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Knapweek, an exotic plant killer

Thought you might find this interesting since it can kill other garden plants.
September 9, 2003
Forensic Botanists Find the Lethal Weapon of a Killer Weed
or over a century, spotted knapweed has been a growing scourge on the
North American landscape, spreading across millions of acres of
prairies, hillsides, roadsides and rangeland  pretty much anywhere it
can get a root in the dirt. Everywhere it spreads, it replaces native
grasses and other plant species to the consternation of conservationists
as well as ranchers, whose cows refuse to eat it.

The weed, which sprouts pink and purple flowers and can grow a spindly
three feet tall, is a European import, thought to have been introduced
in North America as a contaminant in crop seeds or in dirt used as
ship's ballast and then dumped. But scientists have long been baffled by
the plant's appalling effectiveness at driving out other plants.

Now in the current issue of the journal Science, researchers say they
have found spotted knapweed's deadly secret: a potent and previously
unknown poison that it releases through its roots into the soil to kill
off neighboring plants. By eliminating its neighbors, the weed can
appropriate all the water and nutrients that the other plants would have
taken, and it has plenty of new space to spread out in.

Dr. Jorge M. Vivanco, a plant biologist at Colorado State University and
an author of the study, says the toxin acts so quickly that within 10
seconds of contact the neighboring plants' roots begin producing
chemicals that set off a cascade of events that will ultimately kill
their own cells.

"In one hour the roots die," he said. "The whole plant dies in a matter
of days." The substance is such an effective herbicide that, Dr. Vivanco
said, his university had already taken out a patent on it.

Scientists often assume that invasive exotic species are able to thrive
in new environments because they have escaped from their predators and
other enemies at home. But scientists say the new study suggests that
such troublesome imports may also succeed by using potent but
unrecognized methods, like chemical warfare.

"This is a really nice demonstration that other factors come into play,"
said Dr. Sarah Reichard, an invasion biologist at the University of
Washington. "This paper shows that the interactions can be very subtle,
things happening below ground that we really haven't had any knowledge

The notion that plants use poisons to suppress or kill their neighbors 
a phenomenon known as allelopathy  has been around for decades. But
until now, few scientists have had much use for it.

"People have been rather dismissive of the whole subject," said Dr.
Alastair Fitter, an ecologist at the University of York who was not
involved in the study.

Part of the problem was that much of the earliest work was poorly done,
he said in a telephone interview. But as Dr. Fitter wrote in an
accompanying commentary in Science, he believes the new study is so
convincing that it will "now place allelopathy firmly back on center

The researchers found that the roots of the spotted knapweed released
two forms of a chemical known as catechin (pronounced KAT-uh-kin)
identical in all respects except that their molecular structures were
mirror images of each other.

One form, known as +catechin, is also found in green tea and was already
known as an antioxidant, able to neutralize the harmful molecules called
reactive oxygen species that are thought to speed the aging process.

The toxin turned out to be the second form, -catechin, which had
essentially the opposite effect of its mirror image. It induced the
production of harmful reactive oxygen species in neighboring plant
roots, setting off the process that led to cell death.

The finding helps explain the failure of many efforts to fight the
onslaught of spotted knapweed by burning it and then seeding the area
with desired plants.

"What they've seen is that 99 percent of the seeds died, and now we know
why," said Dr. Vivanco. With -catechin soaked into the soil, he said,
susceptible seeds have no chance of making it.

But even though the poison is very powerful, it remained unknown to researchers because everything was happening below ground.
"One plant arrives in a field where there are a lot of native plants,"
Dr. Vivanco said. "The next year you see not one, but actually a patch
of spotted knapweed where the natives were. And if there are still
native plants near it, they don't look so healthy."

Around Missoula, Mont., home of the University of Montana, for example, a diversity of native species once bloomed.
Now after several decades of this subtle underground warfare, the hills
have become a vast monoculture of spotted knapweed, Dr. Vivanco said, as
have millions of acres in that particularly hard-hit state.

The scientists found that the grasses that grow alongside spotted
knapweed in Europe are much better able to resist its toxins than native
North American grasses. Scientists say this suggests that the European
grasses have evolved a resistance to this potent toxin, one that North
American grasses lack.

Since spotted knapweed landed in North America, a century or so ago, it
has spread to nearly every state and has caused a variety of problems.

Eric Lane, the state weed coordinator for Colorado, said the loss of
native plant species curtailed the food supply not only for cattle but
for wild species like elk, many birds and insects. In some states, he
said, the spread of spotted knapweed is so severe that elk herds have
altered migration pathways to avoid vast inedible swaths of it.

The weed has also led to erosion because it does not hold soil as well as native grasses.
In the search for solutions to this green plague, researchers were
excited to discover that the plant Arabidopsis thaliana, whose entire
genome has already been sequenced, is susceptible to -catechin. As a
result, they can see in detail how a plant's genome reacts when its
roots are hit with the toxin.

The scientists found 10 genes that appear to shift into high gear
immediately. Scientists say they hope that by identifying what those
genes are doing, presumably mounting the beginnings of a defense, they
can genetically engineer plants that can more effectively resist the
spotted knapweed's attacks.

Researchers are also testing to see what native plants are resistant to
the -catechin. They hope to develop a list of species that can be used
to revegetate an area after spotted knapweed has been burned.

So far, the researchers have found no native plants that can withstand the poison.

Copyright 2003 The New York Times Company

Bonnie Zone 6+ ETN

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