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UDP- l -rhamnose (UDP-Rha) is an important sugar donor for glycosylation of various cell molecules in plant. Rhamnosides are widely present in different plant tissues and play important biological roles under different developmental or environmental conditions. However, enzymes involved in UDP-Rha biosynthesis and their encoding genes have been identified in few plants, which limits the functional analysis of plant rhamnosides. Here, two UDP-Rha biosynthesis genes, named PpRHM1 (2028 bp) and PpRHM2 (2016 bp), were isolated and characterized from Prunus persica, which is rich sources of flavonol rhamnosides. Both recombinant RHM proteins can catalyze the transformation from UDP- d -glucose (UDP-Glc) to UDP-Rha, which was confirmed by LC-MS and formation of flavonol rhamnosides. Biochemical analysis showed that both recombinant RHM proteins preferred alkaline conditions in pH range of 8.0–9.0 and had optimal reaction temperature between 25 and 30 °C. PpRHM1 showed the better UDP-Glc substrate affinity with Km of 360.01 μM. Gene expression analysis showed different transcript levels of both RHMs in all plant tissues tested, indicating the involvement of rhamnosides in various tissues in plant. Such results provide better understanding of UDP-Rha biosynthesis in fruit tree and may be helpful for further investigation of various rhamnose derivatives and their biological functions.

Flavonoids are major polyphenol compounds in plants and contribute substantially to the health-promoting benefits of fruit and vegetables. Peach is rich in polyphenols with flavonols as the main flavonoids. To investigate the regulation of flavonol biosynthesis in peach fruit, two R2R3-MYB transcription factor (TF) genes, PpMYB15 and PpMYBF1, were isolated and characterized. Sequence analysis revealed that the PpMYB15 and PpMYBF1 proteins are members of the flavonol clade of the R2R3-MYB family. Real-time quantitative PCR analysis showed that PpMYB15 and PpMYBF1 transcript levels correlated well with the flavonol content and the expression of flavonol synthase ( PpFLS1) in different fruit samples. Dual-luciferase assays indicated that both PpMYB15 and PpMYBF1 could trans-activate promoters of flavonoid biosynthesis genes, including chalcone synthase ( PpCHS1), chalcone isomerase ( PpCHI1), flavanone 3-hydroxylase ( PpF3H), and PpFLS1. Transient overexpression of 35S::PpMYB15 or 35S::PpMYBF1 both triggered flavonol biosynthesis but not anthocyanin and proanthocyanidin biosynthesis in tobacco leaves. In transgenic tobacco flowers, overexpression of 35S::PpMYB15 or 35S::PpMYBF1 caused a significant increase in flavonol levels and significantly reduced anthocyanin accumulation, resulting in pale-pink or pure white flowers. These results suggest that PpMYB15 and PpMYBF1 are functional flavonol-specific positive regulators in peach fruit and are important candidates for biotechnological engineering flavonol biosynthesis in plants.

Apple is rich in flavonol glycosides, which are believed to contribute to putative health benefits associated with apple consumption. Glycosylation, catalyzed by uridine diphospho-glycosyltransferases (UGTs), is the last step in flavonol biosynthesis, which confers molecular stability and solubility to the flavonol. In the present study, the involvement of two UGTs, MdUGT75B1 and MdUGT71B1, in flavonol biosynthesis in apple was investigated. The major flavonols are quercetin 3-O-glycosides, and UV-B and blue light treatment significantly enhanced the accumulation of quercetin 3-O-galactoside, quercetin 3-O-glucoside, and kaempferol 3-O-galactoside. Transcript levels of MdUGT75B1 and MdUGT71B1 in fruit subjected to different treatments were correlated well with flavonol accumulation. MdUGT75B1 showed flavonol-specific activity with a preference for UDP-galactose as the sugar donor, while MdUGT71B1 using UDP-glucose exhibited a wider substrate acceptance. Thus, MdUGT75B1 and MdUGT71B1 are key UGTs involved in flavonol biosynthesis and may have important roles in regulating accumulation of these health-promoting bioactive compounds in apple.

Bergamottin is a natural furanocoumarin compound with weak polarity. Characterization and quantification of bergamottin were carried out in different fruit tissues of various citrus cultivars. Among the four citrus tissues tested, i.e., flavedo, albedo, segment membrane (SM), and juice sacs (JS) in eight citrus cultivars, the highest bergamottin content was found in the flavedo of Citrus grandis (L.) Osbeck cv. Yongjiazaoxiangyou (YJZXY, 666.54 μg·g−1 DW). A combination of silica gel column chromatography and high-speed counter-current chromatography (HSCCC) was established to efficiently purify bergamottin from the flavedo of YJZXY. Bergamottin showed significant antiproliferative activity on three cancer cell lines, i.e., human liver cancer HepG2, promyelocytic leukemia HL-60, and gastric cancer BGC-823 cells, which showed a marked inhibition effect on these cell lines in a dose-dependent manner. In addition, bergamottin significantly increased glucose consumption in HepG2 cells also in a dose-dependent manner, which is the first report of its potential in anti-diabetes applications.