The Wonder Plant That Could Slash Fertilizer Use

For thousands of years, people from Sierra Mixe, a mountainous region in southern Mexico, have been cultivating an unusual variety of giant corn. They grow the crop on soils that are poor in nitrogen—an essential nutrient—and they barely use any additional fertilizer. And yet, their corn towers over conventional varieties, reaching heights of more than 16 feet.

A team of researchers led by Alan Bennett from UC Davis has shown that the secret of the corn’s success lies in its aerial roots—necklaces of finger-sized, rhubarb-red tubes that encircle the stem. These roots drip with a thick, clear, glistening mucus that’s loaded with bacteria. Thanks to these microbes, the corn can fertilize itself by pulling nitrogen directly from the surrounding air.

The Sierra Mixe corn takes eight months to mature—too long to make it commercially useful. But if its remarkable ability could be bred into conventional corn, which matures in just three months, it would be an agricultural game changer.

All plants depend on nitrogen to grow, and while there’s plenty of the element in the air around us, it’s too inert to be of use. But bacteria can convert this atmospheric nitrogen into more usable forms such as ammonia—a process known as fixation. Legumes, like beans and peas, house these nitrogen-fixing bacteria in their roots. But cereals, like corn and rice, largely don’t. That’s why American farmers need to apply more than 6.6 million tons of nitrogen to their corn crops every year, in the form of chemical sprays and manure.

“All that fertilizer takes a lot of energy to produce, and the excess ends up in places where it distorts the nutrient balance, creating algae blooms and dead zones in waterways,” says Jeremy Yoder, an evolutionary biologist at California State University, Northridge who was not involved in the study. “So self-fertilizing [corn] could substantially cut the cost and environmental impact of a staple crop.” It could also make it easier to grow the crop in developing countries where fertilizer is unaffordable or in areas where soils are poorer.

Crucially, the Davis team involved the Sierra Mixe community throughout their research. They also established legal agreements with the Mexican government to ensure that any benefits from their research—and its subsequent commercialization—would be shared with the community, under the auspices of the Nagoya Protocol, an international framework intended to thwart bio-piracy. Alejandra Barrios, the director of biosafety and biodiversity at Mexico’s environmental agency, repeatedly praised the approach on Twitter, calling it “great work” and a “win-win solution.”

The certificate that ratifies the agreement is the first of its kind to be “issued by the Mexican government, and the first issued to any entity in the U.S. by any country,” says Howard-Yana Shapiro, an agricultural scientist from Mars, Incorporated who initiated the project. “That is as important as the discovery. We’re showing the way of the future.”

Corn, or maize, originated in southern Mexico when it was domesticated from a wild cereal called teosinte. The region is still home to the greatest diversity of the crop, with thousands of unique varieties or “landraces.” In 1980, Shapiro (who was then an independent researcher) was busily collecting these landraces on behalf of the Mexican government when he heard about the giant, mucus-covered corn.

From the start, he suspected that the plant might fix its own nitrogen, and that the shining mucus was somehow involved. But with the technology of the time, he had no way of testing his hunch. And without that evidence, other scientists were rightfully skeptical, including the team at Davis. “I would talk about this maize, and Alan would say, ‘It’s not possible,’” Shapiro recalls. “I said it is possible but I just don’t know how to prove it. And I took him to Sierra Mixe for a visit in 2008. He was gobsmacked. He said, ‘I take it all back. There’s something going on here.’”

To find out what was really happening, the team used modern DNA-sequencing techniques to show that the mucus contains microbes that belong to nitrogen-fixing families, and that carry nitrogen-fixing genes. They also chemically analyzed the mucus to show that it provides its resident microbes with exactly the conditions they need to thrive—an all-you-can-eat buffet of sugar, and protection from oxygen.

Next, they used five different tests to confirm that the microbes really are fixing nitrogen, that the nitrogen moves into the corn, and that the corn gets a lot of its nitrogen—anywhere from 30 to 80 percent—in this way. All five techniques have their own shortcomings, but together “they all pointed to the same conclusion,” Bennett says. “We’ve been working on this for 10 years and we have a high degree of confidence that the results we report are correct.”

Others agree: “It was a very ambitious study that was really well done, and the results should be believed,” says Michael Kantar, a botanist at the University of Hawai’i at Mānoa. Yoder points out that the team hasn’t identified the specific nitrogen-fixing microbes, but beyond that, “I think it’s pretty convincing,” he told me. “Aerial-root mucilage that hosts nitrogen-fixing microbes is, quite honestly, a thing I’d have called a little far-fetched if I saw it on an episode of Star Trek,” he added on Twitter.

Scientists have spent years trying to create nitrogen-fixing cereal crops through genetic engineering, with little progress to show for it. But since we now know that at least one type of corn can fix nitrogen naturally, the ability could potentially be moved into conventional varieties through classical crossbreeding, mucus transplants, or both. These methods might make the final produce more publicly acceptable than a genetically edited crop.

For now, Bennett and his colleagues want to identify the genes that allow the Sierra Mixe corn to produce its mucus-coated aerial roots, and attract the right bacteria. They also want to take a closer look at the microbes themselves. “We’ve isolated thousands, but of those, we don’t know if there are two species that are really important or a hundred,” Bennett says.

And Shapiro, with the blessing of the Sierra Mixe community, is trying to find a company to take charge of commercialization. “It probably won’t be Mars Inc., ’cause we’re not a maize company,” he says, “but I’m trying to find the right partner.”

Kantar cautions that it’s too early to say if there are any big implications for food security, because the team hasn’t shown that the resulting corn can fix enough nitrogen to grow at commercially useful scales. It’s also unclear if the genes behind the ability come with any drawbacks. But “if these questions can be resolved, this may provide a way to significantly reduce fertilizer use worldwide, which would have hugely beneficial environmental effects,” he says.

Kristin Mercer from Ohio State University is similarly cautious.  ‘This corn has been likely doing a very good job ensuring some level of food security for families in the region for a long time,” she says. “If one were to think about capturing that beneficial diversity and distributing it more widely, a number of potential issues arise.” Are there intellectual property issues around dispersing the genetic variation underlying this trait into public or private breeding programs? Would it be a good approach to create varieties of nitrogen-fixing corn for other areas where poor farmers live and asking them to buy those varieties?

“It is easy to jump from describing the amazing biology of these genetic resources stewarded by farmers in the region for millennia to trying to solve the world’s massive, intractable problems—but that is stickier than it may seem,” Mercer adds.