Molecular Approaches for Elucidating the in situ Activities of Bacterial Biological Control Agents

Certain strains of rhizosphere-inhabiting bacteria suppress plant pathogenic bacteria and fungi through processes of resource competition or antibiosis. The scientific literature provides numerous and compelling examples that resource competition and antibiosis occur among microorganisms coinhabiting the rhizosphere, but their importance in determining the outcome of microbial interactions is neither absolute nor entirely predictable. Moreover, these processes are not likely to occur uniformly in time or space; instead, they take place only in those microhabitats where environmental and biotic factors favor their occurrence. Consequently, an understanding of factors influencing resource competition and antibiosis has become a subject of active research during the past several years. Tools are now available to identify factors influencing the expression of antibiotic biosynthesis genes, to assess the chemical nature of habitats that bacteria occupy in the rhizosphere, and to determine if resource availability to microorganisms is influenced by their coinhabitants in the rhizosphere. Utilizing such tools, investigators have gained further appreciation for the diverse interactions that occur between bacterial antagonists and target plant pathogens. Here, we highlight examples from our research utilizing molecular approaches to evaluate mechanisms involved in the suppression of soilborne diseases by strains of fluorescent pseudomonads.