The Rhizobium-legume symbiosis is a complex process that involves an exchange of different signals between the bacterium and its host. Plant flavonoid compounds secreted by the root function both to attract the bacteria and to induce production of rhizobial Nod factor, a lipochito-oligosaccharide that triggers hair root curling and nodule formation (Ehrhardt et al. 1995; Ardourel et al. 1994; Roche el al. 1992). Once trapped inside the curled root hair, the bacteria elicit an extended invagination of the root hair cell membrane called an infection thread. The rhizobia travel down the infection thread into the developing nodule and are released into nodule cells where they differentiate into the nitrogen-fixing bacteroid form. This paper summarizes our laboratory's efforts to understand the rhizobial functions required for nodule invasion and for the establishment of a chronic infection of the plant cells within the nodule.

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