hyper-sensitivity to ABA in germination and an increased resistance to drought, suggesting that AtMKK1 is a key mediator in the ABA signaling cascades.;Superoxide dismutases (SODs) play important roles in plant adaptive response to biotic and abiotic stresses but little is known about their upstream signaling cascades leading to their gene expressions. We report that salt-induced gene expression of the iron superoxide dismutases, FSD2 and FSD3, were mediated by MKK5, one of Arabidopsis MAPK kinases. FSD2 and FSD3 expressions were remarkably increased in response to salt treatment but were blocked in MKK5 null plants, mkk5. Using the transient expression assay in protoplasts, we found that MKK5 was also activated in response to salt stress. The over-expression of MKK5 in wild type plants enhanced the plant salt tolerance. In contrast, mkk5 null mutant plants exhibited hypersensitivity to salt stress and in germination on salt-containing media. These data demonstrate that MKK5 is a key signal transducer of salt stress in Arabidopsis. Moreover, we identified that MPK6 was also involved in the MKK5-mediated iron superoxide dismutases (FSD) signaling pathway in salt stress. The kinase activity of MPK6 was totally arrested in MKK5 null plant-mkk5, whereas the activity of MPK3 was only partially blocked. MKK5 interacted with the AtMEKK1 protein that was also involved in the salt-induced FSD signaling pathway. These data strongly suggest that salt-induced FSD2 and FSD3 expressions are mediated by AtMEKK1 via MKK5-MPK6-coupled signaling. It is suggested that there is a complete MAP kinase cascade (MEKK1, MKK5 and MPK6) that mediates the salt-induced iron superoxide dismutases.;MKK5 is known involved in the oxidative stress-induced copper/zinc superoxide dismutases (Cu/Zn SODs) signaling pathway. Results suggest that MKK5-guided upgrade of CSD1 and CSD2 in transgenic plants is an effective new approach to improve plant productivity under stress conditions. (Abstract shortened by UMI.)...