Hyperacidification of Vacuoles by the Combined Action of Two Different P-ATPases in the Tonoplast Determines Flower Color

Summary The acidification of endomembrane compartments is essential for enzyme activities, sorting, trafficking, and trans -membrane transport of various compounds. Vacuoles are mildly acidic in most plant cells because of the action of V-ATPase and/or pyrophosphatase proton pumps but are hyperacidified in specific cells by mechanisms that remained unclear. Here, we show that the blue petal color of petunia ph mutants is due to a failure to hyperacidify vacuoles. We report that PH1 encodes a P 3B -ATPase, hitherto known as Mg2 + transporters in bacteria only, that resides in the vacuolar membrane (tonoplast). In vivo nuclear magnetic resonance and genetic data show that PH1 is required and, together with the tonoplast H + P 3A -ATPase PH5, sufficient to hyperacidify vacuoles. PH1 has no H + transport activity on its own but can physically interact with PH5 and boost PH5 H + transport activity. Hence, the hyperacidification of vacuoles in petals, and possibly other tissues, relies on a heteromeric P-ATPase pump.