AvrPtoB targets the LysM receptor kinase CERK1 to promote bacterial virulence on plants.

Plant innate immunity relies on a set of pattern recognition receptors (PRRs) that respond to ligands known as pathogen-associated molecular patterns (PAMPs). To overcome such immunity, phytopathogenic bacteria deliver virulence molecules called effector proteins into the plant cell that collectively promote pathogenesis. The vast majority of PRRs controlling PAMP-triggered immunity (PTI) and the mechanisms used by specific effectors to suppress these pathways are mostly unknown. Here, we show that the Arabidopsis LysM receptor kinase CERK1, which is critical for chitin elicitor signaling and resistance to fungal pathogens, plays an essential role in restricting bacterial growth on plants. This is supported by the fact that CERK1 is a target of the bacterial type III effector protein AvrPtoB, which blocks all defense responses through this receptor. AvrPtoB ubiquitinates the CERK1 kinase domain in vitro and targets CERK1 for degradation in vivo. We show that CERK1 is a determinant of bacterial immunity, but its contribution is overcome by bacteria expressing AvrPtoB. Our results reveal a new pathway for plant immunity against bacteria and a role for AvrPtoB E3-ligase activity in suppressing PTI.