© John M. Randall/The Nature Conservancy, Wisconsin Department of Natural Resources.
Image: Nevada Bureau of Land Management.
Plant images courtesy Vivanco lab
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
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
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