Regulation of Symbiotically Important Functions by Quorum Sensing in theSinorhizobium meliloti–Alfalfa Interaction

The establishment of the symbiosis between the nitrogen-fixing Sinorhizobium meliloti and alfalfa relies on a molecular dialog between the two partners. Rhizobia are attracted by flavonoids present in the root exudates. Bacterial Nod factors, together with exopolysaccharides, are responsible for proper recognition, nodule formation, and invasion of the host. It has been shown that the successful invasion of alfalfa requires the down regulation of motility, attachment and colonization of the root surfaces, and the production of exopolysaccharides. In the present chapter, we review how these symbiotically-relevant functions are regulated by quorum sensing in S. meliloti and how rhizobia respond to these signals.