Functionally homologous WRKY proteins regulate vacuolar acidification in petunia and trichome development in Arabidopsis

In petunia, loci PH1 to PH7 are involved in an unknown vacuolar acidification pathway since disruption of these loci results in an increased vacuolar pH and bluish flowers. PH4 encodes a MYB protein, which regulates, together with regulators of the anthocyanin biosynthesis pathway (AN1, AN2 and AN11), the transcription of (unknown) structural genes involved in vacuolar acidification. Here we show that PH3 encodes a WRKY transcription factor and is transcriptional regulated by AN1, AN2, AN11 and PH4. PH3 shows high similarity to Arabidopsis TRANSPARENT TESTA GLABRA 2 (TTG2), which is involved in unrelated processes such as trichome development, pro-anthocyanidin accumulation and mucilage production. When expressed in petunia, TTG2 can complement ph3 and reactivate genes involved in vacuolar acidification such as PH5. Whether PH3 can rescue ttg2 is currently being tested. Our results indicate that PH3 and TTG2 are functionally conserved in different species and control different developmental processes.