Lemons are sour. Here’s why.

Two decades ago, after a series of confusing experiments, a research team led by Lincoln Taiz of the University of California Santa Cruz put aside a question many people had been trying to answer: What makes a lemon so sour?

Taiz had a hunch. Other fruits and vegetables have pumps that shuttle protons in and out of small sacs inside cells called vacuoles. Lemons have vacuoles, the source of juice, and their pH was an acrid 2. The pump they were looking for needed to be strong and efficient to maintain such a strong gradient inside the neutral pH of the cell. But Taiz’s team couldn’t find it, and turned their attentions back to other projects.

Twenty years later, a team of researchers thinks they have the answer, and it came to them while studying petunias. They discovered two proton pumps on the flowers’ vacuoles that are smaller and more efficient than any found before. The pumps, PH1 and PH5, are part of the P-class of ATPases, enzymes that use ATP to actively move ions across gradients. The research team found the pumps while studying petal color in petunias, which changes depending on the acidity of their cells’ vacuoles. (Nat. Comm. 2019, DOI: 10.1038/s41467-019-08516-3).

“Together, they constitute a ‘super proton pump’,” says Francesca Quattrocchio, one of two lead researchers on the project at the University of Amsterdam. The team started to wonder if the same mechanism that changes petunias’ colors could also be also responsible for lemons’ acidity.

Searching the genomes of citrus fruit, Quattrocchio and the team found their versions of PH1 and PH5. They gathered several varieties of lemons, oranges, and other citrus with the help of plant geneticists at the University of California Riverside. In sweet fruits, the proteins were all but absent. In sour varieties, PH1 and PH5 proteins were present in higher amounts. And it appears that PH1’s and PH5’s efficiency is how they can pack a juice vacuole full of protons—unlike other classes of pumps found on vacuoles, which use up to 4 ATPs to get a single ion across, PH1 and PH5 use only 1.

But interestingly, says Quattrocchio, the mutations that lead to the loss of PH1 and PH5 proteins in sweeter fruits aren’t within those genes themselves but in genes whose proteins control their expression—transcription factors. Agricultural scientists who want to turn a lemon sweet, or an orange back to its native sour, might target these transcription factors and other regulators. This sort of genetic manipulation could be faster than traditional breeding, and will require fewer plants, says Quattrocchio.

“All citrus fruits are sour. The ones that are not have been selected by people in millenia of breeding to make them pleasant in taste,” she says.

While PH1 and PH5 seem to be the main drivers of citrus acidity, they aren’t the only pumps moving ions across gradients in lemon vacuoles, says Ronald Koes, the other project leader from the University of Amsterdam. The movement of other molecules results in the formation of citric acid, among other compounds, giving the fruit its sour flavor.

For his part, Taiz, long retired, is stoked that those confusing experiments from 20 years ago now seem to make sense. “It absolutely answers the question” of why lemons are so sour, he says. “It’s great.”