Vegetables with genetically engineered pest resistance are already appearing on supermarket shelves, but scientists have had a much harder time controlling the sizes and crop yields of many vegetables. Now German researchers have created bulkier potatoes by adding a gene from yeast that changes their sugar metabolism--potentially a first step toward more productive potato crops. The discovery, described in next month's issue of Nature Biotechnology, may also help researchers learn how plants regulate their storage of carbohydrates.
Most plants harness the sunlight hitting their leaves to create sucrose, a sugar. Many then convert the sugar to starch for long-term energy storage in so-called sink organs, such as potatoes, yams, and other tubers. Before a tuber can convert sucrose into starch, however, enzymes have to break down the sucrose into a simpler sugar, glucose. By supplementing these enzymes, Lothar Willmitzer, a plant physiologist at the Max Plank Institute for Plant Molecular Physiology in Golm, Germany, thought he and his colleagues could speed up starch production and build a bigger potato.
The team endowed a potato plant with a gene for a yeast enzyme that breaks down sucrose. They found that if the yeast enzyme, called invertase, was expressed in the space between cells, called the apoplasm, potato size increased dramatically, by as much as three times, but each plant had fewer tubers, canceling out any overall gain. When invertase was expressed inside cells (the cytosol), tuber size decreased, although each plant produced more of them.
The team can't explain these different effects, but the experiment "shows the potential for manipulating the size of sink organs," says team member Richard Trethewey, and that could include anything from bell peppers to bananas. "If it can be expanded to other systems, it will be very important," says William Park of Texas A&M University. The researchers themselves say they are most interested in the result with cytosolic invertase, even though it yielded smaller spuds, because it may give clues to how plants divvy up sucrose between immediate energy needs within cells and the plant's starch reserves in tubers and other sink organs--a central question in plant physiology.