Genetic engineering may be anathema to many card-carrying environmentalists, who typically favour organic farming over other forms of agriculture. But to overcome the threat posed by climate change — while continuing to feed 7.7 billion mouths (and counting) — many scientists say it’s time to fully embrace the three most controversial letters in the food industry: GMO.

Their goal is to radically shrink the carbon footprint of global crop cultivation by doing away with the need for synthetic fertilizers, which account for about 5% of humanity’s total greenhouse gas emissions. Through extensive DNA manipulations, scientists are optimistic they can engineer a self-fertilizing relationship between crop species and root-dwelling microbes, obviating the need for artificial fertilizer.

This kind of mutually beneficial symbiosis already exists in nature: Soybeans, chickpeas, and other legumes form intimate relationships with rod-shaped bacteria called rhizobia. These bacteria transform nitrogen from the air into a form plants can use, through a process known as nitrogen fixation. In return, the legumes feed the nitrogen-nabbing microbes a steady diet of plant sugars, and the lives of the two partners become intimately intertwined.

No comparable interdependence exists between bacteria and corn, wheat, rice, or any other cereal crop critical to modern diets and livestock feeds. Responsible for around two-thirds of all farmland worldwide, these agricultural staples are so widespread that a global landmass the size of Canada gets doused with synthetic fertilizers just to raise their yields. This chemical treatment would be superfluous, however, if scientists could build a legume-like symbiosis into the crops.

“We need to use the tools of synthetic biology to manipulate interactions between beneficial microbes and plants,” says Jean-Michel Ané, an agronomist at the University of Wisconsin–Madison who studies plant-microbe interactions. “It’s not a problem that’s going to be solved without genetic engineering.”

To overcome the threat posed by climate change, many scientists say it’s time to embrace the three most controversial letters in the food industry: GMO.

Plants need nitrogen to make two important building blocks of life: amino acids, which are stitched together to form proteins, and chlorophyll, which traps the sun’s energy to power photosynthesis. For millennia, farmers met their crops’ nitrogen demands either by spreading nutrient-rich manure on their fields or by rotating their crops, alternatively planting legume crops that were plowed under the soil to help fertilize the cereal crops that followed.

The arrival of synthetic fertilizers, pesticides, and high-yield crop varieties during the Green Revolution of the mid-20th century largely replaced these practices, ushering in a new era of bountiful food production — but at a high environmental cost.

Each year, farmers scatter more than 100 million metric tonnes of nitrogen fertilizer created through an industrial process that burns copious amounts of fossil fuel. According to the International Fertilizer Association, fertilizer production consumes an estimated 1.2% of the world’s total energy. Complicating matters for the planet: Fertilizer runoff often ends up in groundwater, rivers, and streams, where it contaminates aquatic ecosystems and threatens biodiversity. Or it evaporates into the air in the form of nitrous oxide, a greenhouse gas that’s nearly 300 times more heat-trapping than carbon dioxide.

The organic path

To wean agriculture off its dependence on synthetic fertilizers, many environmentalists advocate a return to organic farming practices. “We already have the solutions that we need,” says Dana Perls, senior food and technology policy campaigner with Friends of the Earth, an environmental advocacy organization based in Berkeley, California. “The most sustainable, least risky, and healthiest way to provide food for people across the world is using organic, regenerative, and ecological agricultural systems.”

Yet, organic farming tends to produce lower yields than conventional agriculture and might never feed the whole planet without extensive deforestation to make room for more farmland. Such deforestation would not only negate the climate benefits of forgoing fertilizer, but boost overall emissions owing to the loss of the forests’ carbon storage capacity, according to a December 2018 analysis published in the journal Nature.

“We have this tradeoff,” says Adrian Müller, an environmental policy expert at the Research Institute of Organic Agriculture in Switzerland. The only way for organic farming not to encroach on forests, Müller’s research has shown, is through a massive conversion to plant-based diets and a 50% reduction in global food waste to make up for lower yields.

The only way for organic farming not to encroach on forests is through a massive conversion to plant-based diets and a 50% reduction in global food waste.

