Metabolic Profiling and Transcriptome Analysis of Mulberry Leaves Provide Insights into Flavonoid Biosynthesis

Flavonoids are widely distributed in mulberry leaves and have been
recognized for their beneficial physiological effects on the human
health. Here, we analyzed variations in 44 flavonoid compounds among
91 mulberry resources. Metabolic profiling revealed that O-rhamnosylated flavonols and malonylated flavonol glycosides, including
rutin and quercetin 3-O-(6″-O-malonylglucoside) (Q3MG), were absent from Morus
notabilis and multiple mulberry (Morus
alba L.) resources. Transcriptome and phylogenetic
analyses of flavonoid-related UDP-glycosyltransferases (UGTs) suggested
that the flavonol 3-O-glucoside-O-rhamnosyltransferase (FGRT) KT324624 is a key enzyme involved in
rutin synthesis. A recombinant FGRT protein was able to convert kaempferol/quercetin
3-O-glucoside to kaempferol 3-O-rutinoside
(K3G6″Rha) and rutin. The recombinant FGRT was able to use
3-O-glucosylated flavonols but not flavonoid aglycones
or 7-O-glycosylated flavonoids as substrates. The
enzyme preferentially used UDP-rhamnose as the sugar donor, indicating
that it was a flavonol 3-O-glucoside: 6″-O-rhamnosyltransferase. This study provided insights into
the biosynthesis of rutin in mulberry.