PBS3 protects EDS1 from proteasome-mediated degradation in plant immunity.

Plant immunity is mediated by the positive regulators PBS3 and EDS1 and the negative regulators NPR3 and NPR4. However, the relationships among these important immune regulators remain elusive. Here, we show that PBS3 interacts with EDS1 in both the cytoplasm and the nucleus and is required for EDS1 protein accumulation. NPR3 and NPR4, which function as SA receptors and adaptors of Cullin3-based E3 ligase, interact with and mediate the degradation of EDS1 via the 26S proteasome. We prove that PBS3 inhibits the poly-ubiquitination and subsequent degradation of EDS1 by reducing the association of EDS1 with the Cullin3 adaptors NPR3 and NPR4. Furthermore, we show that PBS3 and EDS1 also contribute to PAMP-triggered immunity (PTI) in addition to effector-triggered immunity (ETI). Our study reveals a novel mechanism, in which plants fine-tune defense responses by inhibiting the degradation of a positive player in plant immunity.