Getting cereal crops to fix atmospheric nitrogen is a long-standing goal for plant researchers and breeders. Now, in the case of maize, this goal has unexpectedly been brought closer, with the discovery of an old Mexican variety able to cover between 30 and 80% of its nitrogen requirements through bacterial symbiosis. Actually, the variety has been known by Native Americans for a long time. “Subsistence farmers in the Oaxaca region have been farming it for hundreds of years as a food crop,” explains Dr Jean-Michel Ané, who studies the features of the plant at the University of Wisconsin in Madison, USA. “It is traditionally sown in ‘clusters’ of five to six seeds and grown without fertiliser, reaching a height of up to five metres. This brought us to the question: How does this giant maize plant satisfy its high nitrogen requirements under conditions like these?”
Eight years of testing have uncovered a trait which was previously unprecedented among the grass family. The Oaxaca maize’s aerial roots secrete a mucus-like gel which is colonised by Azospirillum and Herbaspirillum bacteria. These genera of bacteria otherwise live on the soil surface and are able to absorb nitrogen from the atmosphere.
Reducing fertiliser needs
The project, in which the University of California, Davis, and Mars Inc. are also involved, has now proved that it is possible to transfer this trait to common varieties through hybridisation. “The goal is to significantly reduce the use of fertilisers, without affecting yields,” says Dr Ané. “Fertilisation before or during sowing won’t change, but it is the subsequent applications that we will be able to cut back on.” Several breeding companies are showing interest in this discovery, says the researcher, who expects a hybrid variety within the next 10 years. “Being able to provide 10% of nitrogen requirements via symbiosis would be a major success. We are hoping to reach 20-30%.”
Important traits to maintain are the high number of stem nodes and aerial roots, as well as the amount of gel produced by each aerial root. Another area of research is the selection of the most effective bacteria for developing an inoculant.
An (almost) unique property
The tropical Oaxaca maize variety produces a sugary mucus which is home to nitrogen-absorbing bacteria. Each aerial root secretes 1.5-2ml of this gel after a half-hour period of rain. The contained bacteria stays vital for up to three days without further water supply. The phenomenon occurs for four months of the year. Previously, only one further example of this mechanism was known about, in the Brazilian giant rhubarb (Gunnera manicata).