Plants rely on innate immunity to thwart off microbial pathogens. Immune responses are triggered when immune receptors recognize microbial molecular patterns. The cytoplasmic kinase BOTRYTIS-INDUCED KINASE 1 (BIK1) plays a central role in relaying signals from multiple upstream immune receptors to diverse substrate proteins. The homeostasis of BIK1 is tightly controlled and is maintained by a regulatory module, in which the calcium dependent protein kinase CPK28 contributes to BIK1 turnover. Whether are CPK28 proteins under further control to maintain BIK1 homeostasis and fine-tune immune signaling remains unknown.

A research group led by Dr. Dongping Lu from Center for Agricultural Resources Research, Chinese Academy of Sciences find that CPK28 proteins undergo a type of modification called ubiquitination that serves as a signal for protein degradation via the 26S proteasome machinery. Interestingly, the bacterial flagellin treatment enhances the ubiquitination and degradation of CPK28 proteins. Two ubiquitin ligases, ARABIDOPSIS TóXICOS EN LEVADURA 31 (ATL31) and ATL6 directly interact with and mediate the ubiquitination of CPK28, which results in the proteasomal degradation of CPK28 and attenuation of CPK28 function. Therefore, ATL31 and ATL6 modulate BIK1 homeostasis and regulate immune signaling via mediating CPK28 degradation. Our work reveals how CPK28 is kept in check to maintain BIK1 homeostasis and to enhance immune responses.