Abstract
Flavonols are one of the largest groups of flavonoids that confer benefits for the health of plants and animals. Flavonol glycosides are the predominant flavonoids present in the model legume Lotus japonicus. The molecular mechanisms underlying the biosynthesis of flavonol glycosides as yet remain unknown in L. japonicus. In the present study, we identified a total of 188 UDP-glycosyltransferases (UGTs) in L. japonicus by genome-wide searching. Notably, 12 UGTs from the UGT72 family were distributed widely among L. japonicus chromosomes, expressed in all tissues, and showed different docking scores in an in silico bioinformatics docking analysis. Further enzymatic assays showed that five recombinant UGTs (UGT72AD1, UGT72AF1, UGT72AH1, UGT72V3, and UGT72Z2) exhibit activity toward flavonol, flavone, and isoflavone aglycones. In particular, UGT72AD1, UGT72AH1, and UGT72Z2 are flavonol-specific UGTs with different kinetic properties. In addition, the overexpression of UGT72AD1 and UGT72Z2 led to increased accumulation of flavonol rhamnosides in L. japonicus and Arabidopsis thaliana. Moreover, the increase of kaempferol 3-O-rhamnoside-7-O-rhamnoside in transgenic A. thaliana inhibited root growth as compared with the wild-type control. These results highlight the significance of the UGT72 family in flavonol glycosylation and the role of flavonol rhamnosides in plant growth.
Highlights
Flavonoids, including flavonols, flavones, anthocyanins, isoflavones, and proanthocyanidins, are multifunctional polyphenolic compounds with important implications for plant, animal, and human health (Dixon & Steele, 1999)
To identify putative UGT genes in the L. japonicus genome, an implementation of the BLAST algorithm was used to search the whole genome sequence; 120 UGTs from A. thaliana were used as queries in this analysis
To evaluate whether UGT72AD1 and UGT72Z2 were functional in other plant species, the genes encoding these two proteins were ectopically expressed in A. thaliana, a species that is abundant in flavonols
Summary
Flavonoids, including flavonols, flavones, anthocyanins, isoflavones, and proanthocyanidins, are multifunctional polyphenolic compounds with important implications for plant, animal, and human health (Dixon & Steele, 1999). Flavonoids have a wide variety of bioactivities in human beings, including retarding aging, inhibiting coronary disease, and coordinating hormone levels in vivo (Hou, 2003; Kim et al, 2006; Yu et al, 2013). Because of these properties, the bioactivities and potential applications of flavonoids have drawn a great deal of research and commercial attention in recent years. A number of putative UGT genes have been identified in the genomes of many plant species, the number of UGTs that have been functionally characterized at both the biochemical and molecular genetic levels remains fairly small, and the majority of the information about these proteins comes from studies conducted with Arabidopsis thaliana (Caputi et al, 2012)
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