Regulatory role of BOTRYTIS INDUCED KINASE1 in ETHYLENE INSENSITIVE3-dependent gene expression in Arabidopsis.

KEY MESSAGE: Arabidopsis BIK1 negatively regulates EIN3-depedent gene expression as an immediate cellular response. BIK1 localizes to the plasma membrane and its autophosphorylation and kinase activity involves in EIN3 repression. BOTRYTIS INDUCED KINASE1 (BIK1) is a multifunctional receptor-like kinase that involves in ethylene-mediated plant defense signaling. The loss of function BIK1 becomes insensitive to ethylene, but it still accumulates a higher level of ETHYLENE INSENSITIVE3 (EIN3) that serves as the key transcription activator in ethylene signaling. To unequivocally elucidate BIK1 function on EIN3 regulation in ethylene signaling, we took a combined approach of transient expression assay and stable expression analysis of BIK1. In our cell-based functional assay BIK1 destabilized EIN3 and down-regulated EIN3-dependent transcription. Membrane localization and autophosphorylation of BIK1 were required for full repression of EIN3 function, but its kinase activity potential compromised such regulatory action. Consistently, the analysis of transgenic plants verified BIK1 function on EIN3 repression. Our findings have clarified that autophosphorylated BIK1 in the plasma membrane negatively regulates EIN3-dependent gene expression. Thus, ethylene insensitivity in bik1 appears to be an indirect or a feedback long-term response.