Bacterial effector modulation of host E3 ligase activity suppresses PAMP-triggered immunity in rice

Abstract

Pathogen effector proteins are delivered to host cells to suppress plant immunity. However, the mechanisms by which effector proteins function are largely unknown. Here we show that expression of XopPXoo, an effector of rice pathogen Xanthomonas oryzae pv. oryzae, in rice strongly suppresses peptidoglycan (PGN)- and chitin-triggered immunity and resistance to X. oryzae. XopPXoo targets OsPUB44, a rice ubiquitin E3 ligase with a unique U-box domain. We find that XopPXoo directly interacts with the OsPUB44 U-box domain and inhibits ligase activity. Two amino-acid residues specific for the OsPUB44 U-box domain are identified, which are responsible for the interaction with XopPXoo. Silencing of OsPUB44 suppresses PGN- and chitin-triggered immunity and X. oryzae resistance, indicating that OsPUB44 positively regulates immune responses. Thus, it is likely that XopPXoo suppresses immune responses by directly interacting with and inhibiting a positive regulator of plant immunity.