"Nitrogen, nitrogen, everywhere, nor any drop to drink."
If wheat, rice, and corn crops could rhyme using ancient mariner tongue, that might be their unfertilized catchphrase. Despite nitrogen-based molecules being essential for plant growth, and despite nitrogen gas—N2—making up nearly four-fifths of the atmosphere, nitrogen in its abundant N2 form is off-limits for many plants.
The result of evolutionary mapping by European and Australian scientists, published this week in Nature Communications, however, is offering fresh hope for long-running efforts to coax cereal crops into accessing the natural nitrogen around them. That could help wean agriculture off its expensive and environmentally destructive nitrogen-based fertilizer addiction.
Flowering plants evolved a mysterious genetic precursor to enter into a partnership with nitrogen-fixing bacteria at just one remarkable moment in their evolutionary history.
The vice-like bonds that hold the two nitrogen atoms together in an N2 molecule are too strong for plants to break apart, which is why larger nitrogen-based molecules are applied as fertilizer. Many flowering plants have developed a work-around. Some species, including soybeans, alfalfa, and peas, are wedded to nitrogen-fixing bacteria in symbiotic relationships. The microbes are ushered up from the soil through plants' roots to form nodules, where N2 is absorbed by the bacteria and "fixed" into useful ammonia. In exchange, the plant provides the bacteria with photosynthesized sugars.
Figuring out how to transfer this ability to corn, wheat, and rice crops, all of which are flowering plants, is a high priority for agricultural researchers. "Transferring this symbiosis to non-fixing crops is a huge techno dream," says Toby Kiers, a mutualism researcher at VU University Amsterdam and an author of the new paper. "People have been trying to for decades—and failing."
Kiers was part of a team that compiled a database of nitrogen-fixing plants, then mapped their evolution with the aid of a family tree of 32,223 species of flowering plants.
The team concluded that flowering plants evolved a mysterious genetic precursor to enter into a partnership with nitrogen-fixing bacteria at just one remarkable moment in their evolutionary history—more than 100 million years ago. They speculate that the precursor might have been a tweak to the ancient mycorrhizal relationship between plants and fungi. The nitrogen-fixing relationship with bacteria was gained, lost, and regained as flowering species evolved over subsequent eons.
The researchers identified non nitrogen-fixing modern species, which, based on their evolutionary history, may still harbor the precursor.
In the chart below, each white and gray band represents 50 million years of evolutionary history. The star shows the point where the cryptic precursor is thought to have sprung into existence. A "fixer" can fix nitrogen; a "stable fixer" is more likely to sustain the ability down the road.
(Chart: Nature Communications)
"Now that we know which plants have a high likelihood of containing the precursor, we can compare their genetic codes," Kiers says. "By doing this across all flowering plants, we can determine what these plants have in common. Once the cryptic code is known, it will still be difficult, but at least more possible, to introduce it into cereal crops."