It is one of the Grails of agronomy: allowing the main cereals – wheat, maize, barley or rice – which constitute half of the calorie intake of humanity, to capture nitrogen from the air as legumes do this, thus limiting the spreading of nitrogenous fertilizers (nitrates), costly in energy and harmful to the environment and the climate. A step in this direction has been taken by researchers from the universities of Oxford, Cambridge and Norwich, as well as from MIT, who present their results in PNAS of April 11.
Philip Pool (Oxford) and his colleagues had the idea of combining two genetically modified organisms, a barley and a bacterium, to create a virtuous symbiosis. The barley genome has been manipulated to make it produce a molecule, rhizopin, which serves as a signal to trigger a cascade of reactions in the bacteria Azorhizobium caulinodans, present in the roots of the plant, in order to make it capture nitrogen from the air. The bacterium had, for its part, been previously modified to react conditionally to rhizopin.
One of the objectives pursued was that this association be beneficial only to the plant of interest, in this case barley, and that atmospheric nitrogen (N2) captured, transformed into ammonia (NH3) by the bacteria, is not used to fertilize the “weeds”. This is indeed what has been observed. But there is still a long way to go before this proof of concept changes farming practices. The researchers note, for example, that the nitrogenase activity, ensured by an enzyme which contributes to the production of ammonia, is still “sub-optimal” in the genetically modified micro-organism compared to so-called “wild” bacteria.
“Very tricky” approach
Philip Pool considers this work to be “a key part of a larger effort to transfer root nodulation (specific to legumes) and nitrogen fixation to cereals. This was only possible thanks to a great collaborative effort bringing together the work carried out by multiple laboratories over many years. »
The microbial ecologist Yves Dessaux, former director of research at the CNRS, whose work is cited in the article PNASwill not say otherwise. “These are ideas we were already working on twenty-five years ago,” he recalls. He couldn’t chase them. “lack of funding” and in a context where GMOs were not in vogue. He greets the approach “very cunning” of his Anglo-Saxon colleagues. But note that several obstacles will still have to be overcome.
“It will be necessary to verify that this really brings a plus to the plant in conditions where the nitrogen supply by the soil is limiting”, he notes, for example. In fact, cereals have other supply circuits for this fertiliser, which may be less “costly” in terms of energy for them and for the bacteria involved in this capture. We will also have to see if Azorhizobium caulinodans, placed “against its will” in the service of barley, will not inactivate this function to restore its natural metabolism which is less energy-consuming for it. Furthermore, this new genetically modified symbiosis will most likely require regular bacterial inoculations into the soil, with the subterranean microcosm being ruthless with newcomers. As for regulatory obstacles, we can also guarantee that they will not be lacking.