A discovery 10 years ago that seemed too good to be true required eight years of public-private collaboration of researchers at the University of Wisconsin–Madison, the University of California, Davis, and Mars Inc., before they were confident to publicly share their findings.
The news could eventually be a game-changer for corn-producers worldwide, with reduced production costs while also addressing environmental challenges. Researchers say further research is required to determine if the trait can be bred into commercial cultivars of corn, the world’s most productive cereal crop.
Establishing that corn plants were actually incorporating nitrogen from the air was technically challenging, said Jean-Michel Ané, a professor of bacteriology and agronomy at UW–Madison and a co-author of the new study.
“It took us eight years of work to convince ourselves that this was not an artifact,” said Ané, whose lab specializes in studying and quantifying nitrogen fixation. “Technique after technique, they’re all giving the same result showing high levels of nitrogen fixation in this corn.”
Ané and Howard-Yana Shapiro, the chief agricultural officer at Mars, a senior fellow in the Department of Plant Sciences at UC Davis and co-author of the report, identified varieties of tropical corn from Oaxaca, Mexico, that can acquire 30 to 80 percent of the plant’s nitrogen needs through copious globs of mucus-like gel out of arrays of aerial roots along its stalk.
Researchers determined the gel harbors bacteria that converts atmospheric nitrogen into a form usable by the plant, a process called nitrogen fixation. The effectiveness depends on environmental factors such as humidity and rain.
|Nitrogen-fixing corn varieties secreting large amounts of sugar-rich gel as they grow in Madison, Wisconsin. (Jean-Michel Ané)|
Nature provides the solution
Scientists have long-sought corn that could fix nitrogen, with the goal of reducing the crop’s high demand for artificial fertilizers, which are energy intensive, expensive and polluting. Shapiro identified the indigenous varieties of corn in a search for cultivars that might be able to host nitrogen-fixing bacteria.
“It has been a long-term dream to transfer the ability to associate with nitrogen-fixing bacteria from legumes to cereals,” Ané said.
The corn is grown in the Sierra Mixe region of Oaxaca in southern Mexico, part of the region where corn was first domesticated by Native Americans thousands of years ago. Farmers in the area grow the corn in nitrogen-depleted soils using traditional practices with little or no fertilizer.
The corn is striking, with the nitrogen-fixing varieties standing over 16 feet tall and developing up to 10 sets of thick aerial roots that never reach the ground. Under the right conditions, these roots secrete large amounts of sugar-rich gel, providing the energy and oxygen-free conditions needed for nitrogen-fixing bacteria to thrive and complete the transition of nitrogen.
Ané and Shapiro used five different techniques across experiments in Mexico and in Madison to confirm that the Sierra Mixe corn’s gel was indeed fixing nitrogen from the air and that the plant could incorporate this nitrogen into its tissues.
The gel secreted by the corn’s aerial roots appears to work primarily by excluding oxygen and providing sugars to the right bacteria, side-stepping complex biological interactions. The research team was even able to simulate the natural gel’s effects with a similar gel created in the lab and seeded with bacteria.
The simplicity of the system provides inspiration to researchers looking to identify or create more crop plants with this trait.
“What I think is cool about this project is it completely turns upside down the way we think about engineering nitrogen fixation,” said Ané, adding that previous research has focused on root nodules in the soil for nitrogen fixation. “This corn showed us that nature can find solutions to some problems far beyond what scientists could ever imagine,” he said.