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a herbicide for organic agriculture perhaps



Poacher turned gamekeeper

A poisonous plant is proving a virtuous weedkiller, says Paul Simons

Paul Simons
Thursday September 19, 2002
The Guardian

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 thuggery.

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 a century.

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 poisoning itself.

"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

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