Abstract
UDP-glycosyltransferases (UGTs) constitute the largest glycosyltransferase family in the plant kingdom, regulating many metabolic processes by transferring sugar moieties onto various small molecules. However, their physiological significance in plants remains largely unknown. Here, we reveal the functions and mechanisms of two Arabidopsis UGT genes, UGT73C3 and UGT73C4, which are strongly induced by Pseudomonas syringae pv. tomato (Pst) DC3000. Overexpression of these genes significantly enhanced plant immune response, whereas their loss of function in double mutants led to increased sensitivity to pathogen infections. However, single mutants showed no obvious alteration in pathogen resistance. To further investigate the regulatory mechanisms of UGT73C3/C4 in plant immunity, we conducted comprehensive secondary metabolome analyses and glycoside quantification. Overexpression lines accumulated higher levels of pinoresinol diglucosides than wild-type plants, both before and after Pst DC3000 treatment, whereas double mutants accumulated lower levels. Furthermore, in vitro and in vivo experiments demonstrated that UGT73C3 and UGT73C4 can glycosylate pinoresinol to form pinoresinol monoglucoside and diglucoside. Moreover, pinoresinol glycosylation promotes the plant immune response by increasing reactive oxygen species production and callose deposition. Additionally, the transcription factor HB34 was found to activate UGT73C3 and UGT73C4 transcription and play a key role in plant immunity. Overall, this study reveals a novel pathway in which UGT73C3/C4-mediated pinoresinol glycosylation, regulated by HB34, enhances the plant immune response.