'suicide Response' In Plants Gives Colorado State Researchers Important Clues On Preventing Bacterial Invasion In Crops
Tuesday, January 6, 1998
FORT COLLINS--Colorado State University scientists have made
an important breakthrough in studies of bacteria that cause
disease in crops worldwide but whose basic functions have eluded
researchers for years.
The breakthrough proves that plants use an arsenal of gates
and passwords to defend themselves against a group of gram-
positive bacteria--known as club-shaped bacteria--the same way
they defend themselves against other types of bacteria. The
discovery could ultimately lead to the development of
genetically-engineered crops that resist a wide range of
diseases.
The study centered on bacteria that causes ring rot in
potatoes, a disease that spawns millions of dollars in prevention
and treatment costs on American farms each year. A single plant
with symptoms of ring rot infection in a field of potatoes can
cost a farmer as much as $80,000 in lost revenue. Because ring
rot spreads so quickly and potato plants have zero tolerance for
the disease, potatoes grown in the United States must be
inspected and certified disease-free before being shipped out of
state or overseas.
When this bacteria encounters its potato plant host, a
series of complex interactions take place that signal the
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bacteria to invade and infect the plant. In their study, Colorado
State researchers found that plants that are not specific hosts
for these gram-positive bacteria display a much different
reaction known as a suicide response. Like an internal
quarantine, the plant protects itself from bacterial invasion by
killing its own cells and surrounding the bacteria, thus cutting
off the potential for disease. The Colorado State project
documented this response in tobacco plants exposed to the potato
ring rot bacteria.
While the suicide response has been documented in plants
exposed to another group of well-known bacteria called gram-
negative bacteria, Colorado State researchers were the first to
document this reaction in plants exposed to gram-positive
bacteria. Plant breeders have successfully engineered plants
resistant to diseases caused by gram-negative bacteria, which has
been a focus of research for a number of years. But gram-positive
bacteria have remained a mystery, in part because they are
difficult to grow in a laboratory setting and because they grow
very slowly, which hinders research.
Yet gram-positive bacteria are a particularly troublesome
class of bacteria that cause a wide range of diseases in a number
of crops, including wheat, potatoes, alfalfa and apples. Once a
crop is infected with a disease caused by a gram-positive
bacteria, it is almost impossible to combat or is too costly to
treat with chemicals.
"This is an exciting find because the suicide response has
been used as a tool for cloning of disease resistance genes from
tomato, lettuce and rice," said Carol Ishimaru, a lead researcher
on the project and associate professor of bioagricultural
sciences and pest management at Colorado State. "Now that we know
this suicide response occurs in plants exposed to this specific
form of bacteria, we can work on finding ways to develop potato
plants that give this suicide response even when they are invaded
by bacteria for which they are hosts."
Ishimaru said the suicide response is the result of a
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sophisticated interplay between plant and bacteria that works
like this: When a disease-causing bacteria invades a plant, the
plant will only display a suicide response if it has the
resistance genes for that bacteria. The plant host manufactures
products encoded by resistance genes that recognize specific
antivirulence genes, or genes that fight infection, made by the
bacteria. In plants that aren't hosts for bacteria--like the
tobacco plants used in the Colorado State study--the same defense
reaction is triggered but it is not clear what activates the
response.
The next phase of research at Colorado State will
investigate the cues gram-positive bacteria give off that
activates the suicide response and why plants do not express the
suicide response when they are exposed to bacteria for which they
are hosts.
"This has led to the identification of genes in gram-
negative bacteria that are required for the bacteria to cause
disease," Ishimaru said. "We hope there will be features in the
ring rot bacteria that are common to gram-negative bacteria as
well. If we find common traits, we may be able to develop ways to
prevent disease in crops affected by forms of gram-positive
bacteria."
Ishimaru has collaborated on the research with a number of
other Colorado State faculty, including Dennis Knudson, Susan
Knudson, and Penelope Bauer, all of the department of
bioagricultural sciences and pest management; and Nora Lapitan,
professor of soil and crop sciences.
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