Effects of Carbon Quantity and Plant Diversity on Pathogen Suppression by Soil-borne Streptomyces

Members of the genus Streptomyces are some of the best candidates for biological control of soil-borne pathogenic bacteria. Antibiotic production by Streptomyces is thought to be crucial for this strain’s ability to compete with other closely related soil microorganisms. Furthermore, Streptomyces is able to not only inhibit the growth of its competitors, but also that of many soil-borne plant pathogens. My thesis assessed Streptomyces performance under varying levels of soil resources (carbon componds that serve as energy sources) and in plots with varying degrees of plant diversity that, in turn, affects soil resources. This project aims to deconvolute the confounding effects of these environmental variables on plant pathogen suppression by Streptomyces. I characterized soils from low-diversity plant communities (monoculture) and high-diversity plant communities (polyculture (16-species)), while providing fungicide treatment to the leaves of the plant, which has been found to increase plant productivity, to half of the plots. I found that Streptomyces associated with plant monoculture versus plant polyculture is more likely to be subject to resource competition. In contrast, Streptomyces communities in polyculture are much more complex than those associated with plant monoculture. Therefore, in monoculture agricultural system, Streptomyces strains are likely to help suppress plant disease via antibiotics production, and attempts to control soil-borne plant diseases via antibiotics-producing Streptomyces strains should work well in plant monoculture (but perhaps not in plant polyculture). My findings also suggest that the inhibitory phenotypes of Streptomyces (that differ genetically from non-inhibitory strains, as indicated by our 16S sequencing data) are induced by low soil carbon quantity. Finally, Streptomyces responds to low plant diversity and low carbon levels in the soil by not only producing more antibiotics but also increasing their own resistance to antibiotics