The next time you bite into a supermarket tomato and are less than impressed with the taste, blame aesthetics. A new study reveals that decades of breeding the fruits for uniform color have robbed them of a gene that boosts their sugar content.
The finding is "a massive advance in our understanding of tomato fruit development and ripening," says Alisdair Fernie, who studies the chemical composition of tomatoes at the Max Planck Institute of Molecular Plant Physiology in Potsdam, Germany.
The tomato originated in South America and is now grown around the world. More than 15 million tons are harvested in the United States alone each year. Farmers pluck the fruits from the vine before they are ripe, and for about 70 years breeders have selected tomatoes that are uniformly light green at that time. This makes it easier to spot the tomatoes that are ready to be harvested and ensures that, by the time they hit supermarket shelves, the fruits glow with an even red color. Wild varieties, in contrast, "have dark green shoulders, and that makes it harder to determine the right time to harvest," says Ann Powell, a plant scientist at the University of California, Davis. Consumers might also find unevenly colored tomatoes less appealing, she suggests.
To find the gene behind the color change, Powell and colleagues crossed cultivated varieties of tomatoes with wild species. By selecting those plants with dark green shoulders and crossing them back with the cultivated varieties, they narrowed down the culprit to a region on chromosome 10. Using the recently completed tomato genome sequence they then identified the gene as SlGLK2—a so-called transcription factor, which controls when and where other genes are switched on or off.
In wild tomatoes, SlGLK2 increases the formation of chloroplasts, the compartments in plant cells that carry out photosynthesis. Chloroplasts use a green pigment, chlorophyll, to capture the sunlight plants need to grow. A higher number of chloroplasts gives wild tomatoes their darker green color. Even though chloroplast formation and chlorophyll synthesis are among the most important developmental processes in biology, little is known about them, says David Francis, a tomato geneticist at Ohio State University, Wooster, who was not involved in the work. This is a "significant and important" finding, he says. "The authors have described a gene that helps regulate the process."
In most tomatoes on supermarket shelves, however, SlGLK2 is inactive. "We looked at about a dozen varieties, one from Asia, some from Europe, and all of them had the same mutation," says Powell. The researchers do not know where the mutation—a string of bases consisting of seven As, where there should only be six—came from initially. But it might have arisen independently multiple times, says Powell, because repetitions of the same letter of the genetic alphabet are prone to errors.
While the mutation was beneficial to farmers, it's not such a sweet deal for consumers. Chloroplasts use the light energy they capture to convert carbon dioxide and water into sugars. Tomatoes with a mutated SlGLK2 gene not only have fewer chloroplasts, they also sport less sugar. By inserting an intact copy of the gene into tomatoes, the researchers increased the amount of glucose and fructose in ripe fruits by up to 40%, the authors write in the paper, published online today in Science. The content of lycopene, an antioxidant that could have significant health benefits, was also increased. Because of regulations the scientists were not allowed to eat these tomatoes, but they suggest that the higher sugar content should make the fruits more palatable. The intact gene could be crossed back into tomatoes by traditional breeding, says Powell.
Francis is not yet convinced that that will improve the taste, however. The increased sugar content might also be due to changes in other parts of the plants, he suggests. (About 80% of the sugar in tomatoes is actually produced in the leaves and transported to the fruit later.) He also cautions that the results obtained in small greenhouse pots might be different from what growers would see in field-grown tomatoes. "The real culprit affecting tomato flavor is a production system that picks tomatoes before they are ripe," because that changes the ripening process, he says, interrupting for instance the conversion of starch to sugar.
Powell agrees that the early harvest affects fruit quality: "Ripening probably doesn't proceed the same way when the fruit is plucked from the vine," she says. There might be more than one reason that supermarket tomatoes pale in comparison.