Phenazine-1-carboxylic acid-producing Pseudomonas spp. of the Inland Pacific Northwest (U.S.)

James A. Parejko

Phenazine antibiotic-producing rhizobacterial strains belonging to the genus Pseudomonas are effective biocontrol agents against soilborne fungal plant pathogens. Beyond the well-studied model phenazine-producing (Phz+) biocontrol strain Pseudomonas fluorescens 2-79, there existed few studies demonstrating phenazine production in root-colonizing members of the `P. fluorescens' lineage. Furthermore, our collective knowledge of these important biological control rhizobacteria lacked a comprehensive analysis of representatives of Phz+ populations indigenous to intensive agricultural regions, most notably the Inland Pacific Northwest (U.S.) dryland agroecosystem. In the first of the studies within this dissertation, a collection of 412 Phz+ Pseudomonas spp. isolates from three cereal fields of east-central Washington State was genotypically and phenotypically characterized. The results revealed four new groups of Phz+ strains, three of which were divergent from P. fluorescens 2-79. These groups were associated with dryland cereal crops with seven different cropping histories, however cropping history shaped the community of genotypes present. Another collection of 497 Phz+ isolates from throughout the Inland Pacific Northwest dryland agroecosystem was taxonomically described by multi-locus sequence analysis and the phenazine biosynthesis gene phzF was cloned from winter wheat at nine sites. Using this approach, it was determined that the four groups of Phz+ strains from the first study are separate Pseudomonas species, two of which have never been described, and agroclimatic conditions influence Phz+ community diversity. The influence of phenazine-1-carboxylic acid (PCA) on biofilm formation under water limitation and biocontrol characteristics against Rhizoctonia solani AG-8 were also determined for selected strains from the Phz+ species. Generally the strains developed high levels of biofilm and responded uniquely to simulated matric or osmotic stress. The importance of PCA production to biofilm formation was variable by strain, although biofilm architecture was significantly impacted in most Phz- mutants. Representative Phz+ strains protected wheat plants from the Rhizoctonia root rot causal agent R. solani AG-8 in greenhouse bioassays with reduced plant protection from Phz- mutants of the same strains. These findings significantly increase our understanding of indigenous Phz+ populations in dryland agriculture and their involvement in an effective cropping system for a more sustainable dryland agroecosystem.

Phenazine-1-carboxylic acid-producing Pseudomonas spp. of the Inland Pacific Northwest (U.S.)

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