How Plants Expand

A plant cell wall is like a sweater. Sort of. Dan Cosgrove plucks at his own sweater and laughs. "It's like a fabric …" He abandons the analogy with a shrug. "A plant cell wall is made of interlinking fibers, that's clear." It's a rigid wall, responsible for both the size and the shape of the cell. And yet it has to expand to let the plant grow. In the tip of a root or shoot, the new-made cells are very small—only 5 microns in diameter. "When they're done growing," says Cosgrove, a professor of biology at Penn State, "they're hundreds of microns in length. That's a huge volumetric increase.

"Animal cells don't do this," he continues. "That's why trees are so big and rats are so little."

blue dyed proteins

The fabric of the plant cell wall is a complicated weave. On a diagram Cosgrove has drawn, the wall has three fibrous components. First is cellulose, organized in "neat arrays of crystals," or "microfibrils," depicted here like bundles of black cables. "Then there's the stuff in between. It's much more disordered, and the strands are long enough that they're woven around the cellulose-" Cosgrove pauses. "But of course, it's too small, there are no techniques for seeing this, what I'm telling you are just imaginings, not based on hard microscope studies." The disordered stuff is a polysaccharide, interpreted in the diagram as long, snaky red threads looping around and linking onto the cellulose. The third component, noted here by a yellow wash, is pectin. "It's another set of long polymers. We're not drawing them individually because it gets too tangled. They make almost a gel-like matrix. And they're not where expansins work."

Expansins, Cosgrove discovered, are the class of proteins that makes cell walls expand, letting the plant grow. "Expansins work at the bonds stitching the red strands to the black bars, the microfibrils," says Cosgrove, whose team has found more than 30 expansin genes. "You see expansins in all kinds of land plants down to mosses and Marchantia—that's liverwort, a very primitive plant. You don't find them in flies, worms, humans, or yeast. This is an invention that plants made somewhere along the line."

And the plants are keeping their secret. "Before we got into this field, everybody thought it would be like a pair of scissors, some sort of cutting enzyme, that cuts the glue that holds the fibers together."

It's not. Expansins, Cosgrove says, "work in a very idiosyncratic way." Instead of cutting, they loosen. They unwind. They unravel. "We don't have any good examples of that kind of protein activity. We don't really even know how to test it.

"We can see it. We can apply it to a plant and see the wall extending," but chemically the cell wall afterwards seems no different than it did before adding the expansin, "except it's shifted. We can't say exactly what biochemical bond has changed. The biochemistry is very intriguing." Cosgrove imagines that the expansin "unglues" the snaky threads of polysaccharides and lets them pull apart a bit before their natural stickiness glues them back together again.

Each part of the plant has its own suite of expansins: the guard cells that open and close the leaf pores, the veins, the root tips, the root hairs, where the petals fall off. "Plant cell types all look different," Cosgrove says; in print he's called it "a menagerie." And since the shape is determined by the wall, Cosgrove suggests, "expansins give rise to specialization of cell forms. These proteins," he adds, "are crucial both for how plants grow and how fibers form. Knowing how they work gives people the power to manipulate that. For instance, to make fibers longer and stronger, or weaker and fall apart better."

Expansins could help, for example, recycle paper. "To make paper, you beat the fibers apart so they spread out. Then you dry them. In recycling, the hard part is to get the fibers to fall apart again. You could possibly use expansins to solvate the paper, to get the fibers to loosen up and make it more readily recyclable."

To do so, though, would require "bucketloads" of expansins, and so far Cosgrove's team has been unable to coax E. coli or yeast or another microorganism to make the proteins. "Most organisms choke on them. They gag on them. They break them down as fast as they're made," he says.

mauve and green dyed cell wall

"Plants make expansins all the time, but not in large quantities." Except for grass pollen. "Grass pollen is making gobs of it."

When Cosgrove's team had sequenced their first expansins, they looked in the plant genome databases for similar genes—the only thing like it was the major allergen in grass pollen. "So we got the protein and tested it, and it had beautiful expansin activity." While it's the physical properties of the protein that make it react with the body's immune system, knowing it was an expansin let Cosgrove see what it did for the plant: "It loosens up the stigmatic walls. When a pollen grain lands," he says, "it has to grow a tube and force and push its way between the maternal cells to deliver the sperm. The pollen tube is pouring out this strange expansin to loosen up the maternal tissue and deliver the sperm more quickly."

Currently, Cosgrove is trying to extract expansin in usuable form from corn pollen, since corn, or maize, is actually a grass. "Ever looked at a maize plant when it's shedding? When the pollen lands, you can hear them, they're so heavy. They're huge. We were collecting kilos of pollen. A single maize plant will give us a gram of pollen itself."

Getting the pollen to release its expansin has been trickier. But if he does manage to come up with "bucketloads" of it, Cosgrove has other "potential commercial uses" in mind besides paper recycling—some of them admittedly far-fetched, but fun to think about. Detergents, for instance. "You've heard of the depilling enzymes in some detergents already?" he asks. "They take off all the little 'pills' on your sweaters—they have an affinity for lumps of fabric. Well, how about a detergent with expansins added? We'll call it 'Expandex.' Wash your clothes in it and they'll gently loosen. We'll have to get the dosage just right." One cup per load, say, and your pants will expand exactly one size.

Daniel J. Cosgrove, Ph.D., is distinguished professor of biology in the Eberly College of Science, 208 Mueller Lab, University Park, Pa 16802; 814-863-3892; dcosgrove@psu.edu. His work is funded by the National Science Foundation, the Department of Energy, NASA, and the USDA.

Last Updated May 01, 2001