Differential ubiquitin modifications of ESCRT ‐I subunits VPS37 ‐1 and VPS28 ‐2 positively regulate plant immunity

Sensing of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors at the cell surface initiates the first layer of host immunity against invading microbial pathogens. In plants, the immune receptor Flagellin Sensing 2 (FLS2) perceives bacterial flagellin to trigger downstream signaling that culminates in PAMP-triggered immunity (PTI). FLS2 protein levels are critical for effective PTI, yet the regulatory mechanisms governing its stability remain to be fully elucidated. Our previous work showed that the Arabidopsis Really Interesting Gene (RING) type ubiquitin ligase XBAT35.2 positively regulates plant immunity against bacterial pathogen Pseudomonas syringae pv tomato by promoting FLS2 stability. In this study, we demonstrate that XBAT35.2 interacts in vivo with two key ESCRT-I complex components, VPS37-1 and VPS28-2. XBAT35.2 ubiquitinates VPS37-1 with K48-linked polyubiquitin chains, leading to its proteasomal degradation, and modifies VPS28-2 with K63-linked chains, reducing its interaction with FLS2. VPS37-1 and VPS28-2 play redundant, negative roles in FLS2-mediated immunity by promoting vacuolar degradation of FLS2, displaying differential effects on distinct FLS2-mediated immune responses. By targeting VPS37-1 and VPS28-2, XBAT35.2 intercepts this degradative pathway, thereby stabilizing FLS2 and enhancing host immunity. These findings uncover a novel regulatory circuit modulating FLS2 abundance and deepen our understanding of controlling the cell-surface receptor homeostasis in plant immunity.