BioWorld International Correspondent

LONDON - The juiciest, sweetest and ripest strawberries have the highest levels of vitamin C - and now scientists know why. Researchers working in Spain have identified a gene that is specifically switched on in ripening strawberries, which encodes an enzyme that helps to synthesize vitamin C.

Consumers' attitudes permitting, markets and supermarkets around the world could soon be stocked up with all kinds of fruit and vegetables containing triple the normal amount of vitamin C. And that common additive, ascorbic acid, listed on the labels of many preserved foods - which is, of course, an alternative name for vitamin C - could in the future be made by fermenting genetically modified microorganisms instead of by a laborious chemical process.

Victoriano Valpuesta, professor of biochemistry and molecular biology at the University of Malaga in Spain, told BioWorld International, "We have shown that by putting this gene into a different plant, the thale cress Arabidopsis thaliana, we can double or triple the amount of vitamin C it contains. We are now working on trying to increase the levels of vitamin C in commercially important plants such as tomatoes and potatoes, although these results are not yet available."

The finding is important, he added, because it demonstrates a completely new and previously unknown pathway for vitamin C synthesis in plants.

Valpuesta and colleagues, together with collaborators at the University of Cordoba in Spain, report their findings in the Jan. 13, 2003, issue of Nature Biotechnology in a paper titled "Engineering increased vitamin C levels in plants by overexpression of a D-galacturonic acid reductase." The group has filed an international patent application on the finding and assigned it to the technology management company Plant Bioscience Ltd., of Norwich, UK, to develop its commercial applications.

Jan Chojecki, managing director of Plant Bioscience, said the company, which is privately owned by the charities The John Innes Centre and The Sainsbury Laboratory, would be filing and maintaining patent applications in parts of the world able to exploit the technology, and identifying development partners that would, under license, develop the technology for consumer benefit.

The discovery came about when the Spanish group was investigating which genes were specifically expressed in ripening strawberry fruit, in an attempt to find out which gene products played an important role in maintaining fruit quality. The genes involved in the synthesis of vitamin C seemed to be likely candidates, as vitamin C is well known as a food preservative, because of its antioxidant properties.

To date, only one metabolic pathway for the synthesis of vitamin C had been described, but the researchers knew that some preliminary studies during the 1950s had suggested that there might be another pathway in strawberries.

Valpuesta said, "By isolating genes that were expressed in red fruit but not in green fruit, we were able to isolate a gene and, subsequently, its protein, and to demonstrate that this protein functions in the synthesis of vitamin C." The researchers have called the gene GalUR. The enzyme it encodes is a D-galacturonic acid reductase. This converts D-galacturonic acid into L-galactonic acid, which in turn can be converted into the immediate precursor of ascorbic acid.

In Nature Biotechnology, Valpuesta and colleagues write that their study "supports previous suggestions that this pathway could constitute a carbon salvage mechanism in certain organs after the breakdown of cell walls, such as occurs during fruit ripening. Pectins are a major cell wall component whose main building blocks . . . release D-galacturonic acid upon hydrolysis."

The novel metabolic pathway also could be harnessed to synthesize vitamin C in bulk. Valpuesta estimates that 80,000 tonnes of vitamin C are manufactured each year worldwide, making this compound the single most important specialty chemical produced commercially. As well as being taken as a dietary supplement, it is an important food additive.

Now, Valpuesta said, the way is open to synthesize vitamin C by inserting the gene into microorganisms such as yeast, followed by fermentation. "This could replace the current commercial synthesis procedure, which involves microbial fermentation followed by several chemical reactions," he added.

The Spanish group also is planning further studies to find out whether the novel synthetic pathway is widespread in other plants.