Molecular composition and development of the plant glycocalyx associated with the peribacteroid membrane of pea root nodules

In root nodules of pea (Pisum sativum), endosymbiotic Rhizobium bacteroids are individually enclosed by a plant-derived membrane, the peribacteroid membrane, creating an organelle-like structure termed the symbiosome. In order to investigate the structure and function of the peribacteroid membrane in plant-microbe surface interactions, monoclonal antibodies were used to identify the major antigenic components of this membrane and to investigate their cellular and tissue distribution. Immunofluorescence studies with isolated intact and ruptured symbiosomes indicated that the relevant antigens were confined to the luminal (inner) face of the peribacteroid membrane. Biochemical analysis indicated that the antibodies recognised carbohydrate epitopes associated with Golgi-derived glycoproteins and glycolipids. These glycoconjugate molecules are collectively termed the glycocalyx. Three different classes of glycocalyx antigens, reacting with different groups of monoclonal antibodies, could be distinguished. Immunolocalisation studies of longitudinal sections of pea nodules revealed enhanced expression of glycocalyx antigens in infected nodule tissue, with the three classes of antigen showing different degrees of expression in different developmental zones of the tissue. One of the classes of antigen was absent from the apical meristematic region of pea nodules but was re-expressed during cell invasion by Rhizobium and the development of peribacteroid membranes. In Phaseolus nodules, as opposed to those of Pisum, this group of antigens was entirely lacking from the central infected tissue.