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Genetically designed grapes with elevated levels of vitamin C may be more
than wishful thinking, according to researchers at the University of
California, Davis, and the University of Adelaide, Australia, who recently
identified an enzyme in grapes that helps convert vitamin C into tartaric
acid, a key acid in winemaking, March 2006.

This discovery about the biochemical pathway by which grapes synthesize
tartaric acid will appear the week of March 20 in the online version of the
Proceedings of the National Academy of Sciences.

"While we're a long way from producing wine that will replace your morning
glass of orange juice or vitamin C tablet, we now have a much better idea of
one way in which the healthfulness of grapes and wine might be enhanced,"
said UC Davis plant pathology professor Doug Cook.

"It's been said that 'acid is the nerve of great wine,'" Cook added, quoting
Richard Geoffroy of Dom Perignon. "Given the importance of tartaric acid in
wine, it is possible that understanding how this pathway is regulated in
grape berries may have practical implications for how we grow grapes and
make wine."

Tartaric acid is the most abundant acid found in grapes. It plays an
important role in the taste of the grape as well as in the flavor, color and
texture of wine, and can sometimes be seen as crystals in the wine or on the
wine bottle's cork. It is relatively rare in other fruits and, unlike other
fruit acids, is synthesized from vitamin C instead of sugars. Scientists had
previously identified the chemical intermediaries in the production of
tartaric acid, but none of the enzymes responsible for this synthesis had
been identified.

UC Davis' Cook, along with graduate student Seth DeBolt and Chris Ford of
the University of Adelaide, set out to identify several potential candidate
enzymes and the genes that code for them by examining the changing levels of
gene expression and the compounds produced in the cultivated wine grape,
Vitis vinifera.

They compared the acid content of 28 related grape species and found one,
Ampelopsis aconitifolia, which had no tartaric acid. This particular grape
species also lacked one of the candidate genes, which the researchers had
shown controls production of the enzyme for one key step in the formation of
tartaric acid.

Interestingly, the grapes on the A. aconitifolia vine accumulate about three
times as much vitamin C as do other grape species.

Together, these findings suggest that it might be possible to manipulate
this newly identified gene and the enzyme it codes for in cultivated grapes
in order to produce grapes rich in vitamin C.

"This project grew from the fact that our colleagues in Adelaide had
biochemical intermediates, but no genes; whereas here at UC Davis we had
databases and freezers full of grape genes resulting from our genomics work,
waiting for a clear hypothesis to test," Cook said. "In the end, we have a
nice piece of novel biochemistry and a candidate gene for this key metabolic
step."

Further research is needed to identify two more important steps in the
biosynthesis of tartaric acid, and to better understand the impact of light
on that process, he noted.

This study was supported by the Australian Government's Cooperative Research
Centre Program and the University of California, and conducted as a
collaboration between scientists at the University of California, Davis, and
the Australian Cooperative Research Centre for Viticulture.

[www.news.ucdavis.edu]

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