Regulation of anthocyanin biosynthesis involved in the formation of marginal picotee petals in Petunia

Marginal picotee is a floral coloration characterized by differently colored apical and basal zones in the petals. To gain insights into the molecular mechanisms underlying endogenous pattern formation due to pigment accumulation, we analyzed the levels of anthocyanin-related compounds as well as transcripts of six genes involved in anthocyanin biosynthesis in different parts of corolla in Petunia hybrida. Two distinct mechanisms for repression of anthocyanin biosynthesis are proposed in cultivars showing different coloration patterns. In the white margin of the corolla of a cultivar showing strong coloration only in the center of the flower, expression of chalcone synthase, which catalyzes the conversion of cinnamic acid derivatives into flavonoids, was repressed and cinnamic acid derivatives were accumulated, while the expression levels of other enzymes were similar, suggesting that reduction in flavonoid biosynthesis by repression of chalcone synthase is involved in the formation of unpigmented zones in the corolla margins. As for petals bearing the reverse contrast pattern, the levels of flavonol and flavonol synthase transcripts were particularly high in the white regions, while the transcript levels of other genes directly involved in anthocyanin biosynthesis were similar between the white and colored regions, suggesting that enhancement of flavonol biosynthesis by enhanced expression of flavonol synthase is at least in part responsible for reduction in anthocyanin level and formation of unpigmented zones in a cultivar showing white corolla centers.