PUB4 Activates Chitin Signaling by Ubiquitination-linked Degradation of The Immune Suppressor FERONIA

Ubiquitination regulates pattern-triggered immune responses in plants, but the underlying mechanisms remain incompletely understood. In Arabidopsis, the E3 ubiquitin ligase PUB4 has been implicated in growth, development, and stress responses, and is known to interact with and be phosphorylated by CERK1, resulting in the positive regulation of chitin-triggered immunity. In this study, we identified ubiquitinated proteins enriched from chitin-treated microsomal fractions and discovered the receptor kinase FERONIA as a potential target of PUB4. FERONIA interacted with PUB4 both in vivo and in vitro, as evidenced by yeast two-hybrid, BiFC, and co-immunoprecipitation assays. An in vitro ubiquitination assay demonstrated that PUB4 ubiquitinates the cytoplasmic domain of FERONIA. Moreover, transient expression assays in wild-type and pub4-2 mutant protoplasts revealed that PUB4 mediates chitin-induced FERONIA degradation. Unlike PUB4, which positively regulates chitin-induced defense responses, FERONIA negatively regulates them. For example, fer4 mutants exhibited elevated basal and chitin-induced ROS and callose levels. This suggests that FERONIA suppresses immune signaling under steady-state conditions, and that PUB4-mediated degradation of FERONIA relieves this inhibition. These findings clearly reveal a novel mechanism by which PUB4 regulates chitin-induced immunity driven by CERK1 through targeted degradation of FERONIA. This highlights the balance between positive and negative regulators of plant immune signaling.