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The genetics of sour

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The acidity of citrus fruits is determined by genetic drivers that influence vacuole function.

Niklas Skur / EyeEm/Getty Images

Biting into citrus fruit can be a lip-puckering experience, and not just with lemons and limes; some oranges are a little sharper than expected.

For some time, scientists have known why that is, and now they know how it comes about.

The why is linked to the fact that the sourness of a fruit depends on the acidity of a membrane-bound organelle called the vacuole, present in every plant cell.

In most such cells, the vacuole is moderately acidic because hydrogen ions are pumped into it, but in some fruits the acidity is more intense because the pumping process is increased.

And that happens, according to new research, because these 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.

Writing in the journal Nature Communications, a Dutch and US research team led by Ronald Koes from the University of Amsterdam, in The Netherlands, report on their study of sweet and sour tasting varieties of citrus fruits including lemons, oranges, limes and Asian pomelos.

They found that the expression of the two genes was reduced in sweet-tasting or “acidless” varieties of these fruits.

Interestingly, this is the same type of transporter, the authors found, that underlies the purple flower colour of petunias.

It’s not the complete answer to the story of sourness, they acknowledge. The causes of the much smaller variations between acidic varieties of different citrus groups, or the even smaller differences within a group, remain unclear.

However, they believe their findings could help fruit breeders select better tasting fruit more quickly.

“This opens the way to develop molecular markers for fruit acidity and taste to speed up the breeding in citrus and other fruit crops, most of which are trees or shrubs with long generation times,” they write.

By testing the DNA of young saplings, they suggest, breeders may be able to predict the sourness of the fruit, rather than having to wait for the fruit tree to mature.

And then they can let us know what we are letting ourselves in for.