LIPOPOLYSACCHARIDE EPITOPE EXPRESSION OF RHIZOBIUM BACTEROIDS AS REVEALED BY IN-SITU IMMUNOLABELING OF PEA ROOT-NODULE SECTIONS

To investigate the in situ expression of lipopolysaccharide (LPS) epit opes on nodule bacteria of Rhizobium leguminosarum, monoclonal antibod ies recognizing LPS macromolecules were used for immunocytochemical st aining of pea nodule tissue. Many LPS epitopes were constitutively exp ressed, and the corresponding antibodies reacted in nodule sections wi th bacteria at all stages of tissue infection and cell invasion. Some antibodies, however, recognized epitopes that were only expressed in p articular regions of the nodule. Two general patterns of regulated LPS epitope expression could be distinguished on longitudinal sections of nodules. A radial pattern probably reflected the local physiological conditions experienced by endosymbiotic bacteria as a result of oxygen diffusion into the nodule tissue. The other pattern of expression, wh ich followed a linear axis of symmetry along a longitudinal section of the pea nodule, was apparently associated with the differentiation of nodule bacteria and the development of the nitrogen-fixing capacity i n bacteroids. Basically similar patterns of LPS epitope expression wer e observed for pea nodules harboring either of two immunologically dis tinct strains of R. leguminosarum by. viciae, although these epitopes were recognized by different sets of strain-specific monoclonal antibo dies. Furthermore, LPS epitope expression of rhizobia in pea nodules w as compared with that of equivalent strains in nodules of French bean (Phaseolus vulgaris). From these observations, it is suggested that st ructural modifications of Rhizobium LPS may play an important role in the adaptation of endosymbiotic rhizobia to the surrounding microenvir onment.