Petal color is a key morphological trait of flowers that influences reproductive success. It is controlled by the pigments produced by the plant, and by the pH at which the pigments are stored. In plants, the anthocyanins-the main class of flower pigments-reside in the vacuoles, an intracet-lular acidic subcompartment. Because the color of a pigment is often pH-sensitive, variations in petal color have been used to identify mutations in pH regulation. In petunias, Verweij et at. now identify PH5 as a P-type proton pump localized to the vacuolar membrane. Similar P-type proton ATPases have previously been found to reside on the cell surface, not on intracellular membranes, which contain their own vacuolar-type proton ATPases. In PH5 mutants, vacuolar acidification is reduced without changing the expression of the anthocyanin pigments or other coloration-related processes, leading to blue flower coloration (shown to the right of a wild-type flower), which is never observed in natural habitats. Expression of the PH5 gene is linked to the same transcription regulation involved in anthocyanin production. This coordination of pigment production and pH regulation of the pigment-containing compartment is an important aspect in maintaining flower and also seed coloration, which also requires PH5 activity during pigment accumulation.
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