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Daniel Cosgrove is revising the cell-wall model

AAAS Fellow Daniel Cosgrove is optimistic that the mysteries of plants—including how their walls slip and slide—can be unlocked. (Photo: Dan Cosgrove)

AAAS Fellow Daniel Cosgrove faced a dilemma in 1992 while studying plant cell walls. The data were just not adding up.

He and his team were investigating the structure of plant cell walls, looking for an enzyme that was believed to be responsible for loosening the cell wall, enabling plants to grow into long blades of grass or towering redwoods.

What they found, however, was not an enzyme at all. It was a protein, which they named expansin, that loosens the connections between some cellulose microfibrils.

This discovery called into question the widely accepted plant cell-wall model. At first Cosgrove was skeptical of his findings, since this was the model he also had worked with for his entire career. But after multiple tests, he eventually was convinced the data were correct.

"You have to have the courage of your conclusions and say, 'Well, this doesn't make sense. I am not going to discount the data ... I think the model is wrong,' " said Cosgrove, who is a biology professor at Pennsylvania State University.

He has been working on revising the plant cell-wall model ever since and is one of many scientists worldwide who are actively debating models of plant cell growth.

At issue is the arrangement of cellulose, the main material of the cell wall. Cellulose microfibrils are made up of long chains of glucose polymers strung end-to-end. They help give the cell wall shape and make up most of the plant's weight. Many manmade products are made from cellulose, including cotton clothing and paper. Cellulose also is a base for cellulosic ethanol, a leading biofuel.

"Humans have been using cellulose for thousands of years, have known about its rudimentary chemistry for 200 years, and we still don't know how plants make it and we are still arguing about the structure," observed Cosgrove.

Historically, many biologists thought that cellulose fibers in the growing walls of plant cells did not come into contact with each other—that essentially, the entire wall was involved in growth mechanics. But Cosgrove and other experts are proposing different arrangements that look more like a fisherman's net, where cellulose microfibrils come into contact with each other only at few key junctions—which may be the main spots responsible for growth.

Cosgrove became fascinated by plants as a boy growing up in Massachusetts. He had a paper route to earn money, which he spent on a monthly subscription to science kits. One kit focused on plants, featuring projects like watching their roots grow through twisted glass tubes, or staining their leaves. Little did the young boy know he'd one day help change the way the world thought about those roots and leaves.

The self-described "science-nerdy kid\ went on to major in botany at the University of Massachusetts, and then earned a Ph.D. from Stanford University. During a postdoc, he developed methods for understanding how water gets into the plant cells as they expand.

"Those results led me to believe all the magic is in the cell wall," he said.

Cosgrove now directs the Center for Lignocellulose Structure and Formation (CLSF), an Energy Frontiers Research Center funded by the U.S. Department of Energy. Along with biochemistry and genetics, the interdisciplinary team uses nuclear magnetic resonance, small-angle neutron scattering, electron microscopy and infrared spectroscopy to try to identify the physical arrangement of atoms in cellulose.

If the group can figure out how the cell wall is specifically structured and expands, it will be much easier to identify the best way to take it apart for biofuel production. While fuel derived from plants has been in production for decades, it is only on a small scale compared to petroleum. If the energy stored in cellulose can be efficiently converted into fuel for cars and trucks, driving could become a more carbon-neutral activity.

While success is still a way off, Cosgrove is optimistic that the mysteries of plants—including how their walls slip and slide—can be unlocked.

"Human society is facing lots of challenges these days," Cosgrove said. "I think plants have a lot of the solutions to those societal needs. To convert that potential to the reality, it requires understanding how plants do their thing."