Ragweed research Scientists shed new light on how soil microbes help and hinder growth of the common weed
LINDELL BEACH, B.C. — Researchers at the University of Illinois have used DNA-based tools to identify soil microbes that can negatively affect the growth of ragweed.
The study also revealed the complicated relationships between plants and micro-organisms.
“The tools let us collect information from the most common micro-organisms in the soil, allowing us to distinguish approximately 300 different species of microbes from a 0.5 gram soil sample,” said Anthony Yannarell, assistant professor of microbial ecology in the university’s natural resources and environmental sciences department
The researchers wanted to see whether soil microbes that had been conditioned by ragweed were beneficial or harmful to ragweed plants.
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“The way to test this is to determine if ragweed plants grow better or worse when exposed to soil that has been exposed to previous generations of ragweed,” said Yannarell.
“This is the ragweed’s home soil, (but) we needed a basis of comparison to decide if ragweed plants do better or worse in their home soil.”
The comparison came from sunflower plants that were being used for growth response studies. The sunflower-conditioned soil became the ragweed’s “away” soil to measure its comparative rate of growth.
“What we found was that ragweed consistently showed poorer growth in its home soil than in its away soil,” said Yannarell.
“So either ragweed attracts microbes that are bad for its own growth or sunflower attracts microbes that are good for ragweed growth, or some combination of both is happening.”
Ragweed is an extremely persistent weed with 15 species across North America, three of which are in Canada: common ragweed, perennial ragweed and giant ragweed.
The spread of this plant not only threatens crop yields when unchecked, but its notorious pollen is also one of the leading causes of hay fever and other allergies. A single plant can produce 5,000 seeds, and each season millions of spores of pollen are windblown over hundreds of kilometres, affecting thousands of sufferers.
Herbicides are used for ragweed control, but giant ragweed has shown resistance to some of these chemicals. The old school idea might have been to add more microbes to the soil, but studies have shown that the approach seldom works.
“These kinds of add-some-beneficial-bacteria-to-the-soil approaches have generally not been effective when tried in the past,” said Yannarell.
“Any bacterium added to the soil needs to overcome a set of challenges in order to proliferate.”
They must be able to survive in the specific soil environment, compete with other organisms already present, avoid soil predators, reproduce and expand to new areas. Not all bacteria can overcome all challenges.
The research team documented three generations of the plants and their interaction not only with soil microbes but indirectly with each other.
Twenty ragweed plants and 20 sunflower plants were individually grown in pots in a greenhouse. The soil community for each species became conditioned with the exclusive presence of either ragweed microbes or sunflower microbes.
“Using the same conditioned soil, we planted 10 of the ragweed pots with ragweed and 10 of the ragweed pots with sunflower,” Yannarell said.
“We did the same switch with the sunflower pots: 10 of them got sunflower again and 10 got ragweed. “
The plants were exposed to each other’s microbes, setting up the indirect interaction. The plants were never together in the same pot.
“What we showed in our experiment was that, after 10 weeks in a greenhouse, the ragweed plants were smaller in their home soil than they were in their away soil,” said Yannarell.
“Thus, these microbes were somehow slowing down the growth of the ragweed plants. They may have been doing this by directly infecting the plants, or maybe they were taking nutrients out of the soil and out-competing the ragweed. Our experiment showed an overall effect, but we won’t know for sure what mechanism was driving this effect until we do more investigation. That’s one of the most exciting things in my mind about our work: our ‘affinity-effect’ analysis, (which) allows us to pinpoint which microbes were most likely the ones responsible for this reduced growth. We can then try to cultivate these microbes and see if we can figure out what they are doing, specifically, to reduce the ragweed growth.”
The affinity-effect analysis is based on a unique connection that microbes have with plants. Some microbes might grow better when exposed to ragweed plants, while others thrive when exposed to sunflowers. Each has a kind of “affinity” for either of these plants. The effect of this affinity is that the microbes may be either good or bad for either of them, therefore increasing or reducing the growth or health of ragweed and sunflower.
In the case of ragweed, the researchers found that microbes with a high affinity for ragweed also had a strong negative effect on the ragweed’s growth.
According to the research team’s report, An Affinity-Effect Relationship for Microbial Communities in Plant-Soil Feedback, published in the journal Microbial Ecology, the ragweed used a broad subset of soil bacteria and fungi that produced consistently negative feedback.
This created a self-regulating feedback loop influencing the plant community.
“For example, plant-soil feedback loops can determine which plant species are abundant in a local community of plants and which are rare,” said Yannarell.
By comparison to ragweed, the microbes in the sunflower affinity–effect relationship showed a positive feedback, meaning that the sunflower is dependent on the composition of the microbial source community for growth.
