Kathy Schwinn

The major pigments responsible for flower colour are flavonoids (particularly anthocyanins) and carotenoids, with betalains occurring in a relatively small number of species. Flavonoids and betalains are water-soluble and generally located in the vacuole. Carotenoids are lipid-soluble, plastid-located terpenoids, which for pigmentation of flowers accumulate in specialized plastids called chromoplasts. The biosynthetic pathways for flavonoids and carotenoids have been characterized

The Rosea1, Rosea2, and Venosa genes encode MYB-related transcription factors active in the flowers of Antirrhinum majus. Analysis of mutant phenotypes shows that these genes control the intensity and pattern of magenta anthocyanin pigmentation in flowers. Despite the structural similarity of these regulatory proteins, they influence the expression of target genes encoding the enzymes of anthocyanin biosynthesis with different specificities. Consequently, they are not equivalent biochemically in their activities. Different species of the genus Antirrhinum, native to Spain and Portugal, show striking differences in their patterns and intensities of floral pigmentation. Differences in anthocyanin pigmentation between at least six species are attributable to variations in the activity of the Rosea and Venosa loci. Set in the context of our understanding of the regulation of anthocyanin production in other genera, the activity of MYB-related genes is probably a primary cause of natural variation in anthocyanin pigmentation in plants.