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Chapter 14 : Cell-to-Cell Communication in Rhizobia: Quorum Sensing and Plant Signaling
Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
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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.
Arrangement of plasmid transfer and replication genes in different plasmids in the Rhizobiacea. The characterized genes involved in plasmid replication (repABC), conjugation (trbBCDEJKLFGHI, traAFBH, traCDG), and regulation of conjugation (traI, traR, traM, bisR) are shown. The traR and traM genes on pRL8JI are in the opposite orientation to those in the other strains. The traABFH and traCDG genes have not been sequenced on pRL1JI and pRme41 but are probably present. The trbBCDEJKLFGHI and traG genes are not present on pRL7JI.
Model for recipient-induced transfer of plasmid pRL1JI from R. leguminosarum bv. viciae. BisR is activated by CinI-made 3-OH-C14:1-HSL from potential recipients (i), resulting in the induction of traR expression. The traR induction of traI (ii) and the resulting TraI-made AHLs induce a positive feedback loop (iii), causing high-level expression of the traI-trb operon; the plasmid-replication repABC operon is induced in parallel with the traI-trb operon. Premature accumulation of TraR is avoided by the expression of the TraR anti-activator TraM (iv). BisR, produced when pRL1JI is present, represses cinI expression (v), thereby preventing 3-OH-C14:1-HSL activation of TraR in pRL1JI-containing strains. The repression of cinI and induction of traR by BisR enable the pRL1JI donor to respond to CinI-made 3-OH-C14:1-HSL from potential recipients. Figure adapted from Danino et al. ( 21 ) with permission from Blackwell Publishing.
Rhizobial quorum-sensing systems