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New findings on how nitrogen-fixing bacteria associate with their host plant could help increase crop yields without using nitrogen fertilisers, say scientists.

Research stemming from this discovery could benefit developing countries, where the quality of the soil and the nitrogen content is often poor.

Currently nitrogen fertilisers are used to supplement these soils, which can be costly and cause ecological problems.

In a paper published last week (1 June) in the journal Science, a team of French and American researchers report that some types of rhizobia ― soil bacteria that provide nitrogen to leguminous plants such as peas and soybeans ― interact with their host plants in previously unknown way.

Rhizobia establish a symbiotic relationship with their host plant ― exchanging nitrogen for nutrients ― by forming nodules, which penetrate the roots and sometimes stems of the plant.

Until last week, it was universally believed that these nodules only appeared when the rhizobia release specific molecules under the control of 'nod' genes.

But these nod genes are absent from some strains of Bradyrhizobium, a type of rhizobia studied by the team. Since the Bradyrhizobium strains still form nodules to establish symbiosis, they must be using different signalling molecules to do so.

"By demonstrating an alternative mechanism used by these rhizobia to enter into symbiosis with the host plant, we have broken the dogma," Eric Giraud, from the Institute of Research for Development in France and lead author of the study, told SciDev.Net. 

Giraud predicts that within ten years, we may be able to associate rhizobia with other plants, including non-leguminous plants. "This could reduce the need for artificial fertilisers, providing both ecological and economic advantages," he says.

Rhizobia already assist several important leguminous crops to grow in nitrogen-poor soils, including peanuts, peas, lentils and soybeans, as well as trees such as carob and acacias.

Link to full paper in Science

Reference: Science 316, 1307 (2007)

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