A Flavonoid MalonyltransferaseMnMaT1, Directly Regulatedby MnATHB51 , Leads to the Biosynthesis of AcylatedFlavonol Glucosides in Morus notabilis

Acylated flavonol glucosides like quercetin 3-O-(6″-O-malonylglucoside) are recognized as the major antioxidative flavonoid components in mulberry leaves. Here, we revealed that O-glycosylated and malonylated flavonols were specifically accumulated in younger leaves of Morus notabilis. A key acyltransferase, namely, MnMaT1, in the flavonol-associated module generated through weighted gene coexpression network analysis (WGCNA), was identified. Subsequent in vitro enzyme assays and in vivo function studies of MnMaT1 revealed its functional role as malonyl-CoA: flavonoid 3/7-O-glucoside-6″-O-malonyltransferase. A homeobox-leucine zipper protein ATHB51 (MnATHB51) in the same module was also identified and found to directly bind to the MnMaT1 promoter and possess transcriptional activity in mulberry leaves. The results indicated that MnATHB51 is a positive regulator governing the acylated flavonol glucosides biosynthesis in mulberry. Our findings elucidate a key transcriptional mechanism in specialized phenylpropanoid metabolism and provide a molecular foundation for the targeted enhancement of bioactive phytochemicals of mulberry.