By Elizabeth PennisiFeb. 26, 2019 , 11:00 AM Inga Spence/Science Source What makes petunias red also makes lemons sour This Lisbon lemon has a molecular pump that helps make it sour and turn its flowers purplish. Ronald Koes could never have guessed that his quest to understand what makes some petunias red—and others blue—would lead him to lemon groves in California. But as a result, this University of Amsterdam geneticist has now answered the long-standing question of why some lemons taste sweet and others, sour. The secret: a powerful molecular pump that makes the cell more acidic.The new work provides a “blueprint” for figuring out which plants in breeding programs have desired colors and flavors, says Harry Klee, a molecular geneticist at the University of Florida in Gainesville who was not involved with the work. It could also lead to more flavorful fruits and more colorful flowers.Because acidity can wreak havoc on a cell’s ability to function, plant cells pump protons—charged hydrogen atoms—into bubbles called vacuoles, sequestering the acidic substances from the rest of the cell. But that acid is necessary for some things: Petunia petals, for example, need high concentrations of protons in their flower cells to color the petals red; otherwise, the petals are blue. 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This pump was more powerful, able to pull enough protons together into vacuoles to keep petunia flowers red. He and his colleagues wondered whether that same pump was active in sour fruits such as lemons.Mikeal Roose, a geneticist and plant breeder at the University of California (UC), Riverside, sent Koes’s team samples of more than a dozen varieties of sweet and sour lemons, oranges, pomelos, and limes. The researchers examined the DNA of each fruit and measured the activity of two genes coding for a pair of proteins that make up the powerful pump. Those genes were very busy in sour plants, but not in sweet plants, he and his colleagues report today in Nature Communications.When the genes aren’t active, the pumps aren’t made, so hydrogen ions don’t accumulate. That makes the fruit sweeter, they conclude. Among different fruit strains, over thousands of years of cultivation, pumps have been turned on and off multiple times, often by breeding that changes how the two genes are regulated. A similar process is likely at play in flowers, shifting their colors to be more red or blue. In fact, the flowers of sour fruits tend to be more purple than those of sweeter fruits, thanks to their ability to retain red pigment.“It is a very satisfying explanation accounting for the differences in ‘sweet’ versus very sour lemons and other citrus fruit,” says Craig Montell, a neuroscientist at UC Santa Barbara, who was not involved with the work.The findings could help researchers and breeders come up with new ways to dial up or down the color of petunias, roses, and other flowers—or change the acidity of fruits, including lemons, grapes, and apples. The ability to know what causes subtle differences in fruit acidity “could be very valuable,” says Roberto Gaxiola, a plant molecular biologist at Arizona State University in Tempe, who called the work “extraordinary.” Maybe extraordinary enough to turn even the sourest of lemons into lemonade.