Aerial roots of Sierra Mixe corn. Photo: Jean-Michel Ané
Getting nitrogen into soils in a form plants can use has long been major problem for farmers and gardeners. But an ancient landrace of corn or maize (Zea mays) has been discovered that gets its nitrogen from bacteria feeding on the slime or mucus its roots produce.
Nitrogen (N) is abundant in the atmosphere and plants need it for their growth, but they’re unable to absorb it from the air. Billions of dollars are spent annually to produce nitrogen-rich fertilizers that plants can absorb through their roots, causing numerous environmental problems, both in its production and its use, so wouldn’t it be wonderful if plants could collect nitrogen on their own? That would certainly reduce the nitrogen pollution that’s become one of the biggest environmental problems around the world.
Of course, some plants do harvest their own nitrogen … with a little bacterial help.
Legumes, especially, including well-known food plants like peas and beans, are renowned for their ability to form symbiotic associations with nitrogen-fixing Rhizobium and Bradyrhizobium bacteria living in nodules on their roots. They give the bacteria a bit of sugar; the bacteria give them nitrogen. Everybody wins! A few other plants, like sea buckthorn (Hippophae rhamnoides) have likewise developed symbioses with nitrogen-fixing bacteria, but the vast majority of plants have to get their nitrogen from the decomposition of plant and animal products … or fertilizer!
Now, however, a strain of corn has been found that also has a symbiotic relationship with nitrogen-fixing bacteria. This tall-growing (16 to 20 feet/4.5 to m high, twice the height of modern corn varieties), odd-looking traditional corn from the Mixes District of Oaxaca (southern Mexico), now being called Sierra Mixe, has been grown for hundreds, possibly thousands, of years. This is one of the areas where the corn was first developed from the wild grass teosinte some 7000 years ago. Sierra Mixe corn is adapted to poor soil, where it thrives without the use of fertilizer. And this it does through nitrogen fixation.
It took nearly 30 years of study fully to understand what was going on, from the discovery of the landrace by Howard-Yana Shapiro (now with Mars, Incorporated, a major sponsor of the study) in the 1980s to today’s team, with Alan B. Bennett at the helm, at the University of California, Davis.
This corn produces numerous pink and green aerial roots from its stem nodes and these are covered a sort of clear, gelatinous mucilage. It turns out nitrogen-fixing bacteria settle in the slime and pull nitrogen from the air, then share it with the corn, contributing from 29% to 82% of the nitrogen its uses.
Now, the question is: can this ability be bred into standard corn? If so, will the nitrogen-fixing capacity be widely adapted (corn is grown in an extensive range of environments all over the globe)? And does it exist in other cereal crops or can it be introduced into them? If so, can its effect be intensified by careful breeding?
Corn is already the most widely produced grain in the world (1,099 million metric tons are grown annually), followed by wheat (734 metric tons) and rice (496 metric tons). If all three produced most of their own nitrogen, that would certainly lighten the load on the environment!
For more information on the study behind this discovery, read Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota.