What's the story, morning glory?

The U-M researchers also found the reverse: plants treated with and susceptible to glyphosate, the active ingredient in the herbicide RoundUp, are also susceptible to damage from insects. This suggests that glyphosate, a herbicide humans have introduced into the environment, can disrupt the co-evolution of plants and their insect herbivores. Their results are published in the journal New Phytologist.

"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 lead author Grace Zhang, a U-M doctoral student working with Regina Baucom, a professor in the Department of Ecology and Evolutionary Biology. "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."

To examine the effects of glyphosate on Ipomoea purpurea -- the common morning glory -- Zhang planted 1,600 morning glory plants in U-M's Matthaei Botanical Gardens. She sprayed half of the plants with glyphosate, then recorded growth traits.

These traits included the amount of damage glyphosate caused the plants. Zhang also recorded insect damage to both the treated plants and control plants, noting herbivorous insects nibbling holes in the center of the leaves, on the margins of the leaves, or scraping cells off the surface of the leaves.

Zhang found that glyphosate impacted the pattern of damage from herbivory: Plants treated with glyphosate incurred more damage from insects compared to the control plants. Glyphosate-treated plants had greater rates of holes chewed through the leaves, as well as nibbles taken from around the margin of the leaves. However, there was a lower proportion of leaves that showed surface feeding.

But the researchers were surprised to find that plants resistant to glyphosate had less insect damage, and plants susceptible to glyphosate had more insect damage.

"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," Zhang said.

The researchers also measured the fitness of the plant, or its likelihood of producing seeds. They found that there was a positive selection acting on glyphosate resistance: that is, the plants that are more resistant to glyphosate tend to be more fit and more likely to produce seeds than those that aren't.

"This is really interesting because the implications are that there's a causal relationship between glyphosate resistance and herbivory resistance," Zhang said. "If higher levels of glyphosate resistance are being selected for, then perhaps higher levels of herbivory resistance will be dragged along with that, when it comes to positive selection."

Zhang and Baucom don't know why plants resistant to glyphosate were also resistant to insect damage, but they do have some guesses. Glyphosate works by attacking what's called the shikimate pathway in plants. The shikimate pathway is a metabolic process responsible for producing chorismate, a precursor to aromatic amino acids such as tryptophan and phenylalanine. Salicylic acid, the bitter compound from which aspirin is derived, is also a product of the shikimate pathway. Plants use these compounds as herbivory defense. Glyphosate stops the production of chorismate, which halts the cascade of herbivory defense.

As far as the plants that are resistant to glyphosate? The researchers theorize that some morning glory plants may have multiple genes working together to form detoxification pathways that can shunt glyphosate away. Detoxing the herbicide allows the plant to produce the precursor molecule, chorismate, responsible for insect defenses. The scientists plan to investigate this line of research in future work.

"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," Zhang said. "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?"