The long and leguminous quest to give crops nitrogen superpowers

This story was originally published by WIRED and is reproduced here as part of the Climate office collaboration.

If crops could be desired, it would be for legumes. Bean plants have a super power. Or more accurately, they share one. They have developed symbiotic relationships with bacteria that convert atmospheric nitrogen into a usable form for these plants – an essential element for building their tissues, photosynthesis and generally maintaining good health. This is called nitrogen fixation. If you look at the roots of a legume, you see nodules that provide these nitrogen-fixing microbes with a home and food.

Other crops – grains such as wheat, rice and maize – do not have such a deep symbiotic relationship, so farmers must use large amounts of fertilizer to provide plants with the nitrogen they need. need. It is very expensive. And the production of fertilizers is not good for the environment. It is not easy to convert atmospheric nitrogen into a form of nitrogen that plants can absorb on their own.

“It takes a lot of energy and very high pressures and temperatures,” says Angela Kent, a plant biologist at the University of Illinois at Urbana-Champaign. “Bacteria do this at ambient temperatures and pressures, so they are quite special. While energy was cheap, it was easy for us to overuse nitrogen fertilizers.

Worse still, once in the fields, the fertilizer releases nitrous oxide, which is 300 times more powerful a greenhouse gas in the form of carbon dioxide. Runoff from fields also pollutes water bodies, leading to toxic algal blooms. It’s a particularly serious problem in the Midwest, where fertilizers flow into the Mississippi River and flow into the Gulf of Mexico, fueling massive blooms every summer. When these algae die, they suck oxygen out of the water, killing any sea creatures unfortunate enough to be in the area and creating a notorious aquatic dead zone which can reach the size of New Jersey. Climate change is only exacerbating the problem, as warmer waters contain less oxygen to begin with.

Given all this nastiness, scientists have long sought to reduce agriculture’s reliance on fertilizers by giving grain crops their own nitrogen-fixing powers. And with the rise of gene-editing technology over the past few decades, this quest has progressed. Last month in the Plant Biotechnology Journalresearchers describe a breakthrough with rice, designing the plant to produce more compounds that encourage the growth of biofilms, which provide a comfortable home for nitrogen-fixing bacteria, much like legumes provide nodules for their partner microbes.

“For 30 or 40 years, people have been trying to get grains to behave like legumes,” says Eduardo Blumwald, a plant biologist at the University of California, Davis, who co-authored the new paper. “The development in this direction is very cruel. You can’t do in the lab what took millions and millions of years.

So what about evolutionary cruelty? Why some plants can – like, say aquatic ferns — fix nitrogen when others cannot?

It’s not that the other species don’t get any nitrogen at all. Cereal grasses use nitrogen already present in the soil — it comes from animal manure, as well as all the life churning in the dirt. (Many different bacterial groups process atmospheric nitrogen, not just legume symbionts.)

But legume bacteria capture abundant nitrogen directly from the air.

“When you have these nodules and you have this symbiotic relationship, it’s a much more efficient way to get atmospheric nitrogen,” says University of Florida ecologist Joshua Doby. “Because otherwise you have to wait for bacteria and other processes in the soil to turn it into ammonium.”

One theory is that the symbiotic relationship with nitrogen began long ago as a bacterial infection, and that these ancestral plants derived a benefit from it that was passed on to future generations. Earlier this year, Doby published a study plants across the United States, finding that there is a greater diversity of nitrogen-fixing species than other species in arid regions. This is true even if the ground is not low in nitrogen. He theorizes that millions of years ago, when these areas were wetter, plants developed the ability to fix nitrogen, which also allowed them to develop thicker cuticles. This trait protected them from drought when the region eventually became arid. They were pre-fitted, basically. Non-fixers, on the other hand, were eliminated by increasing aridity.

Another theory is that legumes could be consumed nitrogen fixers because something in their genome predisposes them to nodule formation.

But before you start feeling sorry for the non-repairers, building nodules and harboring bacteria comes at a major cost. “It turns out to be very expensive energetically,” says Ryan Folk, a biodiversity scientist at Mississippi State University, who co-authored the new paper with Doby. First, a legume has to build these nodules on its roots, and then it has to supply sugars to the bacteria to keep them happy.

“It’s something like 20-30% of the photosynthetic production of legumes that goes to bacteria, so it’s an extraordinary price,” he says. So while it is less efficient for plants to get their organic nitrogen from bacteria already present in the soil, it is also less expensive because symbiotic bacteria are very needy.

