Fig. 4.

H⁺ transporters regulate vacuolar pH, affecting the flower color of petunias (A–E) and I. nil (F–H). (A) In wild-type petunias, PH3 activates PH1 and PH5 expression, and the heteromeric complex of two P-ATPases, PH1 and PH5, mediate hyperacidification. (B) In the ph3 mutant, PH1 and PH5 are not expressed, resulting in an increase of vacuolar pH. This mutant line accumulates cyanidin derivatives that exhibit dull gray flowers. (C) PH1 is necessary for the H⁺ pump activity of PH5, and PH5 alone cannot rescue the ph3 phenotype. (D) The ph3 phenotype is rescued by the co-expression of PH1 and PH5. (E) The rescued phenotype in (D) is canceled by the expression of 35S:NHX1. NHX exchanges cations and H⁺, resulting in an increase in vacuolar pH. (F) Flower buds of the wild-type I. nil show lower vacuolar pH and red petals. (G) During flower opening, flower color changes from red to blue. In the same stage, PURPLE/InNHX1 (purple circle) is accumulated and mediates vacuolar alkalization. (H) The pr mutant shows partial vacuolar alkalization, and red flower buds change into purple flowers. The pH values of petal homogenates are presented, and those estimated from spectra are shown in parentheses. Scale bars represent 1 cm.