That herculean global shift in consumer behaviours and eating habits is unlikely to ever happen. However, several companies have started offering biological alternatives to synthetic fertilizer to reduce the carbon footprint of farming. For example, last year an agricultural startup called Pivot Bio began selling a natural plant probiotic for corn that farmers can add in-furrow. The company claims the bugs can deliver nitrogen to the roots of corn plants to reduce their need for fertilizer.

Another firm called Indigo offers seed varieties of wheat, corn, soy, and rice that are laced with microbes purported to help the crops handle environmental stresses, such as extreme temperatures and water scarcity. By increasing yields, the bacteria help limit how much fertilizer farmers would otherwise need to lavish onto their fields to match the same output.

But the fertilizer savings from these probiotic products are minor — maybe 10–20%, say executives of both companies. At least twice that amount of fertilizer would need to be displaced to make a “significant impact” on global emissions, according to David Kanter, an environmental scientist at New York University who studies nitrogen pollution. The bigger the displacement, the bigger the environmental benefit.

That’s where genetic engineering could come in.

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Transgenic crops often get a bad rap for being more about corporate profits than about benefits to humanity and the environment — and for good reason. Technologies such as Roundup Ready ultimately only spurred the rise of superweeds that required even more toxic pest control, and decimated populations of frogs, insects, and other wildlife, all while making billions of dollars for Monsanto.

But those criticisms may speak more to particular applications of the technology than to the potential of the technology itself. Other genetically modified crops are nowhere near as controversial. The laboratory creation of disease-resistant Rainbow papaya is credited with saving Hawaii’s papaya industry after it was nearly decimated by the ringspot virus in the ’90s. And the introduction of vitamin A-fortified Golden Rice later this year in Bangladesh could help address micronutrient deficiencies in one of the poorest countries of Asia.

What’s more, genetic engineering technologies have seen dramatic advances in recent years. Scientists are now pushing the limits of crop development in ways that were previously impossible — including in the realm of biological nitrogen fixation.

A scientific fixation

For some scientists, the ultimate goal is to create self-fertilizing crops that can fix nitrogen themselves — but that’s a tall order. The DNA for the task can be extracted from microbes, but it doesn’t function the same way when spliced into plants. It’s such an extreme cross-species conversion, in fact, that it’s only worked in Baker’s yeast, a single-celled fungus that, while not a bacterium, is also not a plant.

It may be easier to keep nitrogen fixation under the purview of microbes and instead genetically recreate the interaction found in legumes within the roots of cereal crops. No scientist has yet succeeded in making this dream a reality, but researchers around the world are pushing forward on several of the required steps.

For example, independent teams in the United States and China have discovered ways to pull the genes needed for nitrogen fixation from one microbe, strip out all the non-essential DNA, and insert the simplified — but still functional — gene cluster into a different bacterial species. A similar bioengineering strategy could conceivably grant fertilizer-making capabilities to microbes that already colonize corn or other crops.

Without a reciprocal contribution from the plant, the nitrogen-fixing bacteria would rapidly be supplanted by more selfish bugs. That’s why scientists want to build this dyadic gift-giving into the plant’s DNA.

Alternatively, researchers could take advantage of rare nitrogen-fixing bacteria that already colonize some cereal crops — just not the ones routinely planted on conventional farms. For example, a variety of ancient corn, which grows in the nitrogen-depleted soils of the Sierra Mixe region of Oaxaca in southern Mexico, doesn’t need any fertilizer to flourish. The corn’s secret? Unusual, finger-like aerial roots, each dripping with globs of a sugar-rich gel that teems with nitrogen-fixing microbes, according to a 2018 study led by Alan Bennett, a crop geneticist from the University of California, Davis.

This Oaxacan corn grows just fine with minimal fertilizer further north, both in California and Wisconsin, but the low natural yield of this cultivar makes it unsuitable for mass food or feed production. Within the plant’s goo, however, Bennett and his colleagues discovered a bacterial strain that promotes the growth of conventional corn and potatoes, at least a little bit. And Bennett says the bug’s DNA contains “several clusters” of nitrogen fixation-related genes.

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These Oaxacan microbes (or their genes) could, in theory, be harnessed to help fertilize conventional crops. The problem is that the microbe would quickly vanish from the soil unless it gets something in return for its nitrogenous offering to the plant. As Charles Darwin pointed out, truly altruistic acts between species cannot be maintained by natural selection — and, unlike the ancient corn from Mexico, most crops don’t produce an energy source for their microbes.

