Plants associate with beneficial bacteria that help protect them from pathogens and facilitate nutrient uptake. As a result, they have the potential to supplement pesticides and fertilizers for use in agriculture. However, attempts to use beneficial bacteria in agriculture have had limited success, often due to poor survival of introduced strains. This project will enhance our understanding of the ecological, molecular, genetic, and genomic mechanisms that make some beneficial bacteria strong colonizers of plants and the mechanisms by which they protect plants from pathogens. The project will use synthetic microbial communities consisting of closely related plant-beneficial and pathogenic strains of bacteria, coupled with genetic and genomic approaches to find the molecular mechanisms that allow some beneficial microbes to successfully colonize plants and exclude pathogens. The use of established microbial communities, genome-sequenced community members and high throughput assays will enable rapid screening of many combinations of microbes. The project will also provide training opportunities for a breadth of career stages including graduate students and postdoctoral fellows, as well as undergraduate students through course-based research experiences. With the rise of emergent pathogens, understanding the mechanisms by which bacteria facilitate or prevent disease will aid in rapid and cost-effective solutions. Microbiome community engineering has proven difficult, in part