Mechanisms of Cell and Tissue Invasion by Rhizobium Leguminosarum: The Role of Cell Surface Interactions

Rhizobium invades host cells and tissues as a result of a reorganisation of plant cell wall growth. This leads to the development of an intracellular tunnel, the infection thread, that traverses the host plant cell from one side to the other and provides a channel for the entry of rhizobial cells embedded in an extracellular matrix material secreted by the plant. Some of the components involved in cell surface interactions can be individually identified using monoclonal antibodies as molecular probes. The role of lipopolysaccharide (LPS) in cell and tissue invasion was investigated by examining pea nodules induced by mutants of R. leguminosarum with defects in LPS structure and biosynthesis. We conclude that the correct LPS structure is essential for “invasiveness” of plant cells and tissues; for avoidance of host defence responses; and for physiological adaptation to the endophytic microenvironment. Differentiation of released bacteria into nitrogen-fixing bacteroids is associated with further changes in cell surface composition, as revealed by in situ analysis of LPS epitope variation using monoclonal antibodies: these changes in epitope expression reflect variations in physiological and developmental status for bacteroids in different regions of the same pea nodule. Bacteroid differentiation is preceded by differentiation of the plant-derived peribacteroid membrane (pbm) which encloses the symbiosome compartment. Using a monoclonal antibody that identifies a group of plant membrane-associated inositol-containing glycolipids, we have identified a very early marker for the differentiation of pbm from plasma membrane.