Crop harvests and bacterial blends

Drive down Wisconsin’s county roads on a crisp fall day and you’ll see farmers maneuvering combines across their fields, harvesting bushels upon bushels of corn. Farmers help ensure a bountiful harvest by applying fertilizers that provide crops with vital nutrients.

One such nutrient, nitrogen, makes individual plants stronger and improves crop yields. Until the mid-1900s, farmers turned to animal manure and legumes to provide crops with this essential nutrient, but the adoption of synthetic fertilizers by industrial agriculture in the 1950s changed the game. Though they’ve increased food production worldwide, these fertilizers are often applied in excess of what plants require, leading to surface and groundwater contamination and other environmental issues.

Meanwhile, UW-Madison scientists have been studying nitrogen fixation – the process by which atmospheric nitrogen is converted into a form useable by plants – and looking for alternatives that might help reduce applications of synthetic fertilizers from another angle. In 2018, bacteriology and agronomy professor Jean-Michel Ané and his collaborators identified another potential game-changer for industrial agriculture in the form of a variety of corn from Mexico. The corn uses a viscous and slime-like gel to host bacteria that fix nitrogen on aerial roots, clumps of roots that protrude from the plant’s stalks. Indigenous Oaxacan communities, recognizing that this gel helped their plants thrive in poor soils, have actively grown and selected this corn in their fields for centuries. But for the academic community, it was the first time that they had observed a variety of corn that could acquire a significant amount of nitrogen from the air by partnering with bacteria.

Clear gel globs on bright red aerial roots
Gel exudes from the aerial roots of a special nitrogen-fixing corn variety growing
at West Madison Agricultural Research Station. Photo: Michael P. King.

Since then, Ané and his team have identified and performed experiments in other plants, like sorghum, that also fix nitrogen in this way. Their preliminary tests suggest that approximately 10% of bacteria in the gels help these varieties of corn and sorghum fix 30% to 80% of their nitrogen; the function of the remaining bacteria remains unknown.

Now UW-Madison scientists want to see if they can harness the bacteria to help cereal crops like corn and sorghum fix nitrogen longer and more efficiently.

Design, test, learn, repeat

For this project, Ané has teamed up with assistant professor of biochemistry Ophelia Venturelli. Venturelli models bacteria and other microbial communities on computers and then moves to the lab, where she creates communities in the lab to study how they interact and how those interactions lead to community-level behaviors, such as the production and degradation of compounds that influence plant phenotypes.

Photo of Ophelia S. Venturelli
Assistant professor Ophelia Venturelli.

“These microbial communities are complex, highly dynamic networks that respond to their environment. They have a collective behavior that’s more than the sum of their parts,” said Venturelli. “We want to understand that systems-level function and also its stability, how the system can maintain this function over time.”

So far, the team has isolated bacteria from the gels of corn and sorghum plants. Ané is studying the basic properties of each bacterium and identifying which ones to use in a representative synthetic community. A postdoctoral researcher in the Venturelli Lab, Claire Palmer, will create and study this representative community in the lab, and improve its ability to fix nitrogen using computational modeling.

The models help the team identify which microbes and microbe-microbe interactions play a critical role in nitrogen fixation.

“There’s such a big space of how many random combinations you can put together, that the model is much more efficient at getting us there,” Venturelli explained. “Models help us rationally design communities and identify properties that we wouldn’t be able to by random sampling alone.”

Using the results of their models, Palmer and Venturelli will introduce different diazotrophs – bacteria that fix atmospheric nitrogen gas into a more usable form – into the community. They’ll observe how the microbes within the community affect the diazotrophs’ growth and nitrogen-fixing behavior and then model these interactions.

Venturelli hopes to have enough experimental data by the end of 2021 to start the modeling process and identify microbes and microbe-microbe interactions that are important to nitrogen-fixation. Microbial blends will be applied to cross-bred plants and to native varieties that rely on gels and aerial roots for nitrogen fixation. If all goes well, farmers may witness another shift in the field of fertilizers – one toward gel-based sources of nitrogen.

Written by Catherine Steffel, Ph.D. Read more about UW-Madison’s nitrogen fixation research here.