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One Strong Carbohydrate Kathleen Cason |
Red wine is rich in a mysterious carbohydrate that has been found, among other things, to be central to vertical plant growth. |
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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, weve been on a quest to understand what its 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 molecules structure cause growth defects. RG-II has been known as an obscure, structurally weird thing that plants make, said Malcolm ONeill, 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. Theres even one sugar thats 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-IIs role emerged when ONeill 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. |
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UGA researchers solved a 25-year puzzle by discovering the role of an obscure plant carbohydrate. (left to right) Malcolm ONeill, Peter Albersheim, Stefan Eberhard and Alan Darvill. Without that cross-linking, the cell walls apparently dont have the strength to expand normally and the plant is dwarfed, ONeill 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 dont cross-link so the box or cell wall is more like soggy cardboard. ONeill found that the mutants 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 cant 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 dont allow that to happen, then you dont 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 dont get poisoned even though they drink lots of red wine. The microbes that ferment grapes dont appear to break down RG-II, allowing the carbohydrate to concentrate in the wine. Thats 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, ONeill said, and so it virtually binds all the lead thats 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, email adarvill@ccrc.uga.edu. |
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Research
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