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

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.