What Blumwald and his colleagues did with rice is sort of halfway between the legumes and non-fixing plant strategies. They sifted through the compounds produced by the plant, testing which ones induced biofilm formation.

“When bacteria form biofilms, it’s like a hippie community — they’re cozy, they’re all together, they share things,” Blumwald says.

A complex layer of polysaccharides, proteins and lipids covers the biofilm, which is not permeable to oxygen. This is important because oxygen interferes with bacteria fixing nitrogen from the air – in legumes the nodules keep oxygen out.

The team landed on a biofilm-boosting compound called apigenin. They then used Crispr gene editing to silence the plant’s expression of an enzyme that breaks down this apigenin, allowing more of the compound to accumulate in the plant and extrude into the soil to create a biofilm.

“Then the bacteria started fixing nitrogen from the air to produce ammonium that the plant can absorb,” Blumwald explains. “The proportion of nitrogen fixation relative to the rest of the bacteria near the root increased.” Basically, the rice plant now had its own fertilizer plant, giving it the nitrogen-fixing power that evolution had denied it.

This appears to circumvent a problem with previous attempts to get grain crops to fix their own nitrogen, says Kent, a plant biologist at the University of Illinois’ Urbana-Champaign, who was not involved in the research. People have tried inoculating soils with nitrogen-fixing bacteria in hopes that plants and microbes will form a partnership. But it was difficult, because the soil microbiome is an extremely complex ecosystem of competing bacteria.

“One thing I really liked about this paper is that it looks at modifying plants to better associate with the soil microbiome,” Kent says. “It helps recruit the kind of microbes you want and give them a competitive edge.”

Interestingly, scientists previously discovered a corn variety unique to Mexico that fixes nitrogen in the same way. The tubular roots of maize grow above the ground, sheathing itself in a bizarre mucilage – lots of dripping goo. Like the biofilm around rice roots, this mucilage harbors nitrogen-fixing bacteria. The authors of the maize study believe that it would be possible to reproduce this trait in commercial varieties of maize.

Another problem with previous inoculation attempts, Kent says, is that the introduced bacteria can’t supply all the nitrogen the plants need. A farmer should always apply fertilizer – but excessive fertilizer application can actually overload the natural nitrogen fixers in the soil, sending them into hibernation. The field goes numb, essentially, as the beneficial microbiome short-circuits itself.

A company called Pivot Bio designs nitrogen-fixing bacteria that won’t die out in the presence of added nitrogen. “We break the genetic feedback loop that causes them to go into hibernation when the fields are fertilized,” explains Karsten Temme, CEO and co-founder of the company.

Now they are launching new products in which these microbes are applied directly to seeds of corn, wheat and other grains. (With earlier products, they instead sprayed the bacteria in liquid form when planting seeds.) Currently, the microbes cannot provide all the nitrogen these grains need, so farmers may still need to fertilize. But Temme says the company is improving the effectiveness of microbes.

“What we’re seeing is there’s going to be a progression, where today we’re providing a fraction of that nitrogen,” he says, “and over time we start to provide the majority and eventually all of that nitrogen that the crop needs.”

An effective biological nitrogen fixation system for rice could be “a game-changer in global agriculture”, says Pallavolu Maheswara Reddy, who studies nitrogen fixation in cereals at the Indian Institute of Energy and Resources. This is because the human population is growing rapidly, requiring more food and fertilizer to feed it.

“Since the advent of the green revolution in the mid-1960s, the application of chemical nitrogen fertilizers has increased rice yields by 100-200% to meet the demands of the world’s population,” says Reddy. “Over the next 30 years, we will need to produce almost 50% more rice than is currently produced to meet the food needs of a growing human population.

But even if scientists can just reduce the amount of fertilizer needed for farming, the industry would save some of the energy needed to make these products while reducing both farmers’ costs and the runoff that goes into waterways . This will be particularly important in parts of the world where climate change is making the showers more powerful (a warmer atmosphere in general retains more water), which will remove more fertilizer from the fields.

And just in case you’re worried about leagues of nitrogen-fixing plants spreading out of control with their new superpower, Kent says there’s nothing to worry about. “We don’t see pulses taking over the world,” Kent says. Nitrogen fixation “probably isn’t the trait a plant would need to become a superplant.”