Differentiation of Free-Living Rhizobia into Endosymbiotic Bacteroids

This chapter outlines the complexities of bacterial invasion and bacteroid differentiation. Because the early plant-bacterial signal exchange has been the subject of many excellent recent reviews the chapter emphasizes the later stages about which less is known. The strategies that both bacteria and plant use to maintain the symbiosis and prevent pathogenesis are also discussed. The formation of effective nodules containing differentiated, nitrogen-fixing bacteroids consists of a defined series of stages. In response to chemical signals secreted by plant roots, rhizobia attach to root hairs, which are cells on the root surface that project outward into the soil. Chemotaxis plays an important role in the initial attraction of rhizobia to plant root hairs. The signal transduction pathway leading to the formation of the nodule must have unique characteristics, because nodules are completely different from other normal plant structures. Mutants lacking succinoglycan were first isolated by the inability of colonies to fluoresce on plates containing the laundry whitener Calcofluor. Such mutants still complete Nod factor-dependent events, such as root hair deformation, cortical cell divisions, and infection thread initiation. Immunological studies with monoclonal antibodies have documented changes in lipopolysaccharide (LPS) structure during bacteroid differentiation and when free-living rhizobia are cultured in different media. Knockout strategies are useful because it is easy to map and clone the affected genes and because many functions in bacteroid development are probably not essential for free-living cells.