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Chemical Ecology

Plants on the Warpath
The roots of combat

At some point in the 1970s, people started talking to houseplants. They figured plants would grow better if made the recipients of verbal nourishment and a little extra carbon dioxide. There was an even bigger presumption: Plants were gentle, peace-loving, tolerant organisms. (Hippies, obviously.)

But now we know the sordid truth: Some plants are stone-cold killers.

Consider the spotted knapweed, accursed invader of the American West. The dogma among ecologists is that invasive alien species thrive because they're free of the diseases, insects, and other enemies that keep them in check on their native turf.

Knapweed, however, has a secret weapon: Its roots secrete a chemical that kills other plants. This is known as allelopathy, and it's tough to prove because soil is such a dense stew of microbes, mites, nematodes, and all sorts of chemicals. How do you tweeze a toxin from the mix and know where it came from and what it's doing?

Colorado State University scientists recently managed to identify knapweed's killer chemical. They grew the plant in a sterile liquid, then examined the compounds its roots released into the liquid. When one chemical, catechin, was applied to the roots of other plants, it triggered the production of free radicals, which passed from the roots upward, activating a wave of cell death. In essence, a tiny amount of catechin induced other plants to commit suicide.

"People think plants are innocuous. We're showing that plants can be as mean as any animal," says Jorge Vivanco, leader of the team.

Plants may also use chemicals to "communicate" with one another.

In the case of knapweed, the message is simple: "You die now." (Vivanco's group calls this "negative communication." To say the least.) But sometimes the message, delivered to fellow members of the species, is something along the lines of "mites attacking; shore up your defenses."

For example, when lima bean plants are attacked by spider mites, they call out the cavalry, emitting a chemical distress signal that attracts carnivorous mites that eat the spider mites. The signal inspires nearby uninfested lima bean plants to do the same thing.

Because we humans are so biased toward visual and auditory signals, and don't tend to sniff everything and lick random objects, we don't realize how much the world around us is shot through with chemical warnings.

Plants don't make that mistake. When something crosses them, they take action.

So the next time you take a stroll in a garden, maybe you should be looking over your shoulder.

—Joel Achenbach
    Washington Post staff writer

Web Links

Pollination Tool Kit
Learn about the birds and the bees with the Ecological Society of America's introduction to pollination.

USDA Plants Database
Access fact sheets on thousands of native and naturalized plants in the U.S.

More Articles by Joel Achenbach
Read some of writer Joel Achenbach's columns for the Washington Post.

Free World Map

Agosta, William. Bombardier Beetles and Fever Trees: A Close-Up Look at Chemical Warfare and Signals in Animals and Plants. Helix Books, 1996.
Agosta, William. Thieves, Deceivers, and Killers: Tales of Chemistry in Nature. Princeton University Press, 2001.
Bais, Harsh, and others. "Allelopathy and Exotic Plant Invasion: From Molecules and Genes to Species Interactions," Science (September 5, 2003), 1377-80.
Proctor, Michael, Peter Yeo, and Andrew Lack. The Natural History of Pollination. Timber Press, 1996.


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