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a herbicide for organic agriculture perhaps
Poacher turned gamekeeper
A poisonous plant is proving a virtuous weedkiller, says Paul Simons
Thursday September 19, 2002
What is a weed? "A plant whose virtues have not been discovered," wrote
the American philosopher and poet Ralph Waldo Emerson in 1878. But he
did not reckon on a stroppy weed that makes a powerful weedkiller,
although the plant itself will probably remain on the public enemy list
of weeds for some time.
The spotted knapweed is a tall, spindly thistle running riot over large
parts of western US. There's long been a hunch that the knapweed had
some sort of chemical weapon that assassinated neighbours. In the
knapweed's native Asia, other wild plants are virtually immune to its
bite, but once it arrived in the US, it slaughtered the native flora and
is today one of the biggest botanical thugs.
Scientists struggled to identify the knapweed's weapon because it
rapidly gets lost in the soil. But at Colorado State University in Fort
Collins, the knapweed's roots were grown in a laboratory. The culprit
turned out to be a poison called catechin, and when it was tested on
other plants, it began killing cells within an hour and murdered plants
in about a week.
"We've tried it on about 20 weeds, and it worked against all of them,"
says Jorge Vivanco, who led the team. Even other species of knapweeds
were massacred. Yet the catechin has the extraordinary knack of leaving
grasses, and grass-like crops such as wheat, alone. This could be
because grasses evolved with the knapweed and so adapted to survive its
Catechin is as devastating as powerful man-made weedkillers such as
2,4-D, but the beauty of using such a natural herbicide is that it has
no known adverse effects. After all, it's been in the soil for more than
Plans are under way to culture the roots on an industrial scale, then
bottle the catechin herbicide for the marketplace. One of the weirder
aspects of the spotted knapweed is that it instantly makes and releases
its poison at the slightest hint of a threat: just tapping its leaves
launches an immediate chemical attack from its roots. Another mystery is
how the knapweed stops the catechin coming back into the root and
"It is a clever root to produce, secrete and protect itself from this
chemical, but if you inject it directly into the plant's cells, it kills
it," explains Vivanco. In fact, the knapweed leads a bizarre existence
by producing two mirror images of catechin. One form, known as negative
catechin, is the plant-killing poison, while its mirror twin, positive
catechin, kills off disease-causing bacteria in the soil.
The positive form has been known about for years, as an antioxidant that
gives a health kick to green tea and red wine, and is rumoured to hold
back the ravages of ageing, as well as helping attack cancers and
prevent heart disease. So, for a weed of little virtue, it's turned into
an astonishing story of poacher turned gamekeeper.
Guardian Unlimited © Guardian Newspapers Limited 2002