METABOLITE TRANSPORT ACROSS SYMBIOTIC MEMBRANES OF LEGUME NODULES

Infection of legume roots or stems with soil bacteria of the Rhizobiac eae results in the formation of nodules that become symbiotic nitrogen -fixing organs. Within the infected cells of these nodules, bacteria a re enveloped in a membrane of plant origin, called the peribacteroid m embrane (PBM), and divide and differentiate to form nitrogen-fixing ba cteroids. The organelle-like structure comprised of PBM and bacteroids is termed the symbiosome, and is the basic nitrogen-fixing unit of th e nodule. The major exchange of nutrients between the symbiotic partne rs is reduced carbon from the plant, to fuel nitrogenase activity in t he bacteroid, and fixed nitrogen from the bacteroid, which is assimila ted in the plant cytoplasm. However, many other metabolites are also e xchanged. The metabolic interaction between the plant and the bacteroi ds is regulated by a series of transporters and channels on the PBM an d the bacteroid membrane, and these form the focus of this review.