Interactions in the Root Environment: An Integrated Approach

Abstract

Strains of Rhizobium leguminosarum use a cell density-dependent gene regulatory system to assess their population density. This is achieved by the accumulation of N-acyl-homoserine lactones {(AHLs)} in the environment during growth of the bacteria and these {AHLs} stimulate the induction of various bacterial genes that are up-regulated in the late-exponential and stationary phases of growth. A genetically well-characterised strain of R. leguminosarum biovar viciae was found to have four genes, whose products synthesise different {AHLs.} We have analysed {AHL} production by four genetically distinct isolates of R. leguminosarum, three of bv. viciae and one of bv. phaseoli. Distinct differences were seen in the pattern of {AHLs} produced by the bv. viciae strains compared with bv. phaseoli and the increased levels and diversity of {AHLs} found in bv. viciae strains can be attributed to the rhil gene, which is located on the symbiotic {(Sym)} plasmid and is up-regulated when the bacteria are grown in the rhizosphere. Additional complexity to the profile of {AHLs} is found to be associated with highly transmissible plasmid {pRL1} J1 of R. leguminosarum bv. viciae, but this is not observed with some other strains, including those carrying different transmissible plasmids. In addition to {AHLs} produced by the products of genes on the symbiotic plasmid, there is clear evidence for the presence of other {AHL} production loci. Expression levels and patterns of {AHLs} can change markedly in different growth media. These results indicate that there is a network of quorum-sensing loci in different strains of R. leguminosarum and these loci may play a role in adapting to rhizosphere growth and plasmid transfer.