AtMEK1 mediates stress-induced gene expression of CAT1 catalase by triggering H₂O₂ production in Arabidopsis

Catalase and hydrogen peroxide (H₂O₂) have been extensively studied for their roles in various stress responses. However, little is known about the triggering mechanisms for stress-induced catalase gene expression or about H₂O₂ production as a stress signal. It is reported here that ABA-, drought-, and salt stress-induced gene expression of CAT1 catalase is mediated by AtMEK1, an Arabidopsis MAPK kinase, by triggering H₂O₂ signal production. Both CAT1 expression and AtMEK1 activity were activated by ABA, drought, and salt stresses. The mek1 mutant totally blocked stress-induced CAT1 expression and, interestingly, stress-induced H₂O₂ production was also blocked. Over-expression of AtMEK1 significantly promoted stress-induced CAT1 expression, and also promoted H₂O₂ production. These results conclusively indicate that stress-induced CAT1 expression is mediated by AtMEK1 and, furthermore, that the triggering of H₂O₂ production might be involved in this process, as further proved by the observation that CAT1 expression was induced by applied H₂O₂. Surprisingly, the signalling mechanisms for stress-induced gene expression of CAT2 and CAT3 were very different from that of CAT1. Except for drought stress, expression of CAT2 or CAT3 was also activated by salt stress or ABA treatment, and AtMEK1 was not proved to be involved in the drought-induced expression of CAT2 or CAT3. Further studies showed that stomatal movement was much less sensitive to ABA in AtMEK1 mutant (mek1), and over-expression of AtMEK1 in Arabidopsis increased plant resistance to drought or salt stress, which further demonstrated that AtMEK1 is a crucial mediator in plant stress signal transduction.