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Illumination magazine.
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Abscission in action on the MU campus.

A tree will drop its fruit in order to disperse its seeds. Or a plant may release its diseased leaves to keep the rest of the organism from becoming infected.

“All of these are programmed in the plant’s development or by changes in the environment,” Walker says. Plants come with predetermined “abscission zones,” places where a petal, for example, can safely drop away without injuring the plant.

Walker reaches for a houseplant on the windowsill of his office. He deftly picks off a leaf from the base of its stem. It comes off easily from the vine, right along its abscission zone. Then he rips a piece off the top of another leaf. It leaves a jagged tear. No abscission zone there.

“Abscission zones are a very specialized set of cells.
You’re not ripping cells apart, you’re causing separation between cells,” Walker says.

Abscission zones are formed as plants develop. Like all plant cells, cells in abscission zones stick tightly to each other. But when the abscission process starts, enzymes are formed that dissolve the “glue” holding the cells together.

After the leaf or petal falls, abscission zone cells remaining on the plant may form scar tissue to protect it from disease and keep it from losing moisture.“The plants have evolved this for a reason,” Walker says. But what they do naturally and what people want them to do may be two contrary things. “Wheat, corn, soybeans, citrus, we grow them in a very different way from how they evolved,” he says. “So they do things that we may not want them to do.”

For example, when soybeans are stressed by drought, abscission is triggered and the plants naturally drop their fruit, even though the farmer may be ready to irrigate. When agricultural plants become infected, a farmer may be able to spray them. “But the plant doesn’t know you can do that,” Walker says, so it drops its leaves to keep the disease from spreading.

While many plants engage in abscission, to get to the root of the process, Walker is focusing his investigation on a single perpetrator: Arabidopsis thaliana, a favorite of plant scientists.

“It’s the fruit fly of plants,” Walker exclaims. “It’s the lab mouse.”

Arabidopsis is related to the mustard plant and has seeds as tiny as grains of dust. The plants themselves are so small they can be grown abundantly indoors. “So instead of having fields, we can have growth chambers.” And Arabidopsis grows quickly, taking just six weeks to mature.
“We can have multiple crops a year.”

Arabidopsis was the first plant to have its whole genome sequenced; it has about 30,000 genes in all. And, as with fruit flies, genetic variants are readily available. “There’s a worldwide community that works on them and they share things, so there are a lot of resources out there,” Walker says. “If I am studying a gene I can go on the web and order a mutation of that gene.”

The MU has a long history with Arabidopsis. George P. Redei, a geneticist and professor emeritus at MU, brought its seeds to MU when he immigrated here after World War II (as recounted in Illumination’s Spring 2001 edition).

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Published by the Office of Research.

©2009 Curators of the University of Missouri. Click here to contact the editor.

 

Illumination home. Spring 2009 Table of Contents.