The genes associated with the sour taste of citrus fruits are identified in a paper published in Nature Communications this week.

The sourness of a fruit depends on the acidity of the vacuole (a membrane-bound organelle found in plant cells). In most plant cells, the vacuole is moderately acidic because hydrogen ions are pumped into it. In the juice-producing cells of certain fruits, this pumping process is increased leading to intense acidity in the vacuole. However, it was previously unclear how this was achieved.

Ronald Koes and colleagues compared sweet and sour tasting varieties of citrus fruits including lemons, oranges, pummelos and limes. The authors found that sour varieties express two genes called CitPH1 and CitPH5, which encode transporter proteins that amplify the pumping process, increasing the concentration of hydrogen ions in the vacuole. Expression of these two genes was reduced in sweet-tasting ‘acidless’ varieties of the fruits. This is also the same type of transporter the authors previously found to underlie the purple flower colour of petunias.

The authors conclude that the findings could help fruit breeders to select better tasting fruit more quickly. By testing the DNA of young saplings, breeders may be able to predict the sourness of the fruit, rather than having to wait for the fruit tree to mature.