Analysis of flavonoids and anthocyanin biosynthesis-related genes expression reveals the mechanism of petal color fading of Malus hupehensis (Rosaceae)

Malus hupehensis (Pamp.) Rehder is an indigenous wild crabapple tree from China that has received increased attention, due to its significant ornamental and economical values. The flowers change from red to white during development, a phenomenon that is prevalent within Malus genus. However, there are no reports on the phytochemical aspects of M. hupehensis flower. Here, we investigated the mechanism of color fading through biochemical and molecular analysis of M. hupehensis petals during development. Sixteen flavonoids were detected, and cyanidin-3-galactoside was the major anthocyanin in petals. The levels of most compounds initially increased and then decreased during flower development. The changing profile of total anthocyanins was consistent with the changes in floral color. Analysis of anthocyanin biosynthesis-related genes showed that the expression levels of PAL, CHS, CHI, DFR, FLS, ANS, UFGT, MYB10 and MYB12 were significantly higher at early stages than at later stages, similarly to the change in cyanidin-3-galactoside contents. MYB10 and MYB12 significantly positively correlated with CHS, CHI and DFR. There was a particularly strong positive correlation between MYB10 and ANS, and MYB12 was strongly positively correlated with LAR. Thus, MYB10 appeared to regulate mainly anthocyanin biosynthesis, and MYB12 could influence the expression of multiple structural genes in both the anthocyanin and flavanol pathways. By contrast, MYB6, bHLH33 and WD40 were significantly and positively correlated with one another but negatively correlated with the levels of many compounds and most structural genes, indicating that these three transcription factors could regulate together anthocyanin biosynthesis. Our results revealed the information about the temporal and spatial regulation of anthocyanin biosynthesis in M. hupehensis and provide the diverse flower color intensities and patterning in Chinese crabapple.