Regulation Mediated by N-Acyl Homoserine Lactone Quorum Sensing Signals in the Rhizobium-Legume Symbiosis

Abstract

Soil-dwelling bacteria collectively referred to as rhizobia synthesize and perceive N-acyl-homoserine lactone (AHL) signals to regulate gene expression in a population density-dependent manner. AHL-mediated signaling in these bacteria regulates several functions which are important for the establishment of nitrogen-fixing symbiosis with legume plants. Moreover, rhizobial AHL act as interkingdom signals triggering plant responses that impact the plant-bacteria interaction. Both the regulatory mechanisms that control AHL synthesis in rhizobia and the set of bacterial genes and associated traits under quorum sensing (QS) control vary greatly among the rhizobial species. In this article, we focus on the well-known QS system of the alfalfa symbiont Sinorhizobium (Ensifer) meliloti. Bacterial genes, environmental factors and transcriptional and posttranscriptional regulatory mechanisms that control AHL production in this Rhizobium, as well as the effects of the signaling molecule on bacterial phenotypes and plant responses will be reviewed. Current knowledge of S. meliloti QS will be compared with that of other rhizobia. Finally, participation of the legume host in QS by interfering with rhizobial AHL perception through the production of molecular mimics will also be addressed.

Deciphering the Symbiotic Significance of Quorum Sensing Systems of Sinorhizobium fredii HH103

“…As commented above, the QS systems of S. meliloti and S. fredii NGR234 are controlling some important traits such as growth rate, surface polysaccharide production, motility, or biofilm formation that, to a greater or lesser extent, are related with an optimal symbiotic performance [22]. In order to decipher the symbiotic relevance of QS systems of S. fredii HH103, these nodulation-related traits were analyzed in the wild-type and mutant strains.…”

“…In some S. meliloti strains the presence of AHLs is necessary for the synthesis of symbiotically important exopolysaccharides (EPS) [22,51]. In this work we have analyzed three S. fredii HH103 important polysaccharides in symbiosis: EPS, capsular polysaccharides (KPS or K-antigens), and lipopolysaccharides (LPS) [52].…”

“…Besides to activate EPS synthesis, the QS systems of S. meliloti also promote repression of flagella synthesis at high population densities, which is related with a lower bacterial motility [22,50]. In order to determine the role of AHL perception in this process, the swimming motility of the HH103 traI, sinI, and traI/sinI mutants in Bromfield medium in the absence or presence of genistein was assayed.…”

“…In the model bacterium S. meliloti, functions regulated by QS include plasmid transfer, production of surface polysaccharides, surface translocation, motility, growth rate, and nodulation [18][19][20][21]. The QS systems TraI-TraR and SinI-SinR(-ExpR) described for S. meliloti are present in other species of Sinorhizobium, with variations at the species and strain levels [18,22]. In S. fredii NGR234, phenotypes regulated by their homologous QS systems, tra and ngr respectively, have been widely characterized: Plasmid transfer, growth rate, sedimentation, motility, biofilm formation, EPS production, and the copy number of the symbiotic plasmid (pSym).…”

“…In S. fredii NGR234, phenotypes regulated by their homologous QS systems, tra and ngr respectively, have been widely characterized: Plasmid transfer, growth rate, sedimentation, motility, biofilm formation, EPS production, and the copy number of the symbiotic plasmid (pSym). Some of these phenotypes are involved in the symbiotic performance with legume hosts whereas others seem to play a role during free-living conditions [22]. The analysis of the S. fredii HH103 and USDA257 genomes reveal also the presence of tra and sin/ngr QS systems [13,14].…”