Characterization of Two Key Flavonoid 3-O-Glycosyltransferases Involved in the Formation of Flower Color in Rhododendron Delavayi

Identification and Functional Characterization of Two UDP-Glycosyltransferases Involved in Narcissoside Biosynthesis in Anoectochilus roxburghii

Flavonoid rutinosides, a group of bioactive compounds in Anoectochilus roxburghii, contribute greatly to the plant's beneficial effects on human health. However, the glycosylation mechanism of flavonoid rutinosides in A. roxburghii remains unclear. In this study, two efficient and selective glycosyltransferases, AUTG25 and AUTG23, involved in the biosynthesis of narcissoside, a major flavonoid rutinoside in A. roxburghii, were identified through transcriptome analysis and functional validation. AUTG25 could regioselectively catalyze 3-O-glucosylation of isorhamnetin to produce isorhamnetin 3-O-glucoside, while AUTG23 could further catalyze 6"-O-rhamnosylation to generate narcissoside. Both AUTG25 and AUTG23 exhibited high positional and sugar donor selectivities in the catalytic reaction. Homology modeling and site-directed mutagenesis showed that H20, E83, E385, and F143 in AUTG25 and E280, E89, D188, W327, D369, and Y191 in AUTG23 may be critical for their catalytic functions. Transient expression in Nicotiana benthamiana finally confirmed that AUTG25 possesses flavonol-3-O-glucosyltransferase activity and AUTG23 has flavonol-3-O-glucoside (1→6) rhamnosyltransferase activity. This study clarified and provided candidate UDP-dependent glycosyltransferase genes for narcissoside biosynthesis in A. roxburghii.

Identification of the UGT Family and Functional Validation of MwUGT2 in Meconopsis wilsonii

Flower color is one of the most ornamental values of Meconopsis wilsonii, but very limited studies have been reported on its flower color formation. The UDP-glycosyltransferase (UGT) gene family plays a crucial role in plant flower color formation. In this study, the full-length transcriptome data of M. wilsonii was used to identify MwUGTs, focusing on protein physicochemical properties' subcellular localization, and phylogenetic relationships. In addition, sequence analysis, expression pattern analysis, subcellular localization, and functional validation of MwUGT2 were also performed. A total of 26 MwUGTs were identified in full-length transcriptome and clustered into eight subgroups. Phylogenetic analysis and KEGG database annotation showed that MwUGT2 is associated with anthocyanin synthesis and accumulation. Subsequently, based on the expression of MwUGT2 during flower development and in different tissues, it was preliminarily determined that MwUGT2 plays a role in the flower bud stage. Subcellular localization assays suggested that MwUGT2 is present in the nucleus and cytoplasm. Overexpression in Nicotiana tabacum showed that MwUGT2 significantly increased the content of Cyanidin-3-O-glucoside and resulted in dark pink flowers in transgenic plants. In summary, our findings suggest that MwUGT2 plays a crucial role in the biosynthesis of anthocyanin and will also contribute to understanding the mechanisms of flower color formation in M. wilsonii.

Chalcone-Synthase-Encoding RdCHS1 Is Involved in Flavonoid Biosynthesis in Rhododendron delavayi

Flower color is an important ornamental feature that is often modulated by the contents of flavonoids. Chalcone synthase is the first key enzyme in the biosynthesis of flavonoids, but little is known about the role of R. delavayi CHS in flavonoid biosynthesis. In this paper, three CHS genes (RdCHS1-3) were successfully cloned from R. delavayi flowers. According to multiple sequence alignment and a phylogenetic analysis, only RdCHS1 contained all the highly conserved and important residues, which was classified into the cluster of bona fide CHSs. RdCHS1 was then subjected to further functional analysis. Real-time PCR analysis revealed that the transcripts of RdCHS1 were the highest in the leaves and lowest in the roots; this did not match the anthocyanin accumulation patterns during flower development. Biochemical characterization displayed that RdCHS1 could catalyze p-coumaroyl-CoA and malonyl-CoA molecules to produce naringenin chalcone. The physiological function of RdCHS1 was checked in Arabidopsis mutants and tobacco, and the results showed that RdCHS1 transgenes could recover the color phenotypes of the tt4 mutant and caused the tobacco flower color to change from pink to dark pink through modulating the expressions of endogenous structural and regulatory genes in the tobacco. All these results demonstrate that RdCHS1 fulfills the function of a bona fide CHS and contributes to flavonoid biosynthesis in R. delavayi.

Genome-Wide Analysis of MYB Transcription Factors and Screening of MYBs Involved in the Red Color Formation in Rhododendron delavayi

Flower color is one of the crucial traits of ornamental plants. Rhododendron delavayi Franch. is a famous ornamental plant species distributed in the mountain areas of Southwest China. This plant has red inflorescence and young branchlets. However, the molecular basis of the color formation of R. delavayi is unclear. In this study, 184 MYB genes were identified based on the released genome of R. delavayi. These genes included 78 1R-MYB, 101 R2R3-MYB, 4 3R-MYB, and 1 4R-MYB. The MYBs were divided into 35 subgroups using phylogenetic analysis of the MYBs of Arabidopsis thaliana. The members of the same subgroup in R. delavayi had similar conserved domains and motifs, gene structures, and promoter cis-acting elements, which indicate their relatively conserved function. In addition, transcriptome based on unique molecular identifier strategy and color difference of the spotted petals, unspotted petals, spotted throat, unspotted throat, and branchlet cortex were detected. Results showed significant differences in the expression levels of R2R3-MYB genes. Weighted co-expression network analysis between transcriptome and chromatic aberration values of five types of red samples showed that the MYBs were the most important TFs involved in the color formation, of which seven were R2R3-MYB, and three were 1R-MYB. Two R2R3-MYB (DUH019226.1 and DUH019400.1) had the highest connectivity in the whole regulation network, and they were identified as hub genes for red color formation. These two MYB hub genes provide references for the study of transcriptional regulation of the red color formation of R. delavayi.