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The dripping gel harbors bacteria that convert atmospheric nitrogen into a form usable by the plant. (Howard-Yana Shapiro)
The dripping gel harbors bacteria that convert atmospheric nitrogen into a form usable by the plant.

Tropical corn acquires its own nitrogen

Public/private collaboration identifies varieties of tropical corn that can acquire significant amount of nitrogen from air by cooperating with bacteria.

Nitrogen is an essential nutrient for plants. While nitrogen makes up 78% of the atmosphere, only legume crops were known to have the ability to use it -- through their association with bacteria.

“Legume crops like soybeans have nodules on their roots that harbor bacteria that can turn nitrogen in the air into a form the plant can use,” Alan Bennett, distinguished professor of plant sciences at the University of California-Davis, said. “For cereal crops like corn, farmers must rely primarily on nitrogen fertilizers.”

A public/private collaboration of researchers at the University of Wisconsin-Madison, the University of California-Davis and Mars Inc. have 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, according to an announcement from the organizations.

To do so, the corn secretes copious globs of mucus-like gel out of arrays of aerial roots along its stalk. This gel harbors bacteria that convert atmospheric nitrogen into a form usable by the plant, a process called nitrogen fixation. The corn can acquire 30-80% of its nitrogen in this way, but the effectiveness depends on environmental factors like humidity and rain, the announcement said.

The findings were reported Aug. 7 in the journal PLOS Biology.

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. Further research is required to determine if the trait can be bred into commercial cultivars of corn.

If this trait can be bred into conventional varieties of corn, it could reduce the need for added fertilizer and increase yields in regions with poor soil. Corn that fixes nitrogen could also help farmers in developing countries that may not have access to fertilizer.

“It has been a long-term dream to transfer the ability to associate with nitrogen-fixing bacteria from legumes to cereals,” said Jean-Michel Ané, a professor of bacteriology and agronomy at the University of Wisconsin-Madison and a co-author of the new study.

Howard-Yana Shapiro, chief agricultural officer at Mars, a senior fellow in the department of plant sciences at the University of California-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 variety 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, the researchers explained. 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 permission from the government of Mexico.

The corn is striking. Most corn varieties grow to about 12 ft. and have just one or two groups of aerial roots that support the plant near its base. However, the nitrogen-fixing varieties stand more than 16 ft. tall and develop up to 8-10 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, the researchers said.

“It has been difficult to identify such a landrace and demonstrate that this nitrogen-fixing association actually contributes to nitrogen nutrition of the plant,” Bennett said. “Our interdisciplinary research team has been working on this for nearly a decade.”

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,” 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.”

The group used five different techniques across experiments in Mexico and Wisconsin 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,” Ané said.

The gel secreted by the corn’s aerial roots appears to work primarily by excluding oxygen and providing 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.

“Engineering corn to fix nitrogen and form root nodules like legumes has been a dream and struggle of scientists for decades,” Ané said. “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.”

“Corn yields in developing countries are one-tenth of those found in the U.S. due both to variety development and access to affordable nitrogen fertilizer,” said co-author Allen Van Deynze, director of research at the University of California-Davis Seed Biotechnology Center. “This discovery opens the door to significantly improving the genetic potential and food security for these countries.”

Other study authors include: Pablo Zamora, Cristobal Heitmann, Alison Berry, Donald Gibson, Kevin Schwartz, Srijak Bhatnagar, Guillaume Jospin, Aaron Darling, Jonathan Eisen, Richard Jeanotte and Bart Weimer at the University of California-Davis; Javier Lopez at the Instituto Tecnologico del Valle de Oaxaca in Oaxaca, and Pierre-Marc Delaux, Dhileephumar Jayaraman, Shanmugam Rajasekar, Danielle Graham and Junko Maeda of the University of Wisconsin-Madison.

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