Leaf senescence is the final leaf developmental process that is regulated by both intracellular factors and environmental conditions. The mitogen-activated protein kinase(MAPK) signaling cascades have been shown to play important roles in regulating leaf senescence; however, the component(s) downstream of the MAPK cascades in regulating leaf senescence are not fully understood. Here we showed that the transcriptions of Zm MEK1, Zm SIMK1, and Zm MPK3 were induced during dark-induced maize leaf senescence.Furthermore, in-gel kinase analysis revealed the 42 k Da MAPK was activated. Zm MEK1 interacted with Zm SIMK1 in yeast and maize mesophyll protoplasts and Zm SIMK1 was activated by Zm MEK1 in vitro. Expression of a dominant negative mutant of Zm MEK1 in Arabidopsis transgenic plants induced salicylic acid(SA) accumulation and SA-dependent leaf senescence.Zm MEK1 interacted with Arabidopsis MPK4 in yeast and High-Im pact activated MPK4 in vitro. SA treatment accelerated darkinduced maize leaf senescence. Moreover, blockage of MAPK signaling increased endogenous SA accumulation in maize leaves. These findings suggest that Zm MEK1-Zm SIMK1 cascade and its modulating SA levels play important roles in regulating leaf senescence.
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