by Matt Simon
If crops could feel envy, it’d be for legumes. Bean plants have a superpower. Or more accurately, they share one. They’ve developed symbiotic relationships with bacteria that process atmospheric nitrogen into a form that’s usable for those plants — an essential element for building their tissues, photosynthesizing, and generally staying healthy. This is known as nitrogen fixation. If you look at a legume’s roots, you’ll see nodules that provide these nitrogen-fixing microbes with a home and food.
Other crops — cereals like wheat, rice, and corn — don’t have such a deep symbiotic relationship, so farmers have to use large amounts of fertilizer to get the plants the nitrogen they need. This is very expensive. And fertilizer production is not great for the environment. It’s not easy to turn atmospheric nitrogen into a form of nitrogen that plants can absorb on their own.
“It takes a lot of energy and really high pressures and high temperatures,” says University of Illinois Urbana-Champaign plant biologist Angela Kent. “Bacteria do this at ambient temperatures and pressures, so they’re pretty special. While energy has been cheap, it’s been easy for us to overuse nitrogen fertilizers.”
Even worse, once it’s on fields, fertilizer spews nitrous oxide, which is 300 times as potent a greenhouse gas as carbon dioxide. Runoff from fields also pollutes water bodies, leading to toxic algal blooms. This is a particularly bad problem in the Midwest, where fertilizer empties into the Mississippi River and flows into the Gulf of Mexico, fueling massive blooms every summer. When those algae die, they suck the oxygen out of the water, killing any sea creatures unfortunate enough to be in the area and creating a notorious aquatic dead zone that can grow to be the size of New Jersey. Climate change is only exacerbating the problem, since warmer waters hold less oxygen to begin with.
Given all that nastiness, scientists have long been on a quest to reduce agriculture’s dependence on fertilizers by giving cereal crops their own nitrogen-fixing powers. And with the rise of gene-editing technology over the past few decades, that quest has been making progress. Last month, in the Plant Biotechnology Journal, researchers described a breakthrough with rice, engineering the plant to produce more compounds that encourage the growth of biofilms, which provide a cozy home for nitrogen-fixing bacteria, much like legumes provide nodules for their partner microbes.
“People for the last 30, 40 years have been trying to make cereals behave like legumes,” says Eduardo Blumwald, a plant biologist at the University of California, Davis who coauthored the new paper. “Evolution in that sense is very cruel. You cannot do in the lab what took millions and millions of years.”
So what’s with the evolutionary cruelty? Why can some plants — like, say aquatic ferns — fix nitrogen while others can’t?
It’s not that other species don’t get nitrogen at all. Cereal grasses use nitrogen that’s already in the soil — it comes from animal manure, as well as all the life churning in the dirt. (Lots of different bacterial groups process atmospheric nitrogen, not just the legumes’ symbionts.)
But the legumes’ bacteria grab abundant nitrogen straight from the air.
“When you have these nodules and you have this symbiotic relationship, it’s a much more effective way of getting atmospheric nitrogen,” says Joshua Doby, an ecologist at the University of Florida. “Because otherwise you have to wait for the bacteria and for other processes in the soil to turn it into ammonium.”
One theory is that the symbiotic nitrogen relationship started out long ago as a bacterial infection, and those ancestor plants derived a benefit that was carried through to future generations. Earlier this year, Doby published a study of plants throughout the United States, finding that there is a greater diversity of nitrogen-fixing species than other kinds in arid regions. That is true even if the soil isn’t nitrogen-poor. He theorizes that millions of years ago, when those areas were wetter, the plants evolved the ability to fix nitrogen, which also allowed them to grow thicker cuticles. This trait protected them against dryness when the region eventually became arid. They were pre-adapted, basically. Non-fixers, by contrast, were weeded out by rising aridity.
Another theory is that legumes might be consummate nitrogen-fixers because something in their genome predisposes them to building nodules.
But before you start feeling sorry for non-fixers, constructing nodules and hosting bacteria comes at a major cost. “It turns out that it’s very energetically expensive to actually do this,” says Ryan Folk, a biodiversity scientist at Mississippi State University who coauthored the new paper with Doby. First, a legume has to build those nodules on its roots, then it has to provide sugars to the bacteria to keep them happy.
“It’s something like 20 to 30 percent of the legumes’ photosynthetic output actually goes to the bacteria, so it’s an extraordinary price,” he says. So even though it’s less efficient for plants to get their organic nitrogen from bacteria already in the soil, it’s also less costly because symbiotic bacteria are super needy.