Metabolite transport across the peribacteroid membrane during broad bean development

A temporal pattern of the peribacteroid membrane (PBM) transport function was studied. Spectrophotometric recording was used for establishing the effect of carbon-and nitrogen-containing substrates (malate, succinate, and glutamate) on the acidification of the peribacteroid space and the intensity of light scattering in the symbiosome suspension from broad bean (Vicia faba L.) root nodules of different age. At the early stages of nodule formation and functioning, PBM is permeable not only for malate and succinate, but also for glutamate, and this permeability fully provides for the active bacteroid division and the nitrogenase complex synthesis in the bacteroids at the expense of the carbon-and nitrogen-containing substrates. Mature nodules are characterized by the greatest nitrogen-fixing activity. In these nodules, PBM is selectively permeable for malate and succinate, but constitutes a barrier for glutamate. Thereby, mutually beneficial relations between the symbiotic partners are achieved. In senescent nodules, a rearrangement of symbiotic interactions is directed toward a minimization of both carbon and nitrogen metabolite consumption by the bacteroids. It is concluded that, in the course of the development of the legume-rhizobia symbiosis, the PBM transport function is changed. This function determines a qualitatively different pattern of symbiotic partner interactions in the following sequence: parasitism-mutualism-commensalism.