Ecology of interspecies signaling among Streptomyces and its relationship to pathogen suppression

University of Minnesota Ph.D. dissertation. May 2013. Major: Plant Pathology. Advisor: Linda Kinkel. 1 computer file (PDF); ix, 84 pages.Interspecies signaling may be defined as the induced change in phenotype of one species by another that is not due to the metabolism of the signal. Although suggested to be a relatively widespread phenomenon, the role of signaling in natural soil communities has not been thoroughly studied. Within Streptomyces communities in soil, understanding the impacts of interspecies signaling on species interactions, and especially on nutrient competition and antagonism, may be key to effective Streptomyces-based suppression of plant pathogens. I evaluated the frequency of signaling interactions and their effect on inhibitory phenotypes of Streptomyces isolated from natural prairies. Signaling among Streptomyces was frequent, and observed in 35% of all interactions. Isolates from the same location in soil were more likely to signal one another than isolates from different locations, suggesting local selection for signaling interactions. Signaling was similarly more frequent between isolates that had similar nutrient use profiles. Finally, closely-related isolates were more likely to increase inhibition towards one another via signaling than distantly-related isolates. In chapter 2, subinhibitory concentrations of antibiotics were studied as signals, specifically in relation to their capacities to shift nutrient use among Streptomyces. We found that some antibiotics altered nutrient use by Streptomyces in ways that could reduce nutrient competition among isolates. Finally, pathogen suppression and signaling were evaluated in soils with different cropping histories. Pathogen suppression by Streptomyces varied significantly among soils, and suppressive activity was positively correlated with bacterial density. Among Streptomyces from these plots, shifts in inhibitory phenotypes in response to signaling by another isolate were very frequent (~ 50% of all interactions). Overall, signaling in Streptomyces is frequent and varies with spatial origin, nutrient overlap, antagonistic phenotype, and genetic relatedness among isolates, as well as soil cropping history. Moreover, some antibiotics have the potential to act as signals that can significantly alter nutrient competition among Streptomyces. Variation in signaling has significant potential to mediate pathogen suppression in soil communities