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

• Hyperacidification of vacuoles involves a mechanism independent from V-ATPases
• Two different of P-ATPases combined can transport protons against a steep gradient
• One of these P-ATPases belongs to a subfamily so far known only in bacteria
• Petal pigmented cells are acidified by this mechanism

SummaryThe 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 P3B-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+ P3A-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.

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