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

The acidification of endomembrane compartments is
essential for enzyme activities, sorting, trafficking,
and trans-membrane transport of various compounds.
Vacuoles are mildly acidic inmost 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 P3BATPase,
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.