BioWorld International Correspondent
LONDON - Supermarkets of the future may one day be stocked with fruits and vegetables that have been genetically manipulated to contain high levels of natural compounds that help protect against heart disease and cancer.
Among them may be tomatoes containing levels of flavonols - antioxidant compounds thought to benefit health - much higher than those usually found in tomatoes, a recent study suggests.
Researchers at Unilever Research, in Sharnbrook, UK, together with colleagues at the University of Exeter in Exeter, UK, and at Plant Research International in Wageningen, the Netherlands, have shown that it is possible to produce tomatoes containing levels of flavonols 78 times greater than normal. The team reports its results in a paper in Nature Biotechnology titled "Overexpression of petunia chalcone isomerase in tomato results in fruit containing increased levels of flavonols."
Martine Verhoeyen, plant science project leader at Unilever Research, told BioWorld International: "We know the government recommends that people should eat five portions of fruit and vegetables a day but most people find this difficult. Making more nutrient-rich fruit and vegetables would be a possible solution, particularly for those groups whose health would benefit from that nutrient."
Unilever Bestfoods, parent company of Unilever Research, is one of the world's largest food companies, and the biggest producer of processed tomato foods. It uses over 1 million tons of tomatoes per annum.
Verhoeyen and her colleagues set out to find a way to boost levels of flavonols in tomatoes. She said, "These compounds are thought to be beneficial for heart health in particular. There is some evidence that they protect against cancer, too, but this is less strong." More research is needed to investigate these effects, she added.
Biochemical analysis of tomatoes showed that the peel of the fruit contained only small amounts of flavonols, because of a bottleneck in production of these compounds. The rate-limiting step appeared to be production of the enzyme chalcone isomerase (CHI), which is involved in biosynthesis of one of the building blocks that make up the finished flavonol molecule.
Verhoeyen explained, "We decided to get around this by .supplementing" the CHI that was already present in tomato with a petunia CHI, by introducing the petunia gene encoding CHI into the genome of the tomato." Petunias have high levels of flavonols in parts of their flowers.
The main flavonol produced in the genetically manipulated tomato was quercetin-rutinoside (quercetin with a glucose and rhamnose sugar attached), which is better known as rutin. Significantly increased levels of kaempferol-rutinoside (kaempferol with a glucose and rhamnose sugar attached), commonly known as isoquercitrin, and of quercetin-3-glucoside (quercetin with only a glucose sugar attached) also were present. Verhoeyen said: "Although most of the increase is in rutin, isoquercitrin is thought to be more bioavailable and it should be possible to increase levels of the latter further."
Breeding the plants showed that the chi transgene was associated with tomatoes containing high levels of flavonols, and that the transgene remained stable for all generations tested so far.
Most of the rutin accumulated in the peel rather than the flesh of the fruit. Using a benchtop scale processor designed to mimic factory production of tomato paste, the team went on to process the flavonol-enriched tomatoes. They found that because processing involves an initial heating step before peel and seeds are removed, much of the flavonol content of the peel "leached" into the paste.
Analysis of the paste obtained showed that it contained 65 percent of the flavonol content of the fresh fruit. Taste tests showed that the paste from the genetically manipulated tomatoes could not be distinguished in taste and flavor from paste made from control tomatoes.
Verhoeyen and her colleagues are currently collaborating with other researchers in Europe. "We want to investigate the relative efficacy of the different flavonols in foods, including our genetically manipulated tomatoes," she said.
So how soon might consumers find these tomatoes in the stores? Verhoeyen pointed out that release of such fruits into the environment is subject to stringent regulatory and environmental procedures. "As these would take some years, there is no question of product launches in the foreseeable future," she said.
Importantly, the consumer also would have to be comfortable with the product derived from the technology used. "To enable consumers to make an informed choice they would need to be supplied with sufficient information on the nature of the tomato modification and the potential health benefits," Verhoeyen concluded.