Identification and Characterization of CtUGT3 as the Key Player of Astragalin Biosynthesis in Carthamus tinctorius L.

Abstract

Safflower (Carthamus tinctorius L.) is a multipurpose economic crop that is distributed worldwide. Flavonoid glycosides are the main bioactive components in safflower, but only a few UDP-glycosyltransferases (UGT) have been identified. Three differentially expressed UGT genes related with the accumulation of 9 flavonoid O-glycosides were screened from metabolomics and transcriptome analysis. Safflower corolla protoplasts were used to confirm the glycosylation ability of UGT candidates in vivo for the first time. The astragalin content was significantly increased only when CtUGT3 was overexpressed. CtUGT3 also showed flavonoid 3-OH and 7-OH glycosylation activities in vitro. Molecular modeling and site-directed mutagenesis revealed that G15, T136, S276, and E384 were critical catalytic residues for the glycosylation ability of CtUGT3. These results demonstrate that CtUGT3 has a flavonoid 3-OH glycosylation function and is involved in the biosynthesis of astragalin in safflower. This study provides a reference for flavonoid biosynthesis genes research in nonmodel plants.

Highly Promiscuous Flavonoid Di-O-glycosyltransferases from Carthamus tinctorius L.

“…Studies have shown that CtUGT3 catalyzes the glucosylation of flavones (apigenin), flavanones (naringenin), flavonols (kaempferol, quercetin, and isorhamnetin), and chalcones (naringenin chalcone), forming their respective O-glucosides. Furthermore, when flavonols were used as aglycones in vitro, CtUGT3 exhibited both 7-OH and 3-OH glycosylation activities simultaneously [24]. The most abundant flavonoid glycosides in plants are flavone Oor C-glycosides and flavonol O-glycosides, with glycosides at the 3-OH or 7-OH positions being the most common.…”

“…Ren et al demonstrated the glycosylation function of candidate UGTs using safflower corolla protoplasts in vivo for the first time. Overexpression of CtUGT3 resulted in a significant increase in the content of kaempferol-3-O-glucoside, indicating its glycosylation activity towards flavonoids with 3-OH and 7-OH moieties [24]. Qi et al provided preliminary evidence for the function of CtUGT4 as a flavonoid-O-glycosyltransferase in safflower by demonstrating that its overexpression led to a noteworthy increase in the content of quercetin-3-O-rutinoside and a trend of increased quercetin-3-O-glycoside content [25].…”