Submitted to: Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 4, 2000
Publication Date: N/A
Interpretive Summary: We are studying the mechanism by which bacteria are able to cause plant disease. Towards this end, we are analyzing the Pseudomonas syringae pv. syringae (the causative agent of brown spot disease of snap bean - an important field disease in Wisconsin) as a model system. We hope to gain an understanding of the molecular biology of the genes and gene products that are required for the disease process. This manuscript describes characterization of the role of the algD gene and alginate production in virulence on the plant. This gene is not required for normal growth or lesion formation in bean plants using laboratory assays. The analysis of this gene illustrates some of the genetic differences between animal and plant pathogenic bacteria. Ultimately this research will provide information of use to breeders and plant molecular biologists that will allow the development of snap bean lines resistant to brown spot disease and reduce the need for chemical control.
Transposon mutagenesis of the bean pathogen P. syringae pv. syringae B728a generated mutants containing TnlacZ insertions within the algD gene that appeared to be positively regulated by the global regulator gacA. Genetic exchange of one of these mutations (algD313::TnlacZ)into wild-type B728a resulted in the loss of alginate production. Subsequent restoration by a plasmid containing an intact alginate biosynthetic region indicated that the TnlacZ insertion was causal to the phenotype. Direct chemical quantitation of alginate demonstrated that the gacS, gacA, and algD genes but not the salA, or the ahlI (HSL) genes are required for alginate production in the B728a genetic background. Reporter studies measuring the expression of algD313::TnlacZ in various genetic backgrounds confirmed that the disruption of gacA results in an 8 to 17-fold decrease in algD::TnlacZ expression, while the gac-dependent regulator salA had no effect. Addition of sorbitol to the medium enhanced algD expression with a maximal induction of 2-fold occurring in 0.6 M sorbitol. This 2-fold induction was independent of the gacA gene, suggesting that the GacS sensor component of this regulatory pair does not respond to sorbitol. Loss of alginate production did not affect the virulence of B728a in laboratory infiltration assays, indicating that the effects of gacS/gacA mutations on pathogenicity do not stem from these genes' effects on alginate production. These results together establish alginate biosynthesis as a phenotype belonging to the salA-independent branch of the gacS/gacA regulon in B728a.