Colorado State University Identifies Natural, Plant-Produced Herbicide

The discovery and isolation of the chemical, called catechin, within spotted knapweed may revolutionize the war against weeds for homeowners and farmers.

"For years, scientists have talked about spotted knapweed releasing this chemical, but they couldn't find it in the soil because it was almost impossible to separate from all the other compounds that naturally occur in soil," said Jorge Vivanco, assistant professor of horticultural biotechnology at Colorado State. "We looked for it in the plant. Spotted knapweed releases catechin into the soil through its roots."

Now that catechin has been identified and isolated, and scientists can capture the chemical in the Department of Horticulture's laboratory, Vivanco and a team of researchers at Colorado State are investigating a wealth of applications for the chemical. They have discovered that the weed produces two types of catechin that are the same chemical compound but the mirror image of each other in structure. One has anti-bacterial properties and the other acts as a natural herbicide.

The chemical acts as a natural herbicide to most other plants, although grasses and grassy-like plants, such as wheat, display some resistance to it. This discovery alone holds much potential. For example, it may mean that specific amounts of catechin could be used on lawns to kill weeds without killing grass or on wheat without damaging the crop. The chemical also is environmentally friendly and has existed in the soil for decades.

Catechin kills other species of knapweed, such as diffuse knapweed, which also is a noxious weed. It is fatal to spotted knapweed only when manually inserted into its cells in a laboratory. In nature, spotted knapweed cells do not permit catechin to reenter the plant once the chemical is produced and released into the soil.

"It is a 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 as it produces it."

The Colorado State team has found that spraying catechin on plants or adding it to soil is as effective as 2,4-D against pigweed, lambs quarters and other common weeds. Catechin usually kills cells within the plants in an hour and kills the plants in about a week, but the team still is investigating the length of time that it remains active in the soil to prohibit plant growth. The researchers are working with commercial companies to make spotted knapweed catechin spray available to consumers within a year or two.

Colorado State researchers also are working to transfer the genes that produce the natural chemical into other plants to give them a built-in defense mechanism against weeds.

Perhaps one of the most promising applications of the discovery is the fact that spotted knapweed has such a complex defense mechanism. Spotted knapweed immediately begins to produce and release chemicals at the slightest hint of a threat or stress. Just tapping its leaves automatically activates the plant's chemical response.

The funding for these projects comes from Colorado State University's Invasive Weeds Initiative.

For more information about this story and other related Colorado State projects, visit Colorado State AgNews at http://www.agnews.colostate.edu.

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Spotted knapweed catechin fact sheet

-- What is catechin?

Catechin is a phytotoxin. Scientifically, catechin in spotted knapweed is even more remarkable than just a natural herbicide. Spotted knapweed produces a compound of two different kinds of catechin. The two different types of catechin are produced and secreted by spotted knapweed at the same time and in the same amounts. They are structurally the same, but one is produced as the mirror image of the other. One catechin, called (-)catechin, is the natural herbicide that is toxic to other plants. The other, called (+)catechin, has antibacterial properties and has been commercially available for several years as an antioxidant and anti-aging compound. Until now, (-)catechin was considered a by-product with no commercial value.

These mirror-image compounds are called racemic and are very rare in nature. They occur in chemical synthesis in laboratory settings, but the spotted knapweed's catechin is one of the first examples of racemic compounds that occur naturally in plants. It's also particularly interesting, notes Jorge Vivanco, Colorado State University horticulture professor, that each compound has a different activity.

-- How long have scientists known that spotted knapweed may secrete chemicals?

This theory was suggested as early as 1832, but scientists have not been able to identify the chemical secreted by spotted knapweed that is responsible for killing other plants.

-- How many different kinds of knapweed are there?

There are 15 different kinds of knapweed. Spotted knapweed is known as Centaurea maculosa in the scientific community. The common names for some other varieties also found in Colorado are meadow knapweed, yellow starthistle, diffuse knapweed, cornflower, squarrose knapweed and Russian knapweed.

-- Is spotted knapweed native to Colorado or other parts of the United States?

Spotted knapweed (Centaurea maculosa) also is known as Asian native spotted knapweed. It is considered a noxious weed and is one of the most economically destructive exotic weeds because it invades western North America and displaces other weeds and crops with chemical warfare. The chemicals that spotted knapweed produces have less effect on plants native to Asia. This lack of a natural predator or check-and-balance system for plant species is typical of why non-native plants are generally invasive.

-- Is there evidence that other plants produce chemicals to fight off competition from other vegetation?

The soil immediately surrounding a plant root constitutes a unique physical, biochemical and ecological environment. That environment is largely controlled by the root system through chemicals secreted into the surrounding soil. Roots typically secrete compounds such as amino acids, organic acids, sugars and proteins. These compounds help the plant regulate microbes in the soil around their roots.

-- How was Colorado State University able to identify and isolate catechin secretions from spotted knapweed?

The Colorado State University Department of Horticulture laboratory was able to develop a system to grow knapweed roots in vitro, or in a fluid in laboratory beakers. In the laboratory, the roots continue to secrete catechin into a sterile liquid.