Modifications of Two ESCRT-I Subunits with Distinct Ubiquitin Chains Regulate Plant Immunity

Sensing of pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) by pattern recognition receptors (PRRs) at the cell surface induces the first layer of host immunity against invading microbial pathogens. The immune receptor FLS2 perceives bacterial flagellin to initiate host immune signaling upon pathogen infections. It has been known that the FLS2 abundance is crucial for plant pattern-triggered immunity. Nevertheless, the underpinning regulatory mechanisms remain largely unclear. In this study, we demonstrate that XBAT35.2 positively modulates the protein level of FLS2. In addition to the Golgi, XBAT35.2 localizes at the plasma membrane and constitutively associates with FLS2, BAK1, and BIK1. Flg22 treatment increases the association of XBAT35.2 with FLS2 and BAK1 but reduces the interaction with BIK1. XBAT35.2 ubiquitinates two key components of the ESCRT-I complex, VPS37-1 and VPS28-2 with K48 and K63-linked polyubiquitin chains respectively, leading to degradation of VPS37-1 and diminished interaction of VPS28-2 with FLS2. Additionally, VPS37-1 and VPS28-2 play redundant, negative roles in FLS2-mediated immunity by promoting vacuolar breakdown of FLS2. Thus, by intercepting the function of VPS28-2 and VPS37-1, XBAT35.2 stabilizes FLS2 for host immunity. Our findings reveal a new regulatory circuit for modulating the FLS2 abundance and deepen our understanding of controlling the homeostasis of cell surface receptors. One-sentence summaryTargeting the ESCRT-I subunits VPS28-2 and VPS37-1 with distinct ubiquitin chains by the E3 ligase XBT35.2 stabilizes FLS2 and positively modulates plant immunity.