NOTE: This story, like all the "GM apples to fight tooth decay" type tales, is getting wide media coverage. But, leaving aside the issue of the safety of tomatoes genetically engineered in this way, the raison d'etre for this new "top super food" seems oddly self-contradictory.

These GM purple tomatoes are supposed to be good for people who eat badly because they will provide them with similar amounts of anthocyanins, which help protect against cancer, as thy would get if they ate blueberries or raspberries or blackberries or blackcurrants or cherries or red cabbage or cranberries or bilberries, etc.

But why would the people this tomato is said to be designed for - described as "fussy eaters who won't change their diet", adopt a tomato that looks like a cross between an orange and a black pudding? 
Purple tomatoes could ward off cancer
New Scientist, 27 October 2008

Tomatoes engineered to be chock-full of purple pigments could have the same cancer-protecting properties as fruits such as blueberries.

The purple pigments are potent antioxidants called anthocyanins that mop up the free-radicals that cause cancer and heart disease. Anthocyanins naturally occur in blueberries, blackberries and blackcurrants, but natural tomatoes only contain negligible amounts of the compounds.

Previous attempts to create tomatoes with higher levels of anthocyanins had achieved low levels of the chemicals in the skin, but not within the flesh of the fruit, says Eugenio Butelli from the John Innes Centre in Norwich, UK, who engineered the purple tomatoes.

The team's purple tomatoes, however, contain high concentrations of anthocyanins throughout the fruit at levels comparable to blackberries and blueberries. Tomato plants have all the necessary genes to create the pigments, but they are normally dormant, so the team inserted two additional genes from the snapdragon flower that trigger these genes to become active.

In front of the two additional genes, the team also added promoter sections of DNA that are activated only when the tomatoes are ripening meaning that only the fruit contain high levels of the pigments, while the leaves or stems of the plants grow as they would without the extra genes.

When fed to 20 mice genetically predisposed to develop cancer, the purple tomatoes increased the average life span by roughly 36 days compared to the 15 mice fed with normal red tomatoes which lived just 146 days on average.

The same two genes from the snapdragon flower should also work in many other types of fruit too, which is good news for fussy eaters who won't change their diet to include more unusual fruit, says Butelli. "It's difficult to change people's dietary habits, but instead they can eat enriched versions of the food they do like," he says.

The team don't think the anthocyanins will change the flavour of the tomatoes. However, before humans can reap the benefits of the added pigments, the tomatoes will need to be subjected to toxicology studies to be sure the snapdragon genes haven't activated the production of other, less beneficial, chemicals.

Journal reference: Nature Biotechnology (DOI: 10.1038/nbt.1506)