title: Inactivation of gacS does not affect the competitiveness of Pseudomonas chlororaphis in the Arabidopsis thaliana rhizosphere.
abstract: Quorum-sensing-controlled processes are considered to be important for the competitiveness of microorganisms in the rhizosphere. They affect cell-cell communication, biofilm formation, and antibiotic production, and the GacS-GacA two-component system plays a role as a key regulator. In spite of the importance of this system for the regulation of various processes, strains with a Gac(-) phenotype are readily recovered from natural habitats. To analyze the influence of quorum sensing and the influence of the production of the antibiotic phenazine-1-carboxamide on rhizosphere colonization by Pseudomonas chlororaphis, a gnotobiotic system based on Arabidopsis thaliana seedlings in soil was investigated. Transposon insertion mutants of P. chlororaphis isolate SPR044 carrying insertions in different genes required for the production of N-acyl-homoserine lactones and phenazine-1-carboxamide were generated. Analysis of solitary rhizosphere colonization revealed that after prolonged growth, the population of the wild type was significantly larger than that of the homoserine lactone-negative gacS mutant and that of a phenazine-1-carboxamide-overproducing strain. In cocultivation experiments, however, the population size of the gacS mutant was similar to that of the wild type after extended growth in the rhizosphere. A detailed analysis of growth kinetics was performed to explain this phenomenon. After cells grown to the stationary phase were transferred to fresh medium, the gacS mutant had a reduced lag phase, and production of the stationary-phase-specific sigma factor RpoS was strongly reduced. This may provide a relative competitive advantage in cocultures with other bacteria, because it permits faster reinitiation of growth after a change to nutrient-rich conditions. In addition, delayed entry into the stationary phase may allow more efficient nutrient utilization. Thus, GacS-GacA-regulated processes are not absolutely required for efficient rhizosphere colonization in populations containing the wild type and Gac(-) mutants.
authors.affiliation: Bereich Mikrobiologie, Department Biologie I, Ludwig-Maximilians-Universität München, D-80638 Munich, Germany.
authors.forename: Heike
authors.initials: H
authors.lastname: Schmidt-Eisenlohr
authors.forename: Astrid
authors.initials: A
authors.lastname: Gast
authors.forename: Christian
authors.initials: C
authors.lastname: Baron
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000378:metabolism; D017360:Arabidopsis / Q000382:microbiology*; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D004251:DNA Transposable Elements; D017753:Ecosystem; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial; D016695:Glycosyltransferases / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D010619:Phenazines / Q000378:metabolism; D018517:Plant Roots / Q000382:microbiology; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism; D011549:Pseudomonas / Q000235:genetics / Q000254:growth & development*; D036226:Seedlings / Q000382:microbiology; D017422:Sequence Analysis, DNA; D015398:Signal Transduction; D012988:Soil Microbiology; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D010619:Phenazines; D014157:Transcription Factors; C420923:phenazine-1-carboxamide; C088386:homoserine lactone; D016695:Glycosyltransferases; D011494:Protein Kinases; D015107:4-Butyrolactone