Not Your Garden-Variety Tomato

27 October 2008 (All day)

John Innes Centre

Seeing purple? Anthocyanin-enhanced tomatoes sport a distinctive purple color and helped mice with cancer live longer.

Raspberries, strawberries, cranberries, ... and tomatoes? Researchers have now engineered a tomato that has the same antioxidants that give berries their red and purple shades and nutritional punch. And at least in mice, these supertomatoes seem to aid in the fight against cancer.

The magenta juice that tints your strawberry smoothies comes from a pigment in the cells called anthocyanin. This pigment also acts as an antioxidant. Cell- and animal-based research has shown that antioxidants can protect against cancer, though clinical trials have been inconclusive. Still, the U.S. Department of Agriculture (USDA) recommends eating at least five servings of fruits and vegetables a day, in part to ensure adequate intake of antioxidants. The message has yet to sink in; studies show that only about 23% of Americans meet those guidelines. So, researchers such as plant geneticist Cathie Martin are trying to engineer fruits and vegetables with enhanced anthocyanin content to get more bang for the bite.

Martin and colleagues at the John Innes Centre in Norwich, U.K., zeroed in on the tomato because it is eaten around the world and used in plant genetics studies. Although the fruit received high marks from nutritionists for its content of another antioxidant, lycopene, tomatoes contain no anthocyanin. Previous attempts to create an anthocyanin-rich tomato using corn genes had failed, Martin says. Through their research on snapdragons, the researchers identified two key proteins in the flowers that might trigger anthocyanin production if inserted in tomato plants. They engineered tomatoes with the genes for those key proteins and watched as purple fruit ripened on the vine. The tomatoes had about 2.8 mg of anthocyanin per gram. Raspberries have about 3.7 mg per gram, by comparison.

To determine if the purple tomatoes could help fight cancer, the researchers tested three groups of tumor-prone mice. Twenty-four mice received a normal diet, 15 ate food mixed with the dust of freeze-dried red tomatoes, and 20 nibbled rations enhanced with purple tomato dust. Mice on the control and red tomato diets lived about 142 days before dying of cancer, whereas mice on the purple tomato diet lived 182.2 days before succumbing, the researchers reported online on 26 October in Nature Biotechnology.

Because tomatoes naturally lack anthocyanin, Martin says the ability to compare the unmodified fruit with an enhanced counterpart will be a boon to researchers trying to tease out the specific nutritional powers of the pigment. "Dietary improvement is the big hope for preventative medicine," she says, but cautions that more research is needed before people can expect purple tomatoes in their grocery stores.

Jeffrey Blumberg, a nutrition scientist at the Tufts University Friedman School of Nutrition Science and Policy in Boston, applauds the researchers' integration of nutrition science and plant biotechnology. But he agrees that researchers will have to answer key questions such as how much anthocyanin a person would actually metabolize from the tomatoes before the work could benefit humans. And no superfood should replace a diet rich in fruits and vegetables, Blumberg notes.

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