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SUMMER 2002
One Strong Carbohydrate
­ Kathleen Cason

A daily glass of red wine seems to be the French antidote to butter, brie and brochettes. In fact, it may remedy even more: Wine is rich in a mysterious carbohydrate that is central to plant growth and may even counteract lead poisoning.

This carbohydrate — one of the most complex found in nature — was discovered nearly a quarter century ago by Alan Darvill and colleagues. Called rhamnogalacturan II or RG-II, it is found not only in red wine but also in the cell walls of all higher plants.

“In a sense, we’ve been on a quest to understand what it’s doing in plants ever since we discovered it,” said Darvill, professor of biochemistry and co-director of the University of Georgia Complex Carbohydrate Research Center (CCRC).

Now that quest has come to an end. In an article published in the journal Science, CCRC scientists presented evidence that RG-II is essential for normal plant growth and that even small changes in the molecule’s structure cause growth defects.

“RG-II has been known as an obscure, structurally weird thing that plants make,” said Malcolm O’Neill, senior research associate at the CCRC. “But we had no idea why plants went to all the effort to make it. There are 50 to 60 enzymes involved, 12 different sugars and 22 different linkages. There’s even one sugar that’s actually not been found anywhere else.”

In normal plant cell walls, two RG-II strands are held together by a boron molecule to form a fishnet-like network that keeps other cell wall components in place.

One clue to RG-II’s role emerged when O’Neill found that a dwarf mutant of Arabidopsis thaliana — a tiny weed related to cabbage and mustard — had normal amounts of RG-II in its cell walls but only half of it was cross-linked by boron.

“Without that cross-linking, the cell walls apparently don’t have the strength to expand normally and the plant is dwarfed,” O’Neill said.

A plant cell is like a water balloon in a cardboard box: The water in the balloon pushes outward and the box pushes inward, creating a strong building block that let plants stand tall. In the dwarf mutant, RG-II strands don’t cross-link so the box — or cell wall — is more like soggy cardboard.

O’Neill found that the mutant’s RG-II not only lacked the sugar L-fucose, but also substituted a different sugar in its place.

“The sugar substitution changes the shape of the molecule,” Darvill said. “As in all molecules — and in all biology — the shapes of molecules control their function.”

In a normal plant, boron binds to RG-II and forms a bridge that holds everything together. In the mutant, a little bit of the structure of the RG-II has been changed and because of the change in shape, it can’t hold the boron quite as well, Darvill said.

“It almost makes this carbohydrate analogous to proteins, where activity depends on their shapes,” Darvill said. “The boron is stuck between two molecules and holds them together. If you don’t allow that to happen, then you don’t get normal plant growth.”

With that finding, a decades-old mystery about the roles of boron and RG-II in plants finally was solved. But more mysteries remained, among them the curious case of RG-II, French wine and lead pipes.

“French wine has had problems with lead because a lot of the French wineries use old fermentation pipes,” said Peter Albersheim, CCRC co-director. “But the French don’t get poisoned even though they drink lots of red wine.”

The microbes that ferment grapes don’t appear to break down RG-II, allowing the carbohydrate to concentrate in the wine. That’s especially important because RG-II actually helps remove heavy metals like lead.

“In studies with scientists at the French agricultural institute, we found that this polysaccharide has a high affinity for heavy metals,” O’Neill said, “and so it virtually binds all the lead that’s in wine.”

Once RG-II binds a toxic metal, that metal is easier to remove. So CCRC and French scientists have patented different methods to make a form of RG-II to suck up heavy metals like lead from wine and even from human blood or tissues — a discovery that one-day could help people exposed to lead paint or other poisons.

For more information, contact adarvill@ccrc.uga.edu.

THE UNIVERSITY OF GEORGIA RESEARCH MAGAZINE : www.researchmagazine.uga.edu