Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2016
Publication Date: 6/20/2016
Citation: Bankhead, S., Thomashow, L.S., Weller, D.M. 2016. Rhizosphere competence of wild-type and genetically-engineered Pseudomonas brassicacearum is affected by the crop species. Phytopathology. 106:554-561.
Interpretive Summary: Some strains of the bacterium Pseudomonas produce the biocontrol antibiotic 2,4-diacetylphloroglucinol (DAPG). These bacteria suppress a wide spectrum of soilborne plant pathogens that attack the roots of food, fiber and ornamental crops. However, DAPG producers are not able to suppress Rhizoctonia root rot of wheat and barley because the Rhizoctonia pathogens are rather insensitive to the metabolite. We inserted into the DAPG producer Pseudomonas strain Q8r1-96 genes for the production of phenazine-1-carboxylic acid (PCA). Rhizoctonia is very sensitive to PCA and the recombinant strain Z30-97 protects wheat against Rhizoctonia root rot. The purpose of this study was to assess the fate of a recombinant biocontrol agent in the soil. When introduced individually into the rhizosphere, the recombinant strain Z30-97 did not colonize barley, pea or bean any better than the wild type Q8r1-96. However, when the wild type and recombinant strain were mixed the competitiveness of each strain was affected by the plant. These studies were conduced in confined conditions and validation of the results will require field studies.
Technical Abstract: 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas brassicacearum Q8r1-96 is a highly effective biocontrol agent of take-all disease of wheat. Strain Z30-97, a recombinant derivative of Q8r1-96 containing the phzABCDEFG operon from P. synxantha (formerly P. fluorescens) 2-79 inserted into its chromosome, also produces phenazine-1-carboxylic acid. Rhizosphere population sizes of Q8r1-96, Z30-97 and 2-79, introduced into the soil, were assayed during successive growth cycles of barley, navy bean or pea under controlled conditions as a measure of the impact of the crop on rhizosphere colonization of each strain. In the barley rhizosphere, Z30-96 colonized less that Q8r1-96 when they were introduced separately, and Q8r1-96 displaced Z30-96 when the strains were introduced together. In the navy bean rhizosphere, Q8r1-96 colonized better than Z30-97 when the strains were introduced separately. However, both strains had similar population densities when introduced together. Strain Q8r1-96 and Z30-97 colonized the pea rhizosphere equally well when each strain was introduced separately, but Z30-97 out-competed and displaced Q8r1-96 when they were introduced together. To our knowledge, this is the first report of a strain of Pseudomonas gaining rhizosphere competence as a result of antibiotic biosynthesis genes from another strain being introduced into its chromosome. When assessing the potential fate of and risk posed by a recombinant Pseudomonas in soil, both the identity of the introduced genes and the crop species colonized by the recombinant strain need to be considered.