title: Mutation of praR in Rhizobium leguminosarum enhances root biofilms, improving nodulation competitiveness by increased expression of attachment proteins.
abstract: In Rhizobium leguminosarum bv. viciae, quorum-sensing is regulated by CinR, which induces the cinIS operon. CinI synthesizes an AHL, whereas CinS inactivates PraR, a repressor. Mutation of praR enhanced biofilms in vitro. We developed a light (lux)-dependent assay of rhizobial attachment to roots and demonstrated that mutation of praR increased biofilms on pea roots. The praR mutant out-competed wild-type for infection of pea nodules in mixed inoculations. Analysis of gene expression by microarrays and promoter fusions revealed that PraR represses its own transcription and mutation of praR increased expression of several genes including those encoding secreted proteins (the adhesins RapA2, RapB and RapC, two cadherins and the glycanase PlyB), the polysaccharide regulator RosR, and another protein similar to PraR. PraR bound to the promoters of several of these genes indicating direct repression. Mutations in rapA2, rapB, rapC, plyB, the cadherins or rosR did not affect the enhanced root attachment or nodule competitiveness of the praR mutant. However combinations of mutations in rapA, rapB and rapC abolished the enhanced attachment and nodule competitiveness. We conclude that relief of PraR-mediated repression determines a lifestyle switch allowing the expression of genes that are important for biofilm formation on roots and the subsequent initiation of infection of legume roots.
authors.affiliation: Department of Molecular Microbiology, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, UK.
authors.forename: Marijke
authors.initials: M
authors.lastname: Frederix
authors.forename: Anne
authors.initials: A
authors.lastname: Edwards
authors.forename: Anna
authors.initials: A
authors.lastname: Swiderska
authors.forename: Andrew
authors.initials: A
authors.lastname: Stanger
authors.forename: Ramakrishnan
authors.initials: R
authors.lastname: Karunakaran
authors.forename: Alan
authors.initials: A
authors.lastname: Williams
authors.forename: Pamela
authors.initials: P
authors.lastname: Abbruscato
authors.forename: Maria
authors.initials: M
authors.lastname: Sanchez-Contreras
authors.forename: Philip S
authors.initials: PS
authors.lastname: Poole
authors.forename: J Allan
authors.initials: JA
authors.lastname: Downie
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D018441:Biofilms / Q000254:growth & development*; D015964:Gene Expression Regulation, Bacterial; D008351:Mannans / Q000378:metabolism; D046228:Microarray Analysis; D009154:Mutation; D009876:Operon; D018532:Pisum sativum / Q000382:microbiology / Q000502:physiology; D055170:Plant Root Nodulation; D018517:Plant Roots / Q000382:microbiology*; D011135:Polysaccharides, Bacterial / Q000502:physiology; D011401:Promoter Regions, Genetic; D016961:Rhizobium leguminosarum / Q000235:genetics* / Q000254:growth & development / Q000502:physiology*; D013559:Symbiosis; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D008351:Mannans; D011135:Polysaccharides, Bacterial; D014157:Transcription Factors; C022901:(1-6)-alpha-glucomannan