The WD40 proteins ANTHOCYANIN11 (AN11) from petunia (Petunia hybrida) and TRANSPARENT TESTA GLABRA1 (TTG1) fromArabidopsis
thalianaand associated basic helix-loop-helix (bHLH) and MYB transcription factors activate a variety of differentiation processes.
In petunia petals, AN11 and the bHLH protein AN1 activate, together with the MYB protein AN2, anthocyanin biosynthesis and,
together with the MYB protein PH4, distinct genes, such asPH1andPH5, that acidify the vacuole. To understand how AN1 and AN11
activate anthocyanin biosynthetic andPHgenes independently, we isolatedPH3 We found thatPH3is a target gene of the AN11-AN1-PH4
complex and encodes a WRKY protein that can bind to AN11 and is required, in a feed-forward loop, together with AN11-AN1-PH4
for transcription ofPH5 PH3 is highly similar to TTG2, which regulates hair development, tannin accumulation, and mucilage
production in Arabidopsis. Like PH3, TTG2 can bind to petunia AN11 and the Arabidopsis homolog TTG1, complementph3in petunia,
and reactivate the PH3 target genePH5 Our findings show that the specificity of WD40-bHLH-MYB complexes is in part determined
by interacting proteins, such as PH3 and TTG2, and reveal an unanticipated similarity in the regulatory circuitry that controls
petunia vacuolar acidification and Arabidopsis hair development.