UGT76B1 and 41 Additional Arabidopsis UDP-Glycosyltransferases Show No Detectable In Vitro Glycosylation Activity Toward N-Hydroxypipecolic Acid

N-hydroxypipecolic acid (NHP) is a key mobile signal in systemic acquired resistance in plants, and its glycosylation has been proposed to regulate immune signaling. Previous studies have demonstrated that the UDP-glycosyltransferase UGT76B1, known as an SA glycosyltransferase in Arabidopsis thaliana, also catalyzes NHP glycosylation. In this study, we re-evaluated NHP glycosylation activity of UGT76B1 using an in vitro enzyme-coupled fluorescence assay that quantitatively detects UDP released during UDP-sugar-dependent glycosylation. Unexpectedly, our biochemical analyses demonstrated that UGT76B1 lacks genuine glycosylation activity toward NHP under the in vitro assay conditions tested, although this system clearly detected UGT76B1 activity toward salicylic acid (SA), as well as the activities of UGT74F1 and UGT72B1 toward SA and hydroquinone, respectively. To explore potential UGTs responsible for NHP glycosylation, we evaluated the enzymatic activities of 41 UGT candidates successfully expressed in Escherichia coli, which are selected based on transcriptomic responses to tenoxicam treatment, molecular docking simulations using AlphaFold3/AutoDock Vina, phylogenetic criteria, and previous reports. Within this selected and successfully expressed UGT panel, none exhibited authentic NHP glycosylation activity, although this does not preclude the possibility that other members of the Arabidopsis UGT family possess NHP glycosyltransferase activity. Our findings challenge the prevailing view that UGT76B1 is the primary glycosyltransferase for NHP in A. thaliana and indicate that NHP metabolism may rely on undiscovered non-canonical enzymes or distinct metabolic pathways that warrant further investigation.