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Columns::January 28, 2002
State budget proposal includes pay raises, other UGA priorities
Former White House chief of staff to speak at annual conference
Computational Center director wins chemistry award
UGA celebrates the life, legacy of Martin Luther King
Student ‘ambassadors’ visit area high schools
Signed, sealed, and delivered
A dollar could have bought a lot more
Vet medicine professor puts the bite on infectious animal diseases
Newsmakers
Administrative Changes
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Campus News
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| (From left): Peter Albersheim, Alan Darvill, Malcolm O’Neill and Stefan Eberhard have found evidence that normal plant growth depends on how the complex carbohydrate RG-II is organized in cell walls. (Photo by Peter Frey) |
Beyond description
Complex Carbohydrate Research Center scientists discover function of complex plant carbohydrate
By Kathleen Cason
kmc@ovpr.uga.edu
Twenty-five years ago, Alan Darvill and colleagues described one of the most complex carbohydrates found in nature. Called rhamnogalacturan II, or RG-II, it is found in plant cell walls.
The carbohydrate is found in all higher plants, and it requires a host of different proteins to manufacture. Yet for more than two decades, scientists haven’t had a clue about its function.
“In a sense, we’ve been on a quest to understand what it’s doing in plants ever since we discovered it,” says Darvill, professor of biochemistry and co-director of UGA’s Complex Carbohydrate Research Center.
With a lot of hard work, the efforts of dozens of scientists around the world--and a bit of luck--that quest has come to an end.
In an article published this past fall in Science, CCRC scientists present evidence that normal plant growth depends on how RG-II is organized in cell walls.
“RG-II has been known as an obscure, structurally weird polysaccharide that plants make,” says Malcolm O’Neill, senior research associate at 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.”
The work of O’Neill, Darvill, Stefan Eberhard and Peter Albersheim shows that normal plant growth depends on the ability of RG-II strands to crosslink with boron and form a network in the cell-wall matrix. Boron crosslinks RG-II strands together in a fishnet-like structure that holds other components in the cell wall in place.
“You can think of the cell wall as a bit like reinforced concrete,” O’Neill says. “Tiny cellulose fibers are the steel rods and the matrix is the concrete. Only in the cell wall, the concrete has the consistency of Jell-O.”
One clue to RG-II’s role emerged when O’Neill found that a dwarf mutant of Arabidopsis--a relative of cabbage and mustard--had normal amounts of RG-II in its cell walls but only half of the RG-II was crosslinked by boron.
“Without that crosslinking, the cell walls apparently don’t have the strength to expand normally and the plant is dwarfed,” he says.
Since the Arabidopsis mutant was known to lack the enzyme that makes the sugar L-fucose, O’Neill says he suspected that RG-II in the mutant would be deficient in fucose. RG-II is made of about 30 sugars arranged in a chain with four sidechains and L-fucose is found on two of the sidechains. O’Neill found that the mutant’s RG-II not only lacked fucose, but also substituted a different sugar in its place.
He also discovered that when mutants received fertilizer containing L-fucose, plant growth was normal. That’s because the plants contained normal amounts of fucose and RG-II molecules occurred as the boron- crosslinked form.
“The sugar substitution changes the shape of the molecule,” Darvill says. “As in all molecules--and in all biology--it’s the shapes of molecules that 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 says. Fertilizing mutants with high levels of boron also reversed dwarfing because the high amount of available boron effectively forced RG-II to crosslink.
“It almost makes this carbohydrate analogous to proteins, where activity depends on their shapes,” Darvill says. “Here’s a carbohydrate that has a shape that has an activity.” This work shows answers to both the role of boron and RG-II in plants, Darvill says.
“The boron is stuck between two molecules and holds them together,” he says. “If you don’t allow that to happen, then you don’t get normal plant growth.”
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