What is the role of the root microbiome in sustaining ecosystem services under auxinic herbicide stress?

A growing body of research is showing that the root microbiome is an important driver of plant health. For example, microbes living within root tissues or in the rhizosphere can biosynthesize natural compounds that promote plant growth or detoxify harmful compounds like heavy metals and pesticides in the surrounding soil. However, in light of recent agricultural intensification, understanding whether the root microbiome influences plant tolerance to synthetic auxin herbicides remains a pertinent question. These herbicides cause damage to broad-leaved plants by mimicking the plant hormone auxin, and they are a common source of pollution in agroecosytems via drift (i.e., when herbicide particles move away from application sites through the air). In particular, it is currently unclear whether the composition of the root microbiome interacts with herbicide exposures to affect plant resilience or the ability of plants to maintain critical root mutualisms, such as those with nitrogen-fixing rhizobial bacteria or arbuscular mycorrhizal fungi. My current USDA-funded postdoctoral research project with the Baucom and James labs at the University of Michigan is addressing these questions using a combination of field observational studies, greenhouse experiments, bioinformatic analyses, and agriculturally-relevant legumes (red clover, white clover, and soybean) as model plant species. With this research, we seek to uncover the genetic mechanisms underpinning how root-associated microbes mediate plant productivity and ecosystem services under auxinic herbicide stress, and thereby gain insight into how the root microbiome could potentially be manipulated to maximize beneficial ecological outcomes in agroecosystems.

Will herbicide pollution cost plants their mutualisms with N-fixing rhizobial bacteria?

Plant mutualistic interactions with soil bacteria called rhizobia provide the key ecosystem service of biological nitrogen fixation. In these interactions, rhizobia transform atmospheric nitrogen into a bioavailable form that plants can absorb in exchange for carbon, which ultimately enhances soil fertility and contributes to terrestrial nutrient cycling. However, off-target pesticide drift is a modern anthropogenic force that plants and their rhizobial partners are commonly exposed to and are likely evolving in response to. In order to understand the consequences of pesticide drift, particularly from the widely-used synthetic auxin herbicide dicamba, on the evolutionary ecology of the plant-rhizobia mutualism, my research team and I conducted greenhouse and growth chamber experiments with the red clover study system during my PhD. Overall, we uncovered that stress caused by herbicide drift often had detrimental effects on the quantity and quality of plant-rhizobia interactions as demonstrated by symbiotic root nodule formation and biological nitrogen fixation. However, genetic variation from rhizobial partners was an especially important driver of mutualism outcomes, more so even than plant genetic variation (Iriart et al., 2024, Iriart et al., 2026). This work provides further evidence that microbial symbionts can modulate the strength of human-mediated stressors on plants and their beneficial ecological functions, highlighting the need for further investigation into the microbial mechanisms that are producing these results.

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