While studying the eco-evolutionary implications of using herbicides, researchers at the University of Michigan found that insects do greater damage to herbicide-treated plants. The team also revealed that the morning glory plant can resist damage from herbivorous insects.
Since plants and insects have co-existed for more than 400 million years, their interaction influences evolutionary trajectories. Such relations lead to community stability. However, these trajectories are altered when human-mediated stressors enter into play.
Chemical herbicides are the most common way to eradicate unwanted weedy plants. Unfortunately, plants have quickly adapted to herbicides, resulting in hundreds of weeds tolerant to some form of herbicide.
“When people think about herbicides and study them in plants, they think about herbicide resistance. Resistance is a big problem, tolerance is a big problem,” said the lead author Grace Zhang.
“But beyond that, we don’t really have a great idea of how herbicides impact plant evolution in general, and also how it might impact how plants interact with other things in their environment.“
Glyphosate is an active ingredient in the herbicide. To study the examine the effects of glyphosate on the co-evolution of plants and their insect herbivores, Grace Zhang experimented with morning glory plants. The common morning glory (Ipomoea purpurea), is an annual weed commonly found in agricultural fields.
In the Matthaei Botanical Gardens of the University of Michigan, Zhang planted 1,600 morning glory plants and sprayed half of the plants with glyphosate. Then she recorded the growth traits of these two sections of plants.
Zhang’s observation reported more damage to the glyphosate-treated plants, than the controlled plants. The treated plants had more holes chewed from the leaves.
Surprisingly, researchers also found that plants resistant to glyphosate had less insect damage. Meanwhile, plants susceptible to glyphosate had more damage from insects.
Zhang commented, “Ecological theory tells us that there might be more likely a tradeoff between these two traits: A plant is more likely to put more energy into protecting itself against one of these versus the other. But we didn’t see that. We saw the opposite.“
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Researchers were surprised to see the relationship between glyphosate resistance and herbivory resistance. They informed that plants resistant to glyphosate appeared to be more fit and more likely to bear seeds.
As to why plants resistant to glyphosate were also resistant to insect damage, researchers had no answer. But they were able to make intelligent guesses.
Glyphosate attacks the shikimate pathway in plants and restricts the production of aromatic compounds. These aromatic compounds are crucial for herbivory defense. With the restricted production of byproducts, glyphosate halts the cascade of herbivory defense.
As for why plants are resistant to glyphosate, researchers suspect multiple genes working together to form detoxification to shunt glyphosate away.
“Studies in the past usually look at the evolution of herbicide resistance in these plants or they’ll look at how applying the herbicide directly to specific insects will affect them,” says Zhang.
“But they haven’t really looked at this question from more of a community level: How does herbicide act as a disruptor on this community level? So that’s what we really wanted to focus on: how does this new disruptive agent that is human-made impact a very natural, long-standing relationship between plants and their insects,” Zhang continued.
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Journal Reference
- Zhang, G. M., & Baucom, R. S. Herbicidal interference: Glyphosate drives both the ecology and evolution of plant-herbivore interactions. New Phytologist. DOI: 10.1111/nph.20238