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Flavonols are a class of flavonoids that play a crucial role in regulating plant growth and promoting stress resistance. They are also important dietary components in horticultural crops due to their benefits for human health. In past decades, research on the transcriptional regulation of flavonol biosynthesis in plants has increased rapidly. This review summarizes recent progress in flavonol-specific transcriptional regulation in plants, encompassing characterization of different categories of transcription factors (TFs) and microRNAs as well as elucidation of different transcriptional mechanisms, including direct and cascade transcriptional regulation. Direct transcriptional regulation involves TFs, such as MYB, AP2/ERF, and WRKY, which can directly target the key flavonol synthasegene or other early genes in flavonoid biosynthesis. In addition, different regulation modules in cascade transcriptional regulation involve microRNAs targeting TFs, regulation between activators, interaction between activators and repressors, and degradation of activators or repressors induced by UV-B light or plant hormones. Such sophisticated regulation of the flavonol biosynthetic pathway in response to UV-B radiation or hormones may allow plants to fine-tune flavonol homeostasis, thereby balancing plant growth and stress responses in a timely manner. Based on orchestrated regulation, molecular design strategies will be applied to breed horticultural crops with excellent health-promoting effects and high resistance.

1.Overexpression of CmPR4Aimproved resistance to Xanthomonas citrisubsp. citri(Xcc) in sweet orange and resistance to Pseudomonas syringaepv. tomatoDC3000 in Arabidopsis.2.The major effector pthA4 secreted by Xccinteracts with CmWRKY75, a positive regulator of CmPR4A, and up-regulates CmPR4Aexpression in Citron C-05 leaves.3.CmSMU2 interacts with CmPR4A to increase ROS accumulation, thereby enhancing resistance to Xccin citrus.

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, PpMYB15and 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 PpMYB15and PpMYBF1transcript levels correlated well with the flavonol content and the expression of flavonol synthase (PpFLS1) in different fruit samples. Dual-luciferase assays indicated that both PpMYB15and PpMYBF1could 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 PpMYB15and PpMYBF1are functional flavonol-specific positive regulators in peach fruit and are important candidates for biotechnological engineering flavonol biosynthesis in plants.

Cultivated hawthorn (Crataegus pinnatifidavar. major), the principal cultivated hawthorn species in China, is rich in phenolic compounds. These phytochemicals are considered essential bioactive ingredients in different organs of cultivated hawthorn, contributing to its health effects. However, there is lack of a comprehensive review that systematically summarizes the chemistry, health effects, and stability of phenolic substances. This review summarizes recent progress in the characterization, classification, and distribution of phenolic compounds in C. pinnatifidavar. major, including procyanidins, flavonoids, phenolic acids, lignans, and others. Importantly, the stability of these compounds was found to be influenced by factors such as the hawthorn development process, postharvest storage, postharvest processing, and the gastrointestinal tract environment. Additionally, the health effects of phenolic compounds in cultivated hawthorn and their underlying mechanisms were discussed. This review provides valuable insights into the utilization of hawthorn resources and the development of the hawthorn industry.

Flavonol glycosides are health-promoting phytochemicals important for human nutrition and plant defense against environmental stresses. Glycosylation modification greatly enriches the diversity of flavonols. Morella rubra, a member of the Myricaceae, contains high amounts of myricetin 3-O-rhamnoside (M3Rha), quercetin 3-O-rhamnoside (Q3Rha), and quercetin 3-O-galactoside (Q3Gal). In the present study, MrUGT78R1 and MrUGT78R2 were identified as two functional UDP-rhamnosyltransferases, while MrUGT78W1 was identified as a UDP-galactosyltransferase. Site-directed mutagenesis identified Pro143 and Asn386 as important residues for rhamnosyl transfer activity of MrUGT78R1, while the two corresponding positions in MrUGT78W1 (i.e. Ser147 and Asn370) also play important roles in galactosyl transfer activity. Transient expression data for these three MrUGTs in Nicotiana benthamianatested the function of MrUGT78R1 and MrUGT78R2 as rhamnosyltransferases and MrUGT78W1 as a galactosyltransferase in glycosylation of flavonols. This work enriches knowledge of the diversity of UDP-rhamnosyltransferase in plantaand identifies two amino acid positions important for both rhamnosyltransferase and galactosyltransferase.