Plants Can Substitute Cell Wall Sugar

June 21, 1996

WRITER: Phil Williams, 706/542-8501,
CONTACTS: Alan Darvill, 706/542-4411,
Peter Albersheim, 706/542-4404,



ATHENS, Ga. -- Let's say you have gone to the theater to see the musical Cats, but allthe costumes are still in another town. At the last minute, the management substitutes dog costumes for the missing cat costumes, hoping no one will notice the difference. Fat chance.

It turns out that something similar occurs in the plant kingdom, a dramatic substitution that allows enzymes to fulfill that old show biz axiom: The show must go on. Only in the plant world, the management seems to get away with it. The show does go on.

The show takes place in plant cell walls. A team of researchers from the University of Georgia has shown for the first time that an enzyme, faced with the absence of a specific sugar it normally incorporates into the cell wall, replaces it with a new sugar that is not a constituent of normal cell walls. And the plant, though damaged, is able to use the new sugar to stay alive and reproduce.

"This was really a surprising result because we thought the cell could not live if its walls lacked such an important component of all plant cell walls," said Dr. Peter Albersheim, co-director of the UGA Complex Carbohydrate Research Center. "But that wasn't the case at all."

The findings were published in the June 21 issue of the journal Science.

Albersheim and Dr. Alan Darvill, co-directors of the Center and long-time colleagues, have been pioneers in the study of plant cells walls. Some years ago they were the first to understand that cell walls did far more than determine cell size and shape as the plant develops. Cell walls are largely composed of long, branched "chains" of specific sugars attached to one another in a defined pattern. These sugars are called polysaccharides.

The scientists discovered that all cell walls of the edible parts of plants are composed of the same six well-defined polysaccharides. They found, too, that well-defined six- to 10-sugar pieces of these wall polysaccharides play a vital role in regulating growth and other essential functions of plant cells.

The research team working on the new discovery of sugar substitution included Dr. Earl Zablackis and Dr. William York of the CCRC, UGA undergraduate Stephen Hantus, and collaborators at the University of Connecticut and Purdue University.

The plant the team studied is a member of the mustard family called Arabidopsis. It is commonly used in the laboratory because its rapid growth allows faster studies of how it and all its parts change over time. Specifically, the researchers studied a chemically mutated version of Arabidopsis called Mur1. This mutant, obtained from their collaborators at Michigan State University, has an apparently normal life cycle but is more brittle and slightly smaller than normal plants.

The only obvious difference in the cell walls of the Mur1 mutant is the absence of a sugar called L-fucose in the above-ground tissues of the plant. However, this finding by the MSU researchers, published three years ago also in the journal Science, was a challenge to the University of Georgia team. Why? Because CCRC scientists had previously developed and published a hypothesis in which L-fucose was a component required for the biological activity of a nine-sugar subunit of one of the six cell wall polysaccharides. The researchers had found that the L-fucose-containing subunit acts as a hormone. (Hormones composed of sugars are called oligosaccharins.)

"Under normal selection, the cell walls of the `wild type' plant always have L-fucose in there," said Darvill. "The absence of L-fucose in the above-ground parts of the plant was the only thing we knew that was different about the mutant."

According to the UGA theory, the plants should not have lived at all when missing such a vital sugar as L-fucose. Instead, they lived and produced seed. What allowed the plants to survive? Was the theory of the CCRC researchers that oligosaccharins are essential for normal growth and development wrong? The answer turned out to be quite elegant and startling.

Dr. Zablackis took a closer look at what had happened, using a growth inhibition bioassay that Stephen Hantus had dramatically improved. Zablackis found that the enzyme responsible for inserting L-fucose in the plant cell wall had done a remarkable thing.

Since L-fucose was not available in the mutated plant Mur1, the enzyme found the next best thing -- a seldom-seen sugar called L-galactose, whose three-dimensional shape, but for an extra oxygen atom, is almost identical to L-fucose. The L-galactose took over the functions of the L-fucose as well as it could -- well enough to keep the plant alive though somewhat deficient.

Just as surprising, the L-galactose sugar the enzyme substituted when it found no L-fucose in the mutant is rarely seen in plants, though its chemical cousin, D-galactose (which does not resemble L-fucose) is always present.

The substitution of one sugar for another was the first ever confirmed for plant cell walls, but Albersheim said the significance of the experiment goes far beyond just understanding a particular process.

"A lot of regulation and control that we don't understand yet are going on here involving these sugars," he said, "but this is strong evidence that oligosaccharins are involved in regulating cell wall growth."

Just why the substitution of L-galactose for L-fucose happens is not yet clear, but Darvill theorized that there might be some flexibility in the enzyme that normally inserts L-fucose into the cell walls. Zablackis put it succinctly.

"People think of enzymes as a rigid, lock-and-key kind of thing," he said. "A key to understanding them may be the fact that they do make substitutions."

University of Georgia

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