The MYB transcription factor PbMYB12b positively regulates flavonol biosynthesis in pear fruit

As a class of natural antioxidants in plants, fruit flavonol metabolites are beneficial to human health. However, the regulatory networks for flavonol biosynthesis in most fruits are largely unknown. Previously, we reported a spontaneous pear bud sport 'Red Zaosu' (Pyrus bretschneideri Rehd.) with a high flavonoid content in its fruit. The identification of the flavonol biosynthetic regulatory network in this mutant pear fruit is crucial for elucidating the flavonol biosynthetic mechanism in fruit. Here, we demonstrated the PbMYB12b positively regulated flavonols biosynthesis in 'Red Zaosu' fruit. Initially, we investigated the accumulation patterns of four major quercetin glycosides and two major isorhamnetin glycosides in the fruit of 'Red Zaosu' and its wild-type 'Zaosu'. A PRODUCTION OF FLAVONOL GLYCOSIDES (PFG)-type MYB transcription factor PbMYB12b was also screened for because of its correlation with flavonol accumulation in pear fruit. The biofunction of PbMYB12b was verified by transient overexpression and RNAi assays in pear fruit and young leaves. Overexpression of PbMYB12b enhanced the biosynthesis of quercetin glycosides and isorhamnetin glycosides by positively regulating a general flavonoids biosynthesis gene PbCHSb and a flavonol biosynthesis gene PbFLS. This finding was also supported by dual-luciferase transient expression assay and transient β-glucuronidase (GUS) reporter assay. Our study indicated that PbMYB12b positively regulated flavonol biosynthesis, including four major quercetin glycosides and two major isorhamnetin glycosides, by promoting the expression of PbCHSb and PbFLS in pear fruit.