Evolution of tonoplast P-ATPase transporters involved in vacuolar acidification

Petunia mutants with blue flowers uncovered a novel proton pump that controls the pH of the central vacuole in the pigmented petal epidermal cells.. The components of this pump are two interacting P-ATPase, PH1 and PH5, residing in the vacuolar membrane (tonoplast). PH5 has high similarity to plasma membrane proton pumps from the P3A-ATPase family, whereas PH1 is the only known eukaryotic member of the P3B-ATPAse family. As there are no indications that such a pump might operate in other species, we investigated the distribution and evolution of PH1 and PH5 homologs in the plant kingdom. Mining of databases and complementation experiments revealed that (functional) PH1 and PH5 homologs are widespread among angiosperms. Phylogenetic analysis suggests that PH5 originated from plasma membrane proton transporters by the acquisition of a new subcellular localization and new regulation mechanism. PH1 shares its origins with bacterial magnesium transporters, suggesting horizontal transfer either from bacteria to plants or vice versa. PH1 is irregularly distributed within plant families, possibly due to repeated loss. The wide conservation of PH1 and PH5 homologs, also in species lacking colored insect- pollinated flowers, suggests that these genes are involved in other phenomena requiring strong(er) vacuolar acidification, and perhaps other endomembrane compartments.