The transcription factor complex that controls several aspects of petunia petal epidermal cell differentiation: a flexible combination!

Petunia petal epidermal cells are specialized to synthesize, accumulate and display pigment molecules (anthocyanins) in the large central vacuole. To display the right anthocyanin colour (reddish), the lumen of this vacuole is acidified by a unique system that involves P-ATPase pumps, able to build high proton gradients across the tonoplast. The anthocyanin and the acidification pathways share the same genetic control by a set of regulators which includes WDR, MYB, bHLH and WRKY proteins that interact in a transcription complex. The specificity of the complex for the promoter of different sets of structural genes belonging to the one or the other pathway as well as the intensity of activation of the targets depend on the combination of such regulators. We have isolated putative orthologues for the WDR, MYB, bHLH and WRKY regulators from different plant species, and verified their function by complementation of the relative petunia mutants. Phenotypic and molecular analysis of the transgenics shows that: - the machinery is conserved among species relatively distantly related to each other (asterids and rosids); - the different components coming from the different species can interact with each other in Y2H; - the different type of MYB factors participating to this trascriptional complex compete for the same binding site as the ectopic expression of one displaces the function of the other(s); - One WRKY factor is required for the activation (in different extent) of all sets of target genes, but while in some species the one only protein fulfils all the tasks, in others, gene duplication resulted in two specialized WRKYs, one contributing to the activation of anthocyanin biosynthesis and the other to pH regulation.