Herbicides disrupt the co-evolution of plants and insects

Have you ever paused to admire a morning glory? Just as their name suggests, these flowers open at sunrise and often close by afternoon or evening.

Despite their fragile appearance, a recent study from the University of Michigan suggests that morning glory plants are quite resilient. The team discovered that these plants can withstand the effects of weed killers and display resistance to damage from herbivorous insects.

In particular, morning glory plants were found to resist glyphosate – the active component in the herbicide RoundUp.

Plant resistance to insect damage

Interestingly, the researchers found that plants which are susceptible to glyphosate are also susceptible to damage from insects. This implies that glyphosate can disrupt the co-evolution of plants and their insect adversaries.

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

“When people think about herbicides and study them in plants, they think about herbicide resistance,” said Zhang.

“Resistance is a big problem, tolerance is a big problem. 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.”

More insect damage on plants with glyphosate

The researchers set up the study with 1,600 morning glory plants in U-M‘s Matthaei Botanical Gardens. They sprayed half of the plants with glyphosate and recorded the plants‘ growth traits.

Noteworthy findings from the study included the fact that plants treated with glyphosate incurred more damage from insects compared to the non-treated (control) plants. However, plants resistant to glyphosate had less insect damage.

While conventional ecological theory would suggest a tradeoff between these two traits, Zhang said the team “saw the opposite.”

The researchers also analyzed the fitness of the plants, i.e., their likelihood of producing seeds. They discovered a positive selection acting on glyphosate resistance: the more resistant the plants are to glyphosate, the fitter they are and the more likely to produce seeds.

Plant genes and insect defenses

But, why are plants that are resistant to glyphosate also resistant to insect damage?

While Zhang and Baucom are yet to confirm, they hint at the role of the shikimate pathway – a metabolic process that glyphosate attacks, impacting the plant’s herbivory defenses.

In contrast, the plants resistant to glyphosate seem to have multiple genes working together. These form detoxification pathways that can wade off glyphosate, thereby maintaining their insect defenses.

Implications for agricultural practices

This study has profound implications for current agricultural practices and the management of herbicide application. Understanding the dual resistance to both glyphosate and herbivory challenges the conventional approach to pest and weed management.

For farmers, this could mean re-evaluating the use of herbicides like glyphosate to mitigate potential unintended ecological repercussions.

The development of crops that can naturally resist both herbicides and insect predation could lead to more sustainable agricultural strategies, reducing the need for chemical inputs while preserving biodiversity in farmlands.

Future research directions

The findings open the door to further exploration into the genetic underpinnings of plant resistance. Future research could investigate the exact genetic pathways involved in this dual resistance, potentially leading to breakthroughs in genetic engineering for crop improvement.

Another avenue of research could focus on the long-term evolutionary consequences of glyphosate use on plant-insect interactions.

Scientists could harness these insights for developing crops that are better suited to withstand both chemical and biological stressors, ensuring food security in an ever-changing environment.

Human-made impacts on nature

“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?”

The study is published in the journal New Phytologist.

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