Without a syrupy gel or some other reciprocal contribution from the plant, the nitrogen-fixing bacteria would rapidly be supplanted by other, more selfish bugs. That’s why scientists want to build this dyadic gift-giving into the plant’s DNA.

Researchers have successfully engineered plant roots to exude an energy supply for soil bacteria in the laboratory working with thale cress, an easy-to-genetically-manipulate weed. Scientists hope to replicate that same strategy in corn, but that still wouldn’t solve the cross-species altruism problem. To engineer a true symbiosis — one in which the plant provides fuel for bacteria and the microbes supply fertilizer in a tit-for-tat fashion — requires designing something akin to a molecular handshake: The two partners must share in the other’s biochemical bounty and only thrive when both parties are present.

Under control

This kind of “synthetic symbiosis” would obviate the need for synthetic fertilizers, notes Philip Poole, a plant microbiologist at the University of Oxford. It would also have the added bonus of mitigating the concerns around genetically modified plants escaping fields and contaminating other crops, he adds. Since any pollen that blew into a neighbouring field would not carry with it the bacteria from its roots, any resulting corn plants would be at a major competitive disadvantage, since they’d be making food for their new root bugs and getting nothing in return. As such, non-modified crops should quickly overtake any intruders.

“What we’re talking about having is a control system,” says Poole. Thanks to scientific advances in recent years, “we certainly have the tools now for the first time to build the control systems to do this,” he adds.

Others remain circumspect. “These technologies are going to be way trickier than people think,” warns Allen Good, a plant scientist at the University of Alberta in Edmonton. “It’s a very complex relationship that you have to build.”

You can get some reasonable results in the lab, but you don’t necessarily get them in the field.

Even what may seem to work initially in greenhouse testing could be hard to scale for industrial applications, says Ray Dixon, a molecular microbiologist at the John Innes Centre in the United Kingdom. He has studied nitrogen fixation for 50 years. “You can get some reasonable results in the lab, but you don’t necessarily get them in the field,” he explains.

Doug Gurian-Sherman, an independent agricultural science and policy consultant, offers a more philosophical and ecological argument against mitigating fertilizer consumption — a problem created by the scientific advances of the Green Revolution — with yet another technological fix. It only further “entrenches the current kind of monoculture, industrial system we have” and perpetuates a “farming landscape that overall is bad for the environment,” he says, noting how intensive single-crop farming has been linked to soil erosion, biodiversity loss and other forms of ecological damage. “The more you go down a path, the more you’re locked into it.”

Yet, rather than adopt the “no agrichemicals, no GMO” absolutism of organic advocates, Gurian-Sherman — who spent over a decade working for non-profit advocacy groups like the Union of Concerned Scientists and the Center for Food Safety — prefers a middle way known as agroecology, which aims to minimize chemical inputs without necessarily eliminating them altogether. Based on scientific principles and best-practices in sustainable farming, agroecology aims to deploy traditional systems of crop rotations and livestock integration, with manure used as compost or fertilizer and crop by-products recycled as animal feed.

Society can’t continue to wait for a hypothetical solution to an urgent problem.

A decade-long experiment led by agronomist Matt Liebman at Iowa State University best captures the promise of this approach. As Liebman has chronicled, diversifying conventional corn-soybean systems with the addition of small grains (oats), other legumes (alfalfa or clover), and cattle, ended up boosting corn yields by a few percentage points while lowering fertilizer use by about 90% — all with no differences in profitability and huge upsides for the surrounding soil and freshwater ecologies.

Gurian-Sherman concedes that genetically engineered crops could eventually play some role in sustainable agriculture. But the technology is still nowhere near ready for prime time, he notes, and society can’t continue to wait for a hypothetical solution to an urgent problem. “Meanwhile, we know that these agroecology systems work,” he says, “and the main barriers are our policies.”

“The issue, ironically, is not that we have not enough technology,” Gurian-Sherman adds. “It’s almost that we have too much.”

Correction: This article originally included the claim that a complete conversion to organic farming would encroach on forests unless there was “a worldwide conversion to vegetarianism.” It has been updated to the more accurate: “a massive conversion to plant-based diets.”