Rhizosphere Competence of Wild-Type and Genetically Engineered Pseudomonas brassicacearum Is Affected by the Crop Species.

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 crop species 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 out-competed 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 Q8r1-96 when they were introduced together. To our knowledge, this is the first report of a recombinant biocontrol strain of Pseudomonas spp. gaining rhizosphere competitiveness on a crop species. When assessing the potential fate of and risk posed by a recombinant Pseudomonas sp. in soil, both the identity of the introduced genes and the crop species colonized by the recombinant strain need to be considered.

Minimal changes in rhizobacterial population structure following root colonization by wild type and transgenic biocontrol strains.

Pseudomonas fluorescens strains 2-79, Q8r1-96, and a recombinant strain, Z30-97, produce the antibiotics phenazine-1-carboxylic acid (PCA), 2,4-diacetylphloroglucinol (DAPG), or both antibiotics, respectively. Rhizosphere colonization by these strains and subsequent alterations of bacterial community structure were assayed over multiple growth cycles of wheat under controlled conditions. While added to soil at just log 4 cells per gram prior to planting, all four strains subsequently colonized germinating wheat roots to levels in excess of log 6.5 cells per g (f.w.). Strain-specific differences in rhizosphere competence were observed, but these were not generally related to the chromosomal insertion of the phz genes. Multiple differences in bacterial community structure were detected among treatments in each cycle; however, the large majority of changes were not consistently related to the abundance of inoculant strains in the rhizosphere nor the genetic make-up of the inoculant strains. Nonetheless, T-RFLP profiles of amplified 16S eubacterial sequences indicated that, when compared to the untreated samples, inoculation with Z30-97 resulted in several shifts in rhizosphere bacterial community structure previously associated with decreased levels of root disease.