Turning a new leaf – Scientists discover promising new uses for knapweed

"When is somebody going to figure out a good use for knapweed?"

The question has been asked rhetorically, over and over, in western Montana, where spotted knapweed, or Centaurea maculosa, has, in a relatively short amount of time, established itself in perhaps the greatest concentration of any place in the world. Estimated to occupy more than 250,000 acres of Forest Service land in Ravalli County alone, spotted knapweed obviously has a competitive advantage over neighboring plants.

By ROD DANIEL Staff Reporter

For almost 100 years scientists have suspected that spotted knapweed had a secret chemical weapon that allowed it to kill other plants. And for the last decade, Ray Callaway, a University of Montana biology professor, has studied this phenomenon – known as allelopathy – in noxious weeds such as spotted knapweed.

Calloway’s research suggested that knapweed’s invasive nature is a result of an exudate, secreted from its roots into the soil, that acts as an herbicide, but lacking the expertise to isolate such a chemical, Calloway’s theory merely existed as a hypothesis.

Recently however, a Colorado State University horticulture professor, Jorge Vivanco, confirmed Calloway’s hypothesis by isolating the chemical exudate in his Fort Collins, Colo., laboratory.

Vivanco’s research found that spotted knapweed, indeed, produces a natural herbicide, but his experiments also revealed a few subtleties that might impact agriculture and weed control far more than anyone ever suspected.

This week, Dr. Vivanco paid a visit to the man who prompted his successful research and Wednesday gave a talk to professors and graduate students in U of M’s Biological Sciences Department titled "Chemical ecology of Centaurea maculosa root exudates."

Vivanco for years has studied the root exudates of plants, he said, attempting to take his laboratory research and apply it to the natural environment, so when he read Calloway’s paper, he felt like he could help further the research of the Missoula professor.

"Roots are always secreting exudates into the soil," he said "These exudates allow the plants to communicate with their environment, and many have properties that affect their environment." He cited basil as an example of a plant that secretes an exudate with antifungal properties.

But capturing a pure sample of these exudates in the soil is very difficult, Vivanco said

"Soil has too many microbes and other contaminants for us to be able to isolate a pure chemical," he said. "Therefore it is almost impossible to capture an exudate in the field."

So beginning in January 2001, Vivanco started with spotted knapweed seeds and grew them in a flask with water, vitamins and nutrients, so the roots were literally floating in the liquid. His goal was to prove that knapweed, if grown in a liquid medium, would secrete a phytotoxin (or herbicide) into the medium.

After the test-tube knapweed plants were a few inches tall, he removed them from their medium and extracted from the liquid a chemical he identified as catechin. The catechin actually existed in two forms – (-)catechin and (+)catechin – which are mirror images of each other.

Vivanco found that the (-)catechin acted as a phytotoxin, much like the common herbicide 2,4-D, when applied in low concentrations to the leaves of the weeds. Also like 2,4-D, it had no effect on grasses unless applied at a very strong concentration. In addition, he found the (+)catechin had anti-bacterial properties.

"Our discovery shows that Centaurea maculosa, by use of this root exudate, gets rid of other plants," he said, "and at the same time, fights off invading microbes in the soil."

Several companies are interested in isolating the (-)catechin and producing it as an environmentally friendly, natural herbicide, Vivanco said. And because it is a naturally occurring chemical, such companies might be eligible for fast-track approval by the Environmental Protection Agency, whose licensing process for new herbicides normally takes years.

So if things go well, a spotted knapweed catechin herbicide could be on the market in a year or two, he said.

But aside from the obvious benefits of using the knapweed exudate as a "green herbicide," Vivanco cited another discovery that may hold even more long-term benefits to agriculture.

The (-)catechin kills other species of knapweed, including Russian and diffuse, and other noxious weeds, such as Dalmatian toadflax and leafy spurge, but it doesn’t kill spotted knapweed.

"The only plant that is 100 percent resistant to it is spotted knapweed," Vivanco said.

Spotted knapweed cells don’t allow the (-)catechin to re-enter the plant once it’s been produced and released into the soil.

"It is a very clever root to produce, secrete and protect itself from this chemical," Vivanco said. "There are only small amounts of catechin inside the root at any given time. It secretes it almost as fast as it produces it."

Vivanco said if scientists could determine which genes in the plant are responsible for producing, secreting and protecting itself from the chemical, they may in the future be able to transfer, or splice, the genes into other plants. Such genetic engineering could give the recipient plants a built-in mechanism against weeds.

For example, Vivanco said, a potato plant with the chemical could secrete it into the soil and suppress competing weeds.

"In 10 or more years we may be able to produce a transgenic plant that can control its own weeds," Vivanco said.

But many unanswered questions remain, he said. For instance, while they know the catechin can exist in the soil for several years, they don’t know if older plants produce the chemical.

They do know, however, that the (-) catechin, when exposed to light, changes to (+)catechin. So according to Vivanco, "If you are going to spray it as an herbicide, you would have to do it at night."

Funding for his research initially came from CSU’s Invasive Weed Initiative, he said, and later more funding was provided by the EPA.

Vivanco said he believes this initial research is important, but acknowledges that much more research needs to be done in the field. And just as he took Ray Calloway’s research and furthered it with his own studies, he welcomes other researchers in other disciplines to move forward with what he has discovered.

"That’s what is so exciting about science," he said, gesturing toward Calloway. "We all can learn from each other."