MYB regulator of “colorless” flavonols underlies the evolution of red flowers in <i>Iochroma</i> (Solanaceae)

Anthocyanins, the pigments that give rise to blue, purple, red, and pink colors in many flowers and fruits, are produced by the deeply conserved flavonoid biosynthesis pathway. The regulation of this pathway is thus fundamental for species differences in color across flowering plants, and a growing body of evidence implicates MYB transcription factors as key players activating or suppressing the production of different pigments. Nevertheless, the potential role of MYBs genes in determining the type of pigment produced (as opposed to the overall amount) is unknown. Here, we demonstrate that a lineage of R2R3 MYBs that is closely related to well-known flavonol regulators (MYB12 members in subgroup 7) is the primary determinant of the shift from blue to red flowers in the genus Iochroma. Similar to its ortholog in Capsicum, this Iochroma MYB12-like gene controls the expression of flavonoid-3'-hydroxylase, the pathway branch point between red and blue pigments, and when down-regulated, results in redirection of flux toward red pigments. These results underscore the importance of transcription factor evolution in generating phenotypic novelty as well as the competitive nature of interactions among flavonoid pathway branches. In addition, our study demonstrates the effectiveness of RNAseq of segregating populations, in combination with other lines of evidence, for identifying novel functional variation.