Abstract
Flavonols and anthocyanins are two widely distributed groups of flavonoids that occurred apart during plant evolution and biosynthesized by shared specific enzymes involved in flavonoid metabolism. UDP-glucose, flavonoid 3-O-glycosyltransferase (UF3GT), is one of the common enzymes which could catalyze the glycosylation of both flavonol and anthocyanidin aglycons simultaneously in vitro. However, whether and how UF3GT paralogous genes function diversely at the biochemical and transcriptional levels are largely unknown. Recently, Fh3GT1 was identified to be a member of UF3GTs in Freesia hybrida. However, its expression patterns and enzymatic characteristics could not coincide well with flavonol accumulation. In an attempt to characterize other flavonoids, especially flavonol glycosyltransferase genes in Freesia, three closest candidate UFGT genes—Fh3GT2, Fh3GT3, and Fh3GT4—were mined from the Freesia transcriptomic database and isolated from the flowers of the widely distributed Freesia cultivar, Red River®. Based on bioinformatic analysis and enzymatic assays, Fh3GT2 turned out to be another bona fide glycosyltransferase gene. Biochemical analysis further proved that Fh3GT2 preferentially glucosylated kaempferol while Fh3GT1 controlled the glucosylation of quercetin and anthocyanidins. In addition, transfection assays demonstrated that Fh3GT2 could be mainly activated by the flavonol regulator FhMYBF1 or the anthocyanin regulator FhPAP1, whereas Fh3GT1 could only be activated by FhPAP1. These findings suggested that Fh3GTs might have functionally diverged in flavonoid biosynthesis at both the biochemical and transcriptional levels.
Highlights
The derivation of the land-dwelling plant life exemplifies a fundamental transition in plant evolution
The UDP-glucose: flavonoid 3-O-glycosyltransferases (UF3GTs) that transfers UDP-activated sugars to flavonoid compounds is considered as a key enzyme for flavonoid modification in vivo to confer stability (Vogt and Jones, 2000; Zhang et al, 2014)
In our determination to understand the Freesia flavonoid glycosylation system, the Freesia flower transcriptomic database was screened focusing on genes that were annotated as UDPglucose: flavonoid 3-O-glucosyltransferase
Summary
The derivation of the land-dwelling plant life exemplifies a fundamental transition in plant evolution. The early plants encountered diverse abiotic pressures from temperature, moisture, and UV radiation (Agati et al, 2012; Mouradov and Spangenberg, 2014; Vries and Archibald, 2018). Plants adopt varied protective measures against the inimical environment. The emergence of the specialized metabolites, flavonoids, is among the most general responses to versatile habitats (Waters, 2003). Flavonoids with typical C6-C3-C6 skeletons are phenolic compounds that are widely distributed in plants (Marais et al, 2006). According to differences of the C3 unit, flavonoids can be further categorized into several subclasses including flavonols, proanthocyanidins, and anthocyanins
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