R2R3-MYB transcription factor PpMYB17 positively regulates flavonoid biosynthesis in pear fruit

Main conclusion PpMYB17 positively regulates flavonoid biosynthesis in pear fruit by activatingPpCHS,PpCHI,PpF3H,andPpFLSin the flavonoid biosynthesis pathway independently of bHLH or WD40 cofactors in the MBW complex. Flavonoids are important secondary metabolites in plants. The flavonoid biosynthesis pathway is regulated by various transcription factors, with MYB transcription factors considered to be the key regulators. However, the regulation of flavonoid biosynthesis in the pear fruit has not been fully characterized. The R2R3-MYB transcription factor PpMYB17 was isolated from 'Red Zaosu' pear fruit and functionally characterized. An exposure to light upregulatedPpMYB17expression in the pear fruit. A phylogenetic analysis indicated PpMYB17 is related to the flavonol regulators. A subcellular localization assay suggested that PpMYB17 is a nuclear protein. Overexpression ofPpMYB17increased the flavonoid content of pear calli and Arabidopsis via the upregulated expression of structural genes in the flavonoid biosynthesis pathway, especiallyFLS. The LC-MS/MS analysis revealed most of the differentially accumulated flavonols, flavanones, flavones, isoflavones, and anthocyanins were significantly more abundant inPpMYB17-overexpressing calli than in wild-type calli. Moreover, PpMYB17 did not interact with PpbHLH3, PpbHLH33, or PpWD40 in a yeast system. Dual-luciferase assays demonstrated that PpMYB17 strongly activates the promoters ofPpCHS,PpCHI,PpF3H,PpFLS, andPpUFGTwhich are key downstream genes in the flavonoid biosynthesis pathway, independently of the PpbHLH3 cofactor. These gene expression changes may enhance flavonoid biosynthesis in pear fruit. The data presented may be useful for further elucidating the flavonoid biosynthesis regulatory network, potentially leading to the development of new pear cultivars that produce fruits with increased flavonoid contents.