Molecule for tomato ripening and sugar content
June 28, 2012 § Leave a comment
Hands up if you’re like me and refuse to eat raw grocery-store tomatoes because they taste like sawdust. But a tastier fruit may be on its way. In a paper just out in Science, researchers describe the molecular changes that make grocery-store tomatoes ripen evenly but cause the fruit’s sugar levels to decrease.
Tomatoes are a major crop. In California, where the research team of Ann Powell at the University of California, Davis, is based, the agricultural industry grows most of the world’s tomatoes for processing around the Davis region. “Throughout the growing season, July through October, we see fields and fields of tomatoes grown and harvested to provide all sorts of products – tomato sauces, catsup and salsas,” says Powell.
Powell, a plant biochemist, has a longstanding interest in the fruit development and ripening process. As a Californian home garderner who grows tomatoes in her back yard, Powell was intrigued by why a tomato bothered to start off as a dark green fruit and then turn another color.
Indeed, this color turn has been the focus of tomato agriculture. For more than 50 years, tomato breeders have selected varieties with an uniform light green before ripening. These tomatoes evenly turn red as they ripen, looking wonderful and luscious under the bright lights in grocery stores.
But what causes this even red color of ripe tomatoes? As Powell and colleagues report in their paper, a transcription factor called GLK2 is the root cause. GLK2 regulates the capacity of cells in fruit to form chloroplasts in response to light. Under normal circumstances, GLK2 increases the fruit’s photosynthetic capacity and helps it produce sugars and lycopene, critical components of a tasty tomato.
However, in fruits that ripen evenly all over, a mutation shuts down GLK2. So agricultural tomato breeders, in their quest for fruit that are uniformly green before they ripen, select for poor development of chloroplasts, the workhorses in photosynthesis, in the fruit. The selection compromises the accumulation of sugars and lycopene. It’s a completely unintentional consequence, says Powell, because the link between the uniform ripening trait and accumulation of sugars and lycopene from the fruit’s photosynthetic machinery, until this point, was not known.
About 70 to 80 percent of sugar in fruits is produced by photosynthetic processes in the leaves; the fruit acts as a sugar sink. But the work by Powell and colleagues also highlights that the fruits themselves are capable of photosynthesis and carbon fixation into sugars. Those sugars also accumulate in fruit.
All this has implications for the fruit we eat. “Flavor is a complex trait that involves many compounds. The levels of sugars are only one aspect of how we perceive the flavor of tomatoes,” says Powell. “But the levels of sugar can contribute to the intensity of the flavors.”
Powell says that the wild-type GLK2 trait is available from traditional, wild and heirloom varieties of tomato and can be reintroduced into agricultural production lines. She says the work raises a question as to why most tomato lines that have the regular form of GLK2 confine the expression of the protein to just the shoulder region of the fruit. This does not seem to be the case for peppers, which are closely related to tomatoes. Peppers are normally uniformly light green or uniformly dark green over their entire surface.