The term is “nitrogen fixation.” No, it doesn’t refer to an unhealthy obsession with one of the most common chemical elements on Earth. Rather, it’s a process in which atmospheric nitrogen is converted into a form usable by a plant.
Nitrogen is essential for plants to function, but very few crops can “fix” it on their own. Scientists have long sought corn with this ability, their goal being to reduce the crop’s high demand for artificial fertilizers. Fertilizers are expensive, and excess can run off the soil, impacting water used for swimming, fishing, and drinking. Reducing corn’s need for fertilizer would be a boon for farmers and water quality alike. And now the search for nitrogen-fixing corn may be over.
A team of researchers from CALS; the University of California, Davis; and Mars Inc. has identified varieties of tropical corn from Oaxaca, Mexico, that can acquire a significant amount of the nitrogen they need from the air by cooperating with bacteria. To do so, the corn secretes copious globs of mucus-like gel from arrays of aerial roots along its stalk. This gel harbors bacteria that facilitate nitrogen fixation. The corn can acquire 30 to 80 percent of its nitrogen this way, but the effectiveness depends on environmental factors like humidity and rain.
The team reported its findings in the journal PLOS Biology. It’s a positive development in this area of study, but further research is required to determine whether the trait can be bred into commercial cultivars of corn, the world’s most productive cereal crop.
“It has been a long-term dream to transfer the ability to associate with nitrogen-fixing bacteria from legumes to cereals,” says Jean-Michel Ané, a professor of bacteriology and agronomy at CALS and a co-author of the new study.
Legumes, such as beans, are the only group of crop plants previously known to acquire a significant amount of nitrogen through fixation, which they perform in specialized tissues called root nodules.
Howard-Yana Shapiro, the chief agricultural officer at Mars, a senior fellow in the Department of Plant Sciences at UC Davis, and a co-author of the report, identified the indigenous varieties of corn in a search for cultivars that might be able to host nitrogen-fixing bacteria.
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, conditions that have selected for a novel ability to acquire nitrogen. The biological materials for this investigation were accessed and utilized under an Access and Benefit Sharing Agreement with the Sierra Mixe community and with the permission of the Mexican government.
The corn is striking. Most corn varieties grow to about 12 feet and have just one or two groups of aerial roots that support the plant near its base. But the nitrogen-fixing varieties stand more than 16 feet tall and develop 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 that provide the energy and oxygen-free conditions needed for nitrogen-fixing bacteria to thrive.
Establishing that plants are incorporating nitrogen from the air is technically challenging.
“It took us eight years of work to convince ourselves that this was not an artifact,” says 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.”
The group used five different techniques across experiments in Mexico and 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.
“What I think is cool about this project is it completely turns upside down the way we think about engineering nitrogen fixation,” says Ané.
The gel secreted by the corn’s aerial roots appears to work primarily by excluding oxygen and directing sugars to the right bacteria, sidestepping 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.
Breeding the trait into commercial cultivars of corn could reduce the need for artificial nitrogen fertilizers, which have a host of disadvantages. More than 1 percent of the world’s total energy production goes toward producing nitrogen fertilizer. Developed countries contend with waterways polluted by leaching nitrogen, while adequate fertilizer is often inaccessible or too expensive for farmers in developing countries. Corn that fixes some of its own nitrogen could mitigate these issues, but more research will be required.
“Engineering corn to fix nitrogen and form root nodules like legumes has been a dream and struggle of scientists for decades,” says Ané. “It turns out that this corn developed a totally different way to solve this nitrogen-fixation problem. The scientific community probably underestimated nitrogen fixation in other crops because of its obsession with root nodules.
“This corn showed us that nature can find solutions to some problems far beyond what scientists could ever imagine.”