Approximately 65% of the available nitrogen in the biosphere comes from the reduction of atmospheric nitrogen to ammonium, most of which is derived by symbiotic nitrogen fixation carried out in specialized organs called plant nodules. The formation of nodules is induced on leguminous plants by bacteria belonging to the family Rhizobiaceae. It has been thought that this symbiotic relationship is sustained by the plant providing the bacteria with sources of carbon and energy in return for ammonium. In the April 17 Nature, Emma Lodwig and colleagues from the University of Reading, UK show that the amino acid cycle is more complex, with plants providing amino acids to the bacteroids. This enables the shutdown of ammonium assimilation, and bacteroids cycle amino acids back to the plants for asparagines synthesis (Nature, 422: 722-726, April 17, 2003).

Lodwig et al. examined amino acid cycling in pea bacteroids by the...

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