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Scientists have expressed genes from snapdragon in tomatoes to grow purple tomatoes high in health-protecting anthocyanins.

Anthocyanins are naturally occurring pigments found at particularly high levels in berries such as blackberry, cranberry and chokeberry. Scientists are investigating ways to increase the levels of health-promoting compounds in more commonly eaten fruits and vegetables.

"Most people do not eat 5 portions of fruits and vegetables a day, but they can get more benefit from those they do eat if common fruit and veg can be developed that are higher in bioactive compounds," says Prof Cathie Martin from the John Innes Centre.

Anthocyanins offer protection against certain cancers, cardiovascular disease and age-related degenerative diseases. There is evidence that anthocyanins also have anti-inflammatory activity, promote visual acuity and hinder obesity and diabetes.

Tomatoes already contain high levels of the antioxidant lycopene. Highly processed tomatoes are the best source, or tomatoes cooked in a little oil, which helps to release the lycopene from cells. Flavonoids meanwhile are soluble in water, and foods containing both water soluble and fat-dissolved antioxidants are considered to offer the best protection against disease.

In this study the scientists expressed two genes from snapdragon that induce the production of anthocyanins in snapdragon flowers. The genes were turned on in tomato fruit. Anthocyanins accumulated in tomatoes at higher levels than anything previously reported for metabolic engineering in both the peel and flesh of the fruit. The fruit are an intense purple colour.

The scientists tested whether these elevated levels actually had an effect on health. In a pilot test, the lifespan of cancer-susceptible mice was significantly extended when their diet was supplemented with the purple tomatoes compared to supplementation with normal red tomatoes.

"This is one of the first examples of metabolic engineering that offers the potential to promote health through diet by reducing the impact of chronic disease," says Professor Cathie Martin.

"And certainly the first example of a GMO with a trait that really offers a potential benefit for all consumers. The next step will be to take the preclinical data forward to human studies with volunteers to see if we can promote health through dietary preventive medicine strategies."