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The plant hormone ethylene has been extensively studied for its role in many plant developmental processes. This has led to the discovery of five ethylene receptors that are all involved in a complex signaling pathway that overlaps with several other phytohormone pathways, as well as stress responses. The role that ethylene plays in stress responses makes it a central signaling pathway in plant-microbe interactions. This interface has been well studied from the plant perspective, but there is limited data on how bacteria respond directly to ethylene. This work shows that a plant growth-promoting bacteria, Azospirillum brasilense, has a functional ethylene receptor capable of binding ethylene and inducing a response, making it the first plant-associated bacteria to have a receptor identified. The application of ethylene to A. brasilense alters attachment behavior in both biofilm formation assays as well as root colonization of both Arabidopsis thaliana and tomato. These changes in attachment behavior are likely mediated by a large transcriptomic response to ethylene that leads to an upregulation of central carbon metabolism and a downregulation of genes involved in nitrogen fixation. In agreement with the changes in transcripts, ethylene treated cells also have a distinct metabolic profile when compared to untreated cells, and this change can be detected as early as 8 hours after exposure to ethylene. Together, this data show that ethylene perception by A. brasilense alters the nitrogen metabolism of the cell in a way that inhibits colonization of the root and upregulates carbon metabolism and storage.

Motile bacteria must navigate their environment in constant search of nutrients to sustain life. Thus they have evolved precise and adaptable sensory systems to achieve this goal, making the navigation system of the model bacterium Escherichia coli the best characterized signal transduction pathway in Biology. However, many bacteria have evolved more sophisticated arsenals for sensing and responding to their environment including chemoreceptors to identify novel attractants in the microenvironment. The diazotrophic alphaproteobacterium Azospirillum brasilense inhabits the soil and colonizes the roots of cereals like rice, corn, and wheat. Like most proteobacterial, A. brasilense encodes multiple chemotaxis-like pathways, 4, of which only Che1 has been characterized in detail. Also, of the approximately 50 chemoreceptors encoded within the genome, only the function of AerC and Tlp1 have been determine and their role in energy taxis, the dominant behavior of A. brasilense. In this dissertation, I will describe the characterization of another chemoreceptor, Tlp2, with a sensing domain of unknown function and the role it plays in A. brasilense behavior. I will also describe my work in expanding knowledge of the chemotaxis-like pathway of Che1. Also, the role of Tlp1 in root colonization, chemotaxis, and aerotaxis, the ability to navigate oxygen gradients, has been published. My work will detail the role of the C-terminal PilZ domain, a domain shown to bind the ubiquitous bacterial second messenger cyclic-di-GMP. I will characterize the necessity of c-di-GMP binding to Tlp1 for cells to maintain the ability to remain sensitive to temporal changes in aeration. I will also discuss the novel role c-di-GMP plays in modulating the cell’s ability to remain motile and remain sensitive to addition changes in oxygen availability.