Glucosylation of the signal molecules salicylic acid and N-hydroxypipecolic acid

Xu, Ping (2025): Glucosylation of the signal molecules salicylic acid and N-hydroxypipecolic acid: a complex switch to regulate stress response and pathogen defense. Dissertation, LMU München: Fakultät für Chemie und Pharmazie

Abstract

Salicylic acid (SA) plays a pivotal role in activating plant defense responses against both abiotic and biotic stresses. In Arabidopsis thaliana, the glucosyltransferase UGT76B1 inactivates SA alongside other immune modulators, N-hydroxypipecolic acid (NHP) and isoleucic acid (ILA). Soil-borne microbes can trigger systemic shoot resistance through jasmonic acid– and/or SA-dependent pathways, but the identity of root-derived signals remains unclear. UGT76B1 plays a pivotal role in mediating systemic acquired resistance (SAR) in leaves, while the relevance of its constitutive expression in roots remained enigmatic. This study identified a root-triggered systemic resistance (RSR) mechanism that relies on components of the SAR machinery known from leaves. Unlike the inducible nature of SAR, FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) constitutively produces NHP in roots, while UGT76B1 immobilizes NHP via conjugation. Loss of UGT76B1 in roots increases NHP release, activating shoot defenses. This tightly regulated FMO1/UGT76B1 circuit is modulated by root-associated microbes. Notably, endophytic and (hemi)biotrophic fungi induce UGT76B1 degradation and FMO1 expression, leading to variable NHP release, which differentially regulates shoot defense and growth. Additionally, UGT74F1 and UGT74F2 glucosylate SA, with UGT74F2 being the sole enzyme forming SA glucose esters. UGT76B1, UGT74F1, and UGT74F2 are the principal SA glucosyltransferases (SA-GTs) of Arabidopsis since the triple mutant no longer accumulated SA glucosides. A previous study systematically investigated the roles of three SA-GTs by analyzing SA metabolite levels and transcriptomic profiles of combinatorial loss-of-function mutants. However, their collaborative or independent functions remained unclear. By characterizing responses to both abiotic and biotic stresses, this thesis revealed their unique and overlapping roles. The three SA-GTs exhibit differential, partially overlapping expression patterns. The SA-O-glucoside-forming UGT74F1 and UGT76B1 cannot compensate each other due to differences in their substrate specificities and cellular expression. Transcriptomic analyses and stress-response phenotypes further distinguish their roles: UGT76B1 is central to immune regulation, whereas UGT74F1 and UGT74F2 are primarily involved in abiotic stress responses. Nevertheless, all three enzymes contribute to defense against bacterial pathogens and to tolerance of drought, salt, osmotic, and cold stress. Beyond SA, these enzymes may also act on other substrates. An in silico modeling pipeline was used to predict additional substrates, and these predictions were experimentally validated. Neither UGT74F1 nor UGT74F2 glucosylates NHP or ILA, while nicotinic acid, another UGT74F2 substrate, is not conjugated by UGT74F1 or UGT76B1. Similarly, UGT76B1 does not glucosylate anthranilic acid, a substrate of UGT74F1 and UGT74F2. These findings highlight the substrate specificity of SA-GTs and their distinct biochemical roles.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: NHP, SA, SAR, ISR, RSR, endophytes, (hemi)biotrophs, Pst, growth, defense
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 540 Chemie
• Fakultäten: Fakultät für Chemie und Pharmazie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 27. März 2025
• 1. Bericht­erstatter:in: Schäffner, Anton