The dripping gel from this corn plant harbors bacteria that convert atmospheric nitrogen into a form usable by the plant. (Howard-Yana Shapiro photo)
Madison, WI (August 7, 2018) - 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.
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% to 80% of its nitrogen in this way, but the effectiveness depends on environmental factors like humidity and rain.
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, the world’s most productive cereal crop.
The findings are reported Aug. 7 in the journal PLOS Biology.
Nature provides the solution
“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 UW–Madison 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 over 16 feet tall and develop up to eight or 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.
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 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, which have a host of disadvantages. More than 1% 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,” Ané says.
Nitrogen-fixing corn varieties secreting large amounts of sugar-rich gel as they grow in Madison, WI. (Jean-Michel Ané photos)
The gel harbors bacteria that convert atmospheric nitrogen into a form usable by the plant, a process called nitrogen fixation.
The corn can acquire 30% to 80% of its nitrogen in this way, but the effectiveness depends on environmental factors like humidity and rain.
Future research is required to determine if the trait can be bred into commercial cultivars of corn, the world's most productive cereal crop.
Davis, CA (August 7, 2018) - A multidisciplinary team from the University of California, Davis, the University of Wisconsin–Madison, and Mars, Incorporated have found that an indigenous variety of corn can “fix nitrogen” from the atmosphere, instead of requiring synthetic fertilizers. The team’s findings were published Aug. 7 in the journal PLOS Biology.
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.
“This research has been 40 years in the making and is a significant breakthrough in our attempts to find a more sustainable way of growing corn, one of the world’s key crops,” said co-author Howard-Yana Shapiro, chief agricultural officer at Mars, Incorporated.
Nitrogen is an essential nutrient for plants. While nitrogen makes up 78 percent of the atmosphere, only legume crops were known to have the ability to use it through their association with bacteria. For cereal crops like corn, farmers must rely primarily on nitrogen fertilizers.
Researchers spent years searching for isolated indigenous varieties of corn, or landraces, where corn first originated in Mexico. It was thought that such varieties might associate with nitrogen-fixing bacteria. In the 1980s, Shapiro observed such corn being grown in nitrogen-deficient soil in the Sierra Mixe region near Oaxaca. It was not until the 2000s that new technologies developed to allow them to intensely study this nitrogen-fixing process.
“It has been difficult to identify such a landrace and demonstrate that this nitrogen-fixing association actually contributes to nitrogen nutrition of the plant,” says co-author Alan Bennett, distinguished professor of plant sciences in the College of Agricultural and Environmental Sciences at UC Davis. “Our interdisciplinary research team has been working on this for nearly a decade.”
How it works
The study found that one corn variety grown in the Sierra Mixe region obtains 28-82 percent of its nitrogen from the atmosphere. To do this, the corn grows a series of aerial roots. During certain times of the year, these roots secrete a gel-like substance, or mucilage. The mucilage provides the low-oxygen and sugar-rich environment required to attract bacteria that can transform nitrogen from the air into a form the corn can use.
“Our research has demonstrated that the mucilage found in this Sierra Mixe corn forms a key component of its nitrogen fixation,” says co-author Jean-Michel Ané, professor of Agronomy and Bacteriology in the College of Agricultural and Life Sciences at UW–Madison. “We have shown this through growth of the plant both in Mexico and Wisconsin.”
Hope for sustainable agriculture
Researchers are a long way from developing a similar nitrogen-fixing trait for commercial corn, but this is a first step to guide further research on that application. The discovery could lead to a reduction of fertilizer use for corn, one of the world’s major cereal crops. It takes 1-2 percent of the total global energy supply to produce fertilizer. The energy-intensive process is also responsible for 1-2 percent of global greenhouse gas emissions.
“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,” says co-author Allen Van Deynze, director of research at the UC Davis Seed Biotechnology Center. “This discovery opens the door to significantly improving the genetic potential and food security for these countries.”
“As one of the world’s largest food businesses, Mars is committed to reducing the environmental strain caused by farming,” Shapiro added. “We embarked on this uncommon collaboration decades ago to drive forward a discovery that has the potential to create a lasting and positive impact on sustainable agriculture.”
The municipal authority and community in the isolated village in the Sierra Mixe region were an integral part of this research project. Biological materials were accessed and utilized under an Access and Benefit Sharing (ABS) Agreement with the community and with permission from the Mexican government. An internationally recognized certificate of compliance under the Nagoya Protocol on Access and Benefit Sharing has been issued for such activities.
This groundbreaking ABS Agreement is designed to ensure the equitable sharing of benefits arising out of the utilization of genetic resources, contributing to the conservation and sustainable use of biodiversity.
This research was facilitated through the tremendous cooperation of SEMARNAT and SAGARPA, two agencies of the Mexican government responsible for implementation of the access and benefit-sharing provisions of the Nagoya Protocol.
Other 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 of UC Davis; Javier Lopez, Instituto Tecnologico del Valle de Oaxaca, Oaxaca, Mexico; Pierre-Marc Delaux, Dhileephumar Jayaraman, Shanmugam Rajasekar, Danielle Graham, and Junko Maeda of UW–Madison.
This paper is dedicated to co-author Cristobal Heitmann, whose energy and enthusiasm were a major catalyst in this research. He died while the manuscript was under review, and his research for the paper was part of his M.S. thesis. Read more about Cris and the scholarship in his name.
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Sierra Mixe corn growing in a field in Davis, California, next to a modern conventional corn variety (in foreground). The Sierra Mixe corn can grow up to 16 feet tall. (Alan Bennett photo)