Modulation of Plant Salicylic Acid-Associated Immune Responses via Glycosylation of Dihydroxybenzoic Acids.

Salicylic acid (SA) plays a crucial role in plant innate immunity. The deployment of SA-associated immune responses is primarily affected by SA concentration, which is determined by a balance between SA biosynthesis and catabolism. However, the mechanisms regulating SA homeostasis are poorly understood. In this study, we characterized a unique UDP-glycosyltransferase, UGT76D1, which plays an important role in SA homeostasis and associated immune responses in Arabidopsis (). Expression of was induced by treatment with both the pathogen pv. () DC3000 and SA. Overexpression of resulted in high SA accumulation, significant up-regulation of pathogen-related genes, and a hypersensitive response (HR)-like lesion mimic phenotype. This HR-like phenotype was not observed following overexpression in SA-deficient transgenic or plants, suggesting that the phenotype is SA dependent. Biochemical assays showed that UGT76D1 glycosylated 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA), the major catabolic forms of SA, to their Glc and Xyl conjugates in vitro and in vivo. Moreover, in a mutant background blocked in the formation of 2,3-DHBA and 2,5-DHBA, overexpression did not cause a HR-like lesion mimic phenotype. Following infection with DC3000, knockout mutants displayed a delayed immune response, with reduced levels of DHBA glycosides and SA, and down-regulated SA synthase expression. By contrast, overexpression lines showed an enhanced immune response and increased SA biosynthesis before and after pathogen infection. Thus, we propose that UGT76D1 plays an important role in SA homeostasis and plant immune responses by facilitating glycosylation of dihydroxybenzoic acids.