POSTED 6 MAR 2003
Spotted knapweed is a biennial or short-lived perennial forb of the composite family that grows three to four feet tall. Most stem growth occurs in June. © John M. Randall/The Nature Conservancy, Wisconsin Department of Natural Resources.
Millions of acres of the Great Plains are covered with spotted knapweed, much to the distress of ranchers. Image: Nevada Bureau of Land Management.
Four plant species, untreated (1) and treated (2) with natural herbicide made by spotted knapweed. Notice that only spotted knapweed is immune to the herbicide. Plant images courtesy Vivanco lab
Remember this retort from Oliver North's lawyer during the Iran-Contra hearings? Good point. After all, what could be more passive than a plant?
Now
we hear that about one plant that is anything but inert. Spotted knapweed
mounts a good offense - creating a herbicide that's as powerful as 2,4-D
- to kill nearby plants. The invasive weed also produces a second version
of the same chemical to defend against bacteria in the soil.
Suddenly you understand why spotted knapweed and relatives in the genus Centaurea have been able to carpet 4.3 million hectares of the U.S. and Canadian Great Plains.
The first suggestion that spotted knapweed might release anti-plant agents into the soil dates to 1832. But identifying chemicals in the soil is tough, since there are so many of them.
Jorge Vivanco, an assistant professor of horticultural biotechnology at Colorado State University, decided to approach the problem by looking at plants instead of dirt. He grew spotted knapweed in a sterile liquid, vastly simplifying the task of ferreting out chemicals with biological activity. The tactic made sense, he says, since, "We'd observed from previous investigations that whatever the roots secrete in the soil, they will also secrete" when grown in sterile liquid.
His research team analyzed the resulting chemicals and found that all the anti-plant activity was associated with a single chemical, called catechin minus.
When they sprayed catechin minus on a variety of weeds and crop plants, it proved just as effective as 2,4 D, a potent and toxic chemical herbicide. When sprayed like any other herbicide, Vivanco says, 0.16 pounds of catechin minus will kill the plants on one acre. Normally, he stresses, the active ingredient is secreted in the soil, where it attacks the roots of competing plants, but when sprayed, catechin minus also kills leaf tissue.
Curiously, when Vivanco's team injected catechin minus into cells of spotted knapweed, they quickly croaked. So why doesn't spotted knapweed commit suicide with its "best defense is a good offense" strategy? Apparently because the plant's cells have evolved a way to quickly pump out catechin minus. If proven, this would be one of the first discoveries of the pump-out strategy in plants, Vivanco says. Bacteria commonly defend themselves by pumping out antibiotics.
Only the half of it
Catechin minus turned out to have a mirror-image chemical
that was also biologically active. Catechin plus, also made by the knapweed's
roots, is an antibiotic - bacteria killer - and antioxidant that's also found in green
tea, among other places. (Green tea also contains smaller concentrations
of catechin minus).
In the long struggle for survival in the wild, the two chemicals amount to a double-whammy, says Vivanco. "What we think is going on is that catechin minus acts as an attack compound against other plants, plus it acts as a defensive compound against bacteria. This is how it becomes invasive. It's so good at defending itself against microbes, and very good at being aggressive against other plants."
We're impressed. But there's more. Vivanco suspects that the one-two chemical punch becomes particularly intense when the spotted knapweed is stressed - even just touched by another plant.
More evidence that a plant can be just as aggressive as a sharp-tongued Washington defense lawyer.
Chemical warfare, plant-style
The Vivanco findings are part of revolutionary view
of plants as active agents in the environment. Look beneath the soil surface,
and plant roots are secreting gobs of survival-enhancing chemicals that:
Deter herbivores
As an unnamed biological control company tries to put catechin minus on the market, Vivanco sees no reason to suspect that either catechin is toxic to humans. We have, after all, been drinking them in green tea for centuries.
-- David Tenenbaum
Bibliography
Enantiomeric-Dependent Phytotoxic and Antimicrobial
Activity of Catechin. A Rhizosecreted Racemic Mixture from Spotted Knapweed,
Harsh Pal Bais et al, Plant Physiology, Vol. 128, Apr. 2002, pp. 1173-9.
Credits | Feedback
| Search
©2003, University of Wisconsin, Board of
Regents.