Symbioses are a spectrum of interactions between organisms living in closeassociation. These intimate interactions range from mutualism, in which bothorganisms benefit, to parasitism, where one organism benefits at the expense ofthe other. Horizontal gene transfer is the acquisition of genes independent ofvertical transmission and demonstrably promotes the transition of bacteria fromfree-living to symbiotic. The horizontal acquisition of plasmids allows members ofthe Agrobacterium and Rhodococcus genera to cause disease to plants, many ofwhich are important for the ornamental plant industry. Therefore, the accurate andrapid diagnosis of these pathogens is critical for management, and failure toproperly diagnose or respond can result in severe economic losses. Indeed, somediagnostic methods have the potential to be misleading and assign nonpathogenicRhodococcus as the causative agents of disease. I test the hypothesis thathorizontal gene transfer can elicit the transition of Rhodococcus between beneficialand pathogenic states.I demonstrate that most Rhodococcus isolates are beneficial and promotechanges to the root system, such as those that are frequently associated with plantgrowth promoting bacteria. I further demonstrate that beneficial Rhodococcusstrains transition to pathogens upon the acquisition of a virulence plasmid. Theseare virulent on plants and can cause disease symptoms to both roots and aerialportions of plants. Lastly, I develop reagents for use in a sensitive and specificnovel diagnostic test that accelerates identification of pathogenic strains ofRhodococcus and Agrobacterium. This work highlights the fluidity in the evolutionof plant-associated bacteria and how transitions in symbiotic state can confounddiagnosis.
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