Recent discoveries by Colorado State University scientists differ with traditional theories about invasive weed ecology by offering proof that at least one weed – spotted knapweed – doesn’t take over outside of its native ecology just by being leaner – it’s also meaner. So mean, in fact, that it triggers the plants around it that are competing for resources to destroy themselves.
The study, published this week in Science, improves on modern ecological theory that weeds are invasive because they’re leaner – able to use resources in their non-native ecologies faster and better than native plants. Instead, this study offers proof that chemistry can play a role in weed invasion.
A chemical called catechin from spotted knapweed, which was documented in Colorado State research a year ago as a natural herbicide that is released by the plant’s roots to kill other plants, actually makes native plants that compete with spotted knapweed turn on themselves. The chemical triggers an internal reaction that leads native plants that compete with the weed to self destruct and allows the weed to take over more territory.
"Catechin causes plants to self destruct," said Jorge Vivanco, Colorado State Department of Horticulture and Landscape Architecture professor. "It actually triggers a genetic response within the plant, causing the plant to create oxidants – free radicals – inside the plant as well as triggering genes that cause the plant’s cells to die, and it’s dead within a short period of time."
The new research that pair’s experts in horticulture, biology, chemistry, weed science and genetics sheds new light on the role of natural chemicals in weed invasions.
The team, which includes Harsh Bais and Ramarao Vepachedu, post-doctoral students at Colorado State in the Department of Horticulture and Landscape Architecture; Ragan Callaway,
University of Montana invasive plant ecologist; and Simon Gilroy, a cell biologist from Penn State, found that within an hour of exposure to catechin, genes in plants were activated that caused cells to die. The team also compared levels of catechin in the soil in Colorado and Montana to levels in soil in Europe, where the spotted knapweed is native. The group found that levels of catechin are much higher – four to five times so – in the United States, where spotted knapweed invades hundreds of acres.
When potted plants native to the United States were exposed to catechin through the soil at a chemical level normally found in Europe, they either died or wilted. Potted plants native to Europe that were exposed to the same levels of the chemical weren’t affected.
"At this point, we don’t know why there is less catechin in the soil in Europe," said Vivanco. "But this is strong evidence that chemistry can play a role when weeds invade non-native soil."
In previous research, Vivanco lead a team that discovered that spotted knapweed produces two types of catechin that are the same chemical compound but the mirror image of each other in their structures. One has anti-bacterial properties and the other acts as a natural herbicide.
The researchers also looked at the biochemical reaction within plants to the herbicide catechin by using Arapidopsis – a plant with a simple genetic map that is used as a "lab rat" in plant genetic research – and exposed its roots to catechin. Within 60 minutes, the root was dead, with a wave of death passing up the root from its tip. Catechin stopped the stream of cytoplasm within the root cells and burst root hairs, dropped the plant’s pH level to be more acidic, and triggered stress messengers, called reactive oxygen species, or ROS, which turned on genetic production of oxidants and triggered death. Gene mapping showed that plant genes activated within 10 minutes of exposure to catechin and produced antioxidant-related genes, but within an hour genes involved in stress response and cell death were activated.
The gene mapping also allowed the researchers to identify the genes within spotted knapweed that make it resistant to catechin, allowing it to thrive in soils where it has released the chemical. This discovery may allow scientists to create plants, such as crops that are strangled by spotted knapweed, to become resistant to catechin, stopping the weed’s spread. It is estimated that spotted knapweed has infested the native ecology of more than millions of acres and is listed as a noxious weed in 35 states.
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