Alanine, not ammonia, is excreted from N2-fixing soybean nodule bacteroids

Symbiotic nitrogen fixation, the process whereby nitrogen-fixing bacteria enter into associations with plants, provides the major source of nitrogen for the biosphere, Nitrogenase, a bacterial enzyme, catalyzes the reduction of atmospheric dinitrogen to ammonium, In rhizobialeguminous plant symbioses, the current model of nitrogen transfer from the symbiotic form of the bacteria, call ed a bacteroid, to the plant is that nitrogenase-generated ammonia diffuses across the bacteroid membrane and is assimilated into amino acids outside of the bacteroid, We purified soybean nodule bacteroids by a procedure that removed contaminating plant proteins and found that alanine was the major nitrogen-containing compound excreted. Bacteroids incubated in the presence of N-15(2) excreted alanine highly enriched in N-15. Th, ammonium in these assays neither accumulated significantly nor was enriched in N-15, Th, results demonstrate that a transport mechanism rather than diffusion functions at this critical step of nitrogen transfer from the bacteroids to the plant host. Alanine may serve only as a transport species, but this would permit physiological separation of the transport of fixed nitrogen from other nitrogen metabolic functions commonly mediated through glutamate.