UGT76B1, a promiscuous hub of small molecule-based immune signaling, glucosylates N-hydroxypipecolic acid and controls basal pathogen defense

ABSTRACT Glucosylation modulates the biological activity of small molecules and frequently leads to their inactivation. The Arabidopsis thaliana glucosyltransferase UGT76B1 is involved in conjugating the stress hormone salicylic acid (SA) as well as isoleucic acid (ILA). Here, we show that UGT76B1 also glucosylates N-hydroxypipecolic acid (NHP), which is synthesized by FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) and activates systemic acquired resistance (SAR). Upon pathogen attack, Arabidopsis leaves accumulate two distinct NHP hexose conjugates, NHP-O-β-glucoside and NHP glucose ester, which are oppositely regulated by SA. ugt76b1 mutants specifically fail to generate the NHP-O-β-glucoside, and recombinant UGT76B1 synthesizes NHP-O-β-glucoside in vitro in competition with SA and ILA. The loss of UGT76B1 elevates the endogenous levels of NHP in addition to SA and ILA and establishes a SAR-like, primed immune status without pathogen infestation. The introgression of the fmo1 background lacking NHP biosynthesis into ugt76b1 abolishes the SAR-like resistance phenotype indicating an important function of UGT76B1-mediated NHP glucosylation in balancing the defense status. Our results further indicate that ILA promotes and SA finally executes the NHP-triggered immunity via the glucosyltransferase UGT76B1 as the common metabolic hub. Thus, UGT76B1 controls the levels of active NHP, SA, and ILA in concert to modulate plant immune signaling.