FIGURE 2.3 A generalized flavonoid biosynthetic pathway. In the nucleus, 3 transregulators (a WD40, a bHLH, and a Myb) coordinately affect the expression of multiple genes of the core pathway by binding to their cis-regulatory elements. Enzymes are indicated in the cytosol with black boxes, and the number of candidate genes identified in Aquilegia is indicated. Biochemical intermediates are indicated with light shading in the cytosol, with the substrate for each enzyme to the left and the product to the right. Specific anthocyanins (indicated by darker shading in the vacuole) are glucosides of pelargonidins, cyanidins, and delphinidins. Lines from enzymes to their products in the vacuole indicate side-branch pathways. Enzymes are: CHS, CHI, UFGT, anthocyanin GST (GST), F3′H, F3′5′H, aurone synthase (AUS), isoflavone synthase (IFS), flavone synthase (FNS), FLS, leucoanthocyanidin reductase (LAR), and ANR.

nins: pelargonidins (orange/red), cyanindins (blue/magenta/red), and delphinidins (blue/purple). The production of pelargonidins requires just the core enzymes, but the production of cyanidins and delphinidins depends on 2 enzymes [flavonoid 3′-hydroxylase (F3′H) and flavonoid 3′5′-hydroxylase (F3′5′H)] that add 1 or 2 hydroxyl groups, respectively, on the β-ring of the product of F3H (Grotewold, 2006; Rausher, 2008). Subsequently, DFR, ANS, and UFGT act to produce anthocyanins, which are then transported into the vacuole where they accumulate and produce visual colors (Fig. 2.3).

Rausher (2008) has described general trends in flower color evolution: shifts are generally blue to red or from producing anthocyanin to not, although exceptions do occur. Considering the biochemical pathway for anthocyanins, Rausher pointed out that these trends likely arise because mutations causing loss of function are more likely than those causing