Submitted to: ASM Conference
Publication Type: Abstract Only
Publication Acceptance Date: 5/1/2007
Publication Date: 8/26/2007
Citation: Mavrodi, D.V., Brok-Volchanskaya, V.S., Mavrodi, O.V., Thomashow, L.S. 2007. Role of ssh53, 61, 85, and 127 loci in root colonization by Pseudomonas fluorescens Q8r1-96. ASM Conference.P.89.
Technical Abstract: The goal of our research is to elucidate the molecular basis of the interactions between the host plant and beneficial rhizosphere-inhabiting 2,4-diacetylphloroglucinol (DAPG)-producing Pseudomonas bacteria. Certain DAPG-producing strains of P. fluorescens colonize plant roots and suppress soilborne plant pathogens more effectively than others from which they are otherwise phenotypically almost indistinguishable. Previously, we employed subtractive hybridization to clone 32 independent genetic loci present in the superior colonizer P. fluorescens Q8r1-96 but not in the less rhizosphere-competent strain Q2-87 (Mavrodi et al., 2002). In this study, four of the Q8r1-96-specific loci, designated as ssh53, 61, 85, and 127, were sequenced, analyzed and subjected to gene replacement mutagenesis. As a result, we recovered mutants in ssh53, 85, and 127 genes, while the ssh61 locus, which encodes a putative transcription regulator of the AsnC family and a putative threonine synthase, could not be mutated, thus suggesting that these genes are essential for P. fluorescens Q8r1-96. Sequence analyses also revealed that ssh61 and ssh85 represent cases of metabolic redundancy and encode paralogs of amino acid biosynthetic enzymes. Loci ssh85 and ssh53 form parts of a large gene cluster for cell surface lipopolysaccharide biosynthesis and may have been acquired via horizontal transfer from the opportunistic human pathogen P. aeruginosa. The ssh53, 85, and 127 mutants were characterized phenotypically for 2,4-DAPG production, colony morphology, exoprotease and siderophore production, carbon and nitrogen utilization, and the ability to colonize the rhizosphere of wheat grown in a natural soil. Results of greenhouse experiments revealed that ssh85 and ssh127 are not essential for survival of P. fluorescens Q8r1-96 on wheat roots in non-sterile soil. However, one of the ssh53 mutants was less competitive than Q8r1-96 in the wheat rhizosphere when introduced into the soil in mixed inoculation with the parental strain.