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 aapbra 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 aapbra 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."