Release of a ubiquitin brake activates OsCERK1-triggered immunity in rice.

Abstract

Plant pattern-recognition receptors perceive microorganism-associated molecular patterns to activate immune signalling^1,2. Activation of the pattern-recognition receptor kinase CERK1 is essential for immunity, but tight inhibition of receptor kinases in the absence of pathogen is crucial to prevent autoimmunity^3,4. Here we find that the U-box ubiquitin E3 ligase OsCIE1 acts as a molecular brake to inhibit OsCERK1 in rice. During homeostasis, OsCIE1 ubiquitinates OsCERK1, reducing its kinase activity. In the presence of the microorganism-associated molecular pattern chitin, active OsCERK1 phosphorylates OsCIE1 and blocks its E3 ligase activity, thus releasing the brake and promoting immunity. Phosphorylation of a serine within the U-box of OsCIE1 prevents its interaction with E2 ubiquitin-conjugating enzymes and serves as a phosphorylation switch. This phosphorylation site is conserved in E3 ligases from plants to animals. Our work identifies a ligand-released brake that enables dynamic immune regulation.