The Ubiquitin Ligase PUB22 Targets a Subunit of the Exocyst Complex Required for PAMP-Triggered Responses in Arabidopsis

Plant pathogens are perceived by pattern recognition receptors, which are activated upon binding to pathogen-associated molecular patterns (PAMPs). Ubiquitination and vesicle trafficking have been linked to the regulation of immune signaling. However, little information exists about components of vesicle trafficking involved in immune signaling and the mechanisms that regulate them. In this study, we identified Arabidopsis thaliana Exo70B2, a subunit of the exocyst complex that mediates vesicle tethering during exocytosis, as a target of the plant U-box-type ubiquitin ligase 22 (PUB22), which acts in concert with PUB23 and PUB24 as a negative regulator of PAMP-triggered responses. We show that Exo70B2 is required for both immediate and later responses triggered by all tested PAMPs, suggestive of a role in signaling. Exo70B2 is also necessary for the immune response against different pathogens. Our data demonstrate that PUB22 mediates the ubiquitination and degradation of Exo70B2 via the 26S Proteasome. Furthermore, degradation is regulated by the autocatalytic turnover of PUB22, which is stabilized upon PAMP perception. We therefore propose a mechanism by which PUB22-mediated degradation of Exo70B2 contributes to the attenuation of PAMP-induced signaling.