Phosphorylation of an ethylene response factor by MPK3/MPK6 mediates negative feedback regulation of pathogen-induced ethylene biosynthesis in Arabidopsis

Plants under pathogen attack produce high levels of the gaseous phytohormone ethylene to induce plant defense responses via the ethylene signaling pathway. The 1-aminocyclopropane-1-carboxylate synthase(ACS) is a critical rate-limiting enzyme of ethylene biosynthesis. Transcriptional and post-translational upregulation of ACS2 and ACS6 by the mitogen-activated protein kinases MPK3 and MPK6 are previously shown to be crucial for pathogen-induced ethylene biosynthesis in Arabidopsis. Here, we report that the fungal pathogen Botrytis cinerea-induced ethylene biosynthesis in Arabidopsis is under the negative feedback regulation by ethylene signaling pathway. The ethylene response factor ERF1 A is further found to act downstream of ethylene signaling to negatively regulate the B. cinerea-induced ethylene biosynthesis via indirectly suppressing the expression of ACS2 and ACS6. Interestingly, ERF1 A is shown to also upregulate defensin genes directly and therefore promote Arabidopsis resistance to B. cinerea. Furthermore, ERF1 A is identified to be a substrate of MPK3 and MPK6, which phosphoactivate ERF1 A to enhance its functions in suppressing ethylene biosynthesis and inducing defensin gene expression. Taken together,our data reveal that ERF1 A and its phosphorylation by MPK3/MPK6 not only mediate the negativefeedback regulation of the B. cinerea-induced ethylene biosynthesis, but also upregulate defensin gene expression to increase Arabidopsis resistance to B. cinerea.