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Abscisic Acid—The Plant Stress Hormone
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Postdoctoral research associate Todd Linscott
evaluates cold hardiness of wheat breeding lines after a simulated
freezing test.
(K9211-1)
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Every fall, wheat farmers in the
Pacific Northwest gamble their next year's fortunes: They plant their seeds.
Then nature rolls the dice. Will it rain enough to support a good crop? Will a
sudden freeze kill the plants?
Several scientific discoveries may one day reduce such risks for growers of
wheat—and perhaps of other crops as well. For example, researchers have
long known that a hormone called abscisic acid (ABA) plays a role in how plants
respond to weather stresses, such as cold and drought.
ARS plant physiologist Kay
Walker-Simmons and chemist Sue Abrams, who's with the National Research Council
of Canada, discovered features of the ABA molecule that affect wheat and barley
plants.
"ABA slows seed germination and improves wheat's tolerance to cold and
drought," says Simmons. "We showed that by chemically altering ABA,
we could increase these beneficial activities." Simmons led the ARS Wheat
Genetics, Quality, Physiology, and Disease Research Unit in Pullman,
Washington, until last October. Now she's ARS' national program leader for
grain crops. |
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Postdoctoral research associate Benjamin Rangel
conducts an electrophoresis gel test of candidate genes and DNA
markers associated with sprouting resistance and cold hardiness
in wheat.
(K9210-2)
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Plants normally produce an enzyme
that breaks down ABA. When the ABA levels drop low enough, the hormone can no
longer inhibit germination, and the seeds sprout. But sometimes the plants
would fare better if they delayed germinating. For example, light rains in the
Pacific Northwest might trigger wheat to germinate. But if the weather dries up
before the more substantial rains begin, the tender seedlings could die. If the
seeds waited until more water was available before sprouting, they would be
less likely to suffer from drought.
Abrams modified the part of the ABA molecule that's broken down by the enzyme.
Then she and Simmons demonstrated that chemically blocking the enzyme allows
ABA to stay active longer. This means that some sort of seed treatment with the
altered ABA might provide the extra delay needed to improve wheat survival
during drought conditions.
Earlier, the researchers found the parts of the ABA molecule that regulate
whether a wheat plant germinates prematurely. "Mature wheat kernels can
sprout in the head when it rains just before harvest," Simmons says. Such
rains fall about 1 year in 5 in some areas of the Pacific Northwest and with
varying frequency worldwide. Farmers can sell the sprouted wheat for animal
feed but lose up to half the crop's value. (See "Hormone Snippet to
Kernel: Don't Sprout!" Agricultural Research, April 1995, p. 23.)
Simmons' team also identified an array of genes that appear to partially confer
drought and cold hardiness. They hope to use this information to make better
plant selections for breeding programs. |
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Plant physiologist Kay Walker-Simmons examines
club wheat heads from the Pacific Northwest for preharvest sprouting
damage.
(K9213-2)
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"When plants are stressed, these genes respond to the presence of ABA by
producing proteins that somehow help the plants better withstand cold or
drought," Simmons says. Although the researchers are still unraveling
exactly what the genes and their proteins do, they all seem to bind well with
water.
The findings may have unique applications. Simmons worked with ARS scientists
in Beltsville, Maryland, to show that some of these water-binding proteins
could protect turkey sperm during cold storage. She believes the proteins may
act as buffers, preventing damage from very rapid hydration or dehydration.
"Our next goal was to discover how ABA signals plants to respond to cold
and drought," Simmons says. Her team found a likely trigger: an enzyme
known as protein kinase. "Protein kinases are involved in many signaling
responses. They can cause rapid changes in the structure and function of other
proteins," she says.
Simmons and graduate student Robert Anderberg discovered the first protein
kinase involved in plant responses to ABA. She believes that selecting plants
that make this kinase faster or in greater abundance may enhance the plants'
ability to withstand environmental stress. Her team has cloned the genes that
produce the protein kinases, and ARS has applied for a patent on use of the
genes. Simmons' team chose to study winter wheat plants because of their
importance as a crop and because they tolerate environmental stress better than
many other plants. But because ABA is a key hormone in all plants, their
findings may be useful for improving other crops, as well.
Her group is also contributing to understanding the complete function of the
genes in the wheat seed. As part of the wheat genome project, she's supplying
tissue samples of dormant and ABA-treated seeds to ARS scientists in Albany,
California, for DNA sequencing. The project is funded by the National Science
Foundation and involves ARS and 11 universities. The project aims to identify
all the genes expressed in wheat seeds and to determine their function.
"I've spent most of my career looking at individual genes," says
Simmons. "Now we'll be able to look at hundreds of genes that are
expressed when plants are subjected to environmental stress. That's a whole new
level of research."—By
Kathryn Barry
Stelljes, Agricultural Research Service Information Staff.
This research is part of Plant Biological and Molecular Processes, an ARS
National Program (#302) described on the World Wide Web at
http://www.nps.ars.usda.gov.
Kay Walker-Simmons is the National
Program Leader for Grain Crops, National Program Staff, 5601 Sunnyside Ave.,
Beltsville, MD 20705-5139; phone (301) 504-5560, fax (301) 504-6191. |
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"Abscisic Acid—The Plant Stress
Hormone" was published in the
January 2001
issue of Agricultural Research magazine.
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