Submitted to: International Plant Growth Promoting Rhizobacteria Workshop
Publication Type: Abstract Only
Publication Acceptance Date: February 16, 2009
Publication Date: N/A
Technical Abstract: We have shown that near-identical strains of Pseudomonas fluorescens colonize the roots of wheat at levels that differ markedly, ranging from simple commensalism to a more sophisticated relationship better described as a mutualistic symbiosis. In many biological systems, such interactions are mediated by type III secretion systems (TTSS). Using DNA probes based on sequences spanning different regions of TTSS loci from other plant-associated bacteria, we screened a collection of 32 different genotypes of 2,4-diacetylphloroglucinol-producing P. fluorescens and detected the TTSS apparatus in all of them except for Pf-5 and the closely related strain CHA0. We then sequenced the TTSS locus from strain Q8r1-96 and demonstrated that it carries full-length copies of the structural inner membrane protein HrcV, the ATPase HrcN, and a dedicated sigma factor HrpL, and has overall organization similar to that of the hrp/hrc locus of P. syringae pv. phaseolicola. We created a mutation in the TTSS gene cluster of Q8r1-96 by deleting the region spanning the hrpOP and hrcQR genes. The resultant mutant did not differ from the parental strain under in vitro conditions in colony morphology, growth kinetics, biofilm formation and BIOLOG carbon source utilization profiles, nor in production of DAPG, siderophores, and exoprotease. However, the TTSS mutant was less competitive than Q8r1-96 in the wheat rhizosphere when introduced into raw soil alone or in mixed inoculation (1:1 ratio) with the parental strain, and was impaired in the ability to control the take-all pathogen of wheat.