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Title: AHL-DEFICIENT MUTANTS OF BURKHOLDERIA AMBIFARIA BC-F HAVE DECREASED ANTIFUNGAL ACTIVITY

Author
item ZHOU, H - UNIV OF MASSACHUSETTS
item YAO, F - UNIV OF MASSACHUSETTS
item Roberts, Daniel
item LESSIE, T - UNIV OF MASSACHUSETTS

Submitted to: Current Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2002
Publication Date: 9/1/2003
Citation: Zhou, H., Yao, F., Roberts, D.P., Lessie, T.G. 2003. Ahl-deficient mutants of burkholderia ambifaria bc-f have decreased antifungal activity. Current Microbiology. 47:174-179.

Interpretive Summary: Soilborne plant pathogens cause diseases that result in major economic losses to farmers in the United States. Biological control of plant diseases is an environmentally friendly and promising alternative to the use of chemicals for control of these important soilborne plant pathogens. Biological control can be inconsistent and this inconsistency in performance is hampering widespread commercial application of biocontrol agents. We must develop an understanding of how microbial biocontrol agents suppress soilborne plant pathogens to aide in the development of strategies to overcome inconsistent biological control performance. The focus of this paper was on the importance of the genes bafI and bafR in suppression of damping-off disease of cucumber by the bacterial biocontrol agent Burkholderia ambifaria BcF. The genes bafI and bafR participate in a bacterial global regulatory network that allows cell density dependent regulation of bacterial genes. The bafI and bafR genes from B. ambifaria were sequenced and mutants of B. ambifaria with mutations in these genes constructed. It was demonstrated that these mutants were no longer capable of suppressing damping-off disease of cucumber. This indicates that cell density dependent regulation of genes involved in disease suppression is important for biological control by B. ambifaria. This information will be useful to scientists. Knowledge of the genetic and physiological basis of disease suppression by biocontrol agents will enable scientists to devise strategies to enhance biological control performance.

Technical Abstract: Burkholderia ambifaria BcF, a biocontrol strain from the USDA-ARS Sustainable Agricultural Systems Laboratory in Beltsville, MD, was highly active in formation of N-acyl homoserine lactones (AHLs). We isolated a fragment of the genome of B. ambifaria BcF containing the divergently oriented genes, bafI and bafR, which specify, respectively, AHL synthase and AHL-binding transcriptional activator. AHL-deficient strains, in which the bafI or bafR genes had been inactivated by allelic exchange or by transposon mutagenesis exhibited decreased exoprotease and anti-fungal activity. We also identified spontaneous mutants with similar phenotypes in which the bafIR locus had been deleted. Pulsed-field gel electrophoretic analysis of randomly linearized replicons indicated that strain BcF contains three chromosomes of 3.5, 2.8, and 1.3 Mb. Macrorestriction fragment mapping of the site of transposon insertion in a bafR mutant as well as Southern hybridization experiments indicated that the bafIR locus was located on the 2.8-Mb chromosome.