Between friends and foes – when is it best for plants to avoid vs. interact with soil symbionts? Reflections from an LTER fellow

Isabela Lima Borges is a PhD student in the Department of Integrative Biology at Michigan State University and a member of Sarah Fitzpatrick’s lab.

Isabela in the LTER.
Isabela in the LTER.

All organisms on Earth require others to live, and few, if any, have gone untouched by anthropogenic change in the past century. As an ecologist, I am fascinated by how plants interact with other species, and how those interactions are affected by human interventions. Given plants’ fundamental role as the basis of terrestrial food webs, these interactions are critical for most of the biological processes that humans rely upon. Beyond feeding and sheltering us, plants filter the air we breathe, prevent erosion, and inspire us. Understanding their interactions with other organisms and the effects of human-induced changes in their environment, then, is vital for maintaining those ecosystem services.

The interactions between plants and soil microbes are some of the most important determinants of plant health. Millions of years of coevolution with soil organisms have given plants an impressive array of weapons to use against their microscopic enemies, as well as lures to attracts their friends. One of the best known examples of an underground mutualism, where both partners benefit, is the one between legumes and nitrogen-fixing bacteria. These bacteria, also called rhizobia, provide plant with nitrogen, as essential nutrient for plant growth, and in return receive shelter in root nodules as well as carbohydrates for sustenance. This interaction is a major part of the Earth’s nitrogen cycle and maintaining soil health.

Another important interaction in the soil occurs between plants and nematodes. Like rhizobia, plant-parasitic nematodes live, reproduce, and absorb carbohydrates in plants’ roots. Unlike rhizobia, the nematodes don’t repay their plants hosts with nutrients. This one-sided interaction is often detrimental to plants and causes major agricultural losses across the globe. Unfortunately for plants, the same mechanisms that allow mutualist rhizobia to occupy their roots also allows for parasitic nematodes to establish. The dilemma, then, is whether plants should attract mutualists and suffer the accompanying damage from parasites, or avoid both. When does the benefit provided by the rhizobia outweigh the harm done by the parasite, and vice-versa?

The KBS LTER has a long history of manipulating agricultural plots to simulate different anthropogenic changes. Nitrogen fertilization, in particular, is of great interest for the legume-rhizobia-nematode interaction: if plants can get nitrogen directly from the soil, rhizobia are less needed, and plants might be safer from nematode infection. Nematicide application, another common agricultural intervention, reduces the presence of nematodes in soil and may allow plants to more freely invest in their interactions with rhizobia. Variation in plant diversity, on the other hand, might provide generalist nematodes with more hosts and allow for a growth in their population, leading to higher risk of parasitism on legumes. All of these land use changes are represented in the LTER, setting the stage for wide variation in this three-way interaction.

In the Summer of 2020, I sampled plants and soil from a variety of plots in the LTER MCSE (Main Cropping System Experiment) to quantify that variation and relate it back to decades of different land-use treatments. The sampling involved digging up two common species of weedy legumes, trying to capture as much of their intact roots are possible, and collecting soil cores. I will count root nodules, the structures that house beneficial bacteria, and root knots, similar structures that house nematodes, in sampled plants to determine the extent of their interactions with these soil microorganisms. Along with current and long-term data on soil nitrogen, soil nematode abundance, and plant diversity, these data should shed some light on the environmental conditions that determine when plants benefit the most from interacting with these organisms. 

This study will increase our understanding of how complex plant-microbial interactions depend on different environmental conditions. In the future, this knowledge could be used to inform best management practices for weedy and crop legumes, and predict human-induced changes to legumes in their natural environments. Ultimately, the goal is to provide a fuller picture of how plants and their microbial partners respond to different components of global change and management represented in the KBS LTER.