The Arabidopsis flower at top left produces anthocyanin pigments due to the overexpression of the anthocyanin MYB regulator PAP1. Wild-type Arabidopsis flowers exhibit no pigmentation of petals, as shown at bottom left. The tobacco flowers at right illustrate the overexpression phenotype of Arabidopsis PAP1 in tobacco. PAP1 can function in a heterologous system to increase pigmentation, changing the wild-type tobacco flower from light pink to dark red. The pap1-D mutant (shown in Figure 1 of Borevitz et al. 2383-2393 in this issue) was generated through activation tagging. It contains an insertion of four copies of the 35S enhancers 5.1 kb 3' of the PAP1 transcription start site, resulting in mutants overexpressing the PAP1 gene, as illustrated at bottom right. The molecule diagrammed at center is an anthocyanin, which is the pigment responsible for the purple and red pigmentation. Sinapic acid, at bottom, and trans-4-coumaric acid, at top, are other phenylpropanoids that accumulate in the pap1-D plants.