Amino-acid cycling drives nitrogen fixation

About 65% of the biosphere's available nitrogen is provided by the reduction of atmospheric N₂ to ammonium. Most of this is achieved by symbiotic nitrogen fixation, carried out in specialized plant organs called nodules. Their formation 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, Ludwig and colleagues from Reading University, UK show that the interaction is more complex, with plants providing amino acids to the bacteriods. This allows the shutdown of ammonium assimilation, and bacteriods cycle amino acids back to the plants for asparagines synthesis.

Ludwig et al. examined amino-acid cycling in pea bacteriods by the mutation of two ABC-type amino-acid transporters with broad specificity - aap and bra. Single mutants resulted in a 40-70% reduction in rates of amino-acid uptake by pea nodules. No difference in pea growth was observed in the presence of the mutant bacteriods, however. A double aap bra mutant was also capable of amino-acid synthesis, but phenotypic observations suggested that the plants were unable to fix nitrogen. Further analysis established that the plants were capable of reducing nitrogen in the presence of the aap bra mutants, but that the plants could not acquire ammonium. The authors propose that plants provide bacteriods with amino acids via Aap and/or Bra; in turn, bacteriods can shut down ammonium assimilation. To obtain amino acids the bacteriods secrete ammonium to the plant - thus allowing amino-acid synthesis to occur.

The authors conclude that "the interaction between the symbiotic partners is far more complex than hitherto realized: each has evolved a complete metabolic dependence on the other."