TRANSPORT OF FIXED NITROGEN ACROSS SYMBIOTIC MEMBRANES OF LEGUME NODULES

Symbiotic nitrogen fixation requires an exchange of reduced carbon to the endosymbiont for fixed nitrogen to the host plant. In this review, the general properties of isolated symbiosomes and transport across t he peribacteroid membrane are discussed, with attention focused on the efflux of fixed nitrogen. In Bradyrhizobium symbiosomes, no evidence has been obtained for amino acid transporters on the peribacteroid mem brane, even though bacteroids possess active uptake systems. Studies w ith free-living Bradyrhizobium japonicum suggest that the presence of bacteroid transporters reflects an amino acid poor peribacteroid space . Evidence for the presence of a monovalent cation channel on the peri bacteroid membrane of soybean symbiosomes, capable of transporting amm onium out of the symbiosome, is presented and discussed. This channel is voltage-gated and outwardly rectified, properties well-suited to en sure rapid efflux of NH4+ to the plant cytosol upon energization of th e peribacteroid membrane. We suggest that, in soybean at least, the si mple scheme of malate into the symbiosome and ammonium out represents the major exchange of carbon and nitrogen between bacteroid and plant.