There are a number of so called natural
herbicides but at best they are very selective or expensive.
Scientists have speculated for decades that spotted
knapweed (Centaurea Bieberstenii) is able to spread over large areas because
of secret weapon – an ability to release a chemical that kills surrounding
plants. Until now, they have never been able to put their thumb on
the phenomenon, but recently a Colorado State University horticulture professor
identified and isolated the chemical for the first time. What’s more,
they are using the chemical as a completely natural and environmentally
friendly herbicide to kill other weeds.
Ironically, the weed is one of the most unwanted
plants in the USA where, in many states it is totally prohibited.
The discovery and isolation of the chemical, called
catechin, within spotted knapweed may revolutionise weed control.
“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 other components
that naturally occur in soil,” said Jorge Vivanco, assistant professor
of horticultural biotechnology at Colorado State University. “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. The have discovered that
the weed produces two types of catechin that are the same chemical compound
but the mirror image of each other in their 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 re-enter 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 university team has found that spraying catechin
on plants or adding it to soil is as effective as 2,4-D against 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 researches are working with commercial companies to make
spotted knapweed catechin spray available to consumers within a year or
two.
The 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.
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
catchin 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 may 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.
WHERE DOES IT COME FROM?
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.
