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Kohorn Finding New Pathways in Plant Biology

Bruce Kohorn

Story posted July 26, 2007

It's easy to imagine that scientists embark on their career with a clear destination in mind: They know what aspect of the world fascinates them and they set out on a straight path to unlock its mysteries. In reality, however, the map to scientific discovery looks more like a spider's web of detours, fortuitous wrong turns and occasional dead ends.

Biology Chair Bruce Kohorn has spent more than a decade — first at Duke University and for the past six years at Bowdoin College — wending his way through his own personal fascination with what determines the size and shape of a plant cell.

In June, he won a four-year grant from the National Science Foundation to reveal the step-by-step communication between the cell wall and its nucleus. Kohorn has received NSF support for his research since 1990.

The vacuolar membrane of the cell (green), with the chloroplasts (red). The vacuole and sugars within are partly responsible for the pressure exerted against the cell wall, ensuring cell expansion.

Kohorn's journey took an unexpected turn in 1996 while he was studying photosynthesis, one aspect of plant-cell growth, in mustard plants. He and his colleagues made a major discovery: A certain type of protein is embedded in the cell membrane, with one end inside the cell and one end outside. The interior end is a kinase, a particular protein that can relay messages to other proteins. They named these proteins "Wall Associated Kinases," or WAKs, and went on to prove that they are necessary for cell growth. Remove them, and you stunt the cell's growth. But how do they work?

One theory involves the relationship between the WAK and pectin, the gelatinous outer layer of the cell membrane. If the pectin moves or binds to the WAK, it sends a signal to the inside of the cell. By removing, in turn, the WAKs and the pectin, Kohorn determined that both are required for the cell to grow. Together, they somehow activate the genes that make proteins that in turn produce sugar in the cell. Additional sugar in the cell attracts water from outside the cell membrane, expanding the cell like a water balloon and causing it to grow.

"But how did the gene get turned on, and how many other genes have been turned on or off?" Kohorn wonders. "There's a gap between the gene in the nucleus and the cell membrane. A lot can happen in between. Is there anything else that's changing?"

He is getting help unraveling these mysteries this summer, with the assistance of three research undergraduate fellows working in his lab. They are supported by a grant from the Howard Hughes Medical Institution.

Gene chip (detail), which in its entirety contains 30,000 spots, each representing a single gene. The intensity signifies how much the gene is on. The color shows if the gene is expressed more (red), less (green) or moderately (yellow).

A major part of their summer research is identifying other genes that might be switched on or off by WAKs in response to pectin changes. Kohorn is using current genome technology that allows a researcher to survey 10,000 genes at one time. This "gene chip" technology has led to the identification of genes that are dependent on WAKs and are activated specifically by pectin. Bulgarian student Tanya Todorova '09 and Rhysly Martinez '09 of Oakland, Calif., are helping Kohorn analyze the gene chips.

But a remaining question is how WAKs in the cell membrane influence genes in the nucleus. Based on extensive work by other scientists, Kohorn guessed that WAKs activate a chain of proteins leading to the nucleus. Kohorn has been able to identify two proteins in that chain; both are "MAP (mitogen-activated protein) kinases," which work in a particular way and give clues as to what may be upstream and downstream of them.

Kohorn doesn't know how long the chain is, so he doesn't know how many proteins he's looking for. With the help of Duncan Smith '08, from Nova Scotia, he is employing biochemistry and genetics to isolate proteins that bind to the MAP kinases and also to WAKs, with the hope of piecing together this chain.

"It would be nice if I had an idea what they are and what proteins they produce," he said. "We're narrowing in on just that one pathway."

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