Cell-to-Cell Communication in Rhizobia: Quorum Sensing and Plant Signaling

Abstract

This chapter briefly describes the nodulation process and some of the quorum-sensing regulatory systems that rhizobia use to monitor their population density. Conjugation is common among Rhizobiaceae, and there are very strong selection pressures to optimize growth in the rhizosphere and nodulation competitiveness. The regulation of gene transfer by quorum-sensing regulation is common among rhizobia. Individual rhizobial strains can contain up to four different LuxI-type acyl-homoserine lactone (AHL) synthases and associated regulators plus several other LuxR-type regulators lacking dedicated AHL synthases. Mutations in cinI and cinR delayed and decreased the growth rate of Rhizobium etli; because such altered growth was not observed in R. leguminosarum cinI or cinR mutants, this suggests that different sets of genes are induced via this quorum-sensing regulon in these two Rhizobium species. Bradyoxetin activity was found in extracts of all α-proteobacteria tested. This suggests that compounds similar to bradyoxetin may play an important role, not only in rhizobial symbiosis, but also in other plant- and animal-bacterial interactions. Elegant studies by researchers suggest that quorum sensing modulates both intra- and inter-species cell-cell communications. It was demonstrated that the halogenated furanones modulate LuxR activity through accelerated degradation of the transcriptional activator, rather than by blocking or displacing the binding of the AHL signal. Another potential way to interfere with quorum sensing is through the degradation or inactivation of the AHL signal molecules.