Vacuolar proton pumping: more than the sum of its parts?

Petunia flower colour is dependent on vacuolar pH and is therefore used to study acidification mechanisms. Recently, it was shown that the concerted action of two tonoplast-localised P3-ATPases is required to hyperacidify vacuoles of petunia petal epidermis cells. Here we discuss how steep cross-tonoplast pH gradients may be established in specific cells.