Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2006
Publication Date: 11/20/2006
Citation: Mavrodi, O.V., Mavrodi, D.V., Weller, D.M., Thomashow, L.S. 2006. The role of ptsP, orfT, and sss recombinase in root colonization by Pseudomonas flurescens Q8r1-96. Applied and Environmental Microbiology. 72:7111-7122 Interpretive Summary: Some strains of the bacterium Pseudomonas fluorescens produce the biocontrol antibiotic 2,4-diacetylphloroglucinol (DAPG). These bacteria suppress a wide spectrum of soilborne plant pathogens that cause wilts, damping-off and root diseases of food, fiber and ornamental crops. DAPG-producing Pseudomonas fluorescens also are responsible for the natural suppressiveness of certain soils to diseases such as take-all of wheat. From a worldwide collection of DAPG producers, 22 distinct genotypes (A-T, PfY and PfZ) are known. Genotypes differ significantly in ability to colonize the roots of wheat and pea: some are superior and some are average. Understanding root colonizing ability is important because it directly relates to biocontrol activity against root diseases such as take-all. The purpose of this study was to determine whether the three genes, an sss recombinase gene, ptsP, and orfT contribute to the superior root colonizing ability of wheat and pea by P. fluorescens Q8r1-96 (genotype D). Mutation in the gene ptsP had a strong effect on the colonization ability of Q8r1-96, but mutations in the sss recombinase gene or orfT had minimal effects on colonization. This study is important because it narrowed the field of possible genes and traits that could account for superior colonization by strain Q8r1-96. Understanding the traits involved in root colonization will greatly enhance the use of pseudomonads for biocontrol, which is a sustainable and ecologically-friendly approach to controlling soilborne pathogens.
Technical Abstract: Pseudomonas fluorescens Q8r1-96 produces 2,4-diacetylphloroglucinol (2,4-DAPG), a polyketide antibiotic that suppresses a wide variety of soilborne fungal pathogens, including Gaeumannomyces graminis var. tritici, which causes take-all disease of wheat. Strain Q8r1-96 is representative of the D-genotype of 2,4-DAPG producers, which are exceptional because of their ability to aggressively colonize and maintain large populations on the roots of host plants, including wheat, pea, and sugar beet. In this study, three genes, an sss recombinase gene, ptsP, and orfT, which are important in the interaction of Pseudomonas spp. with various hosts, were investigated to determine their contributions to the unusual colonization properties of strain Q8r1-96. The sss recombinase and ptsP genes influence global processes, including phenotypic plasticity and organic nitrogen utilization, respectively. The orfT gene contributes to the athogenicity of Pseudomonas aeruginosa in plants and animals and is conserved among saprophytic rhizosphere pseudomonads, but its function is unknown. Clones containing these genes were identified in a Q8r1-96 genomic library, sequenced, and used to construct gene replacement mutants of Q8r1-96. Mutants were characterized to determine their 2,4-DAPG production, motility, fluorescence, colony morphology, exoprotease and hydrogen cyanide (HCN) production, carbon and nitrogen utilization, and ability to colonize the rhizosphere of wheat grown in natural soil. The ptsP mutant was impaired in wheat root colonization, whereas mutants with mutations in the sss recombinase gene and orfT were not. However, all three mutants were less competitive than wild-type P. fluorescens Q8r1-96 in the wheat rhizosphere when they were introduced into the soil by paired inoculation with the parental strain.