Plants and pathogenic microorganisms are in constant conflict with each other. Understanding the molecular networks that trigger resistance, along with the molecular networks that pathogens might co-opt to infect susceptible plants, is important for developing the integrated, holistic perspective that is necessary for innovative development of engineered resistance to current and emerging pathogens. The first objective of the dissertation was to increase the understanding of mechanisms by which plants recognize pathogen attack and mount an appropriate defense response. These experiments focused on resistance triggered by the Arabidopsis thaliana R gene, RPP7, which encodes a coiled-coil nucleotide binding-leucine-rich repeat (CC-NB-LRR) protein that activates race-specific resistance to the downy mildew pathogen, Hyaloperonospora parasitica (Hpa). Previously-published genetic epistasis tests have established that RPP7 activates defense responses through a signaling mechanism that does not require accumulation of salicylic acid (SA), or components of the ethylene and jasmonate response pathways. Furthermore, RPP7 is not strongly compromised by mutations in genes associated with defense signal transduction (PAD4, NDR1, NPR1, RAR1). Double mutant combinations of these signal transduction components were analyzed to detect additive or functionally-redundant contributions to RPP7-dependent resistance. Most of the double mutants support an enhanced level of asexual sporulation compared to the single mutant parental lines. Time-course experiments with histochemical stains revealed that these double mutants delay, but do not suppress, the oxidative burst and the hypersensitive response. These results suggest that RPP7 activates multiple signaling pathways, each of which makes incremental contributions to the timing of defense activation. The second objective of the dissertation was to investigate the role that auxin plays in enabling virulent H. parasitica to colonize Arabidopsis. Transcript profiling revealed induction of auxin-associated genes in response to infection of Arabidopsis thaliana by virulent strains of the oömycete pathogen, H. parasitica. Experiments with the DR5 / Ph. D.

Plants have evolved physical barriers and inducible defense responses to combat microbial pathogens. Inducible responses are mediated by R proteins, which recognize invading pathogens. R proteins must be precisely regulated to provide effective resistance, without inhibiting normal plant growth. However, little is known about R gene regulation under defense-inducing conditions. The interaction between the oomycete Hyaloperonospora parasitica and the model plant Arabidopsis thaliana provides an excellent model system to explore R gene regulation. My research focuses on RPP8, a CC-NBS-LRR gene, which provides resistance to the H. parasitica isolate Emco5. Previous work in the McDowell lab suggested that RPP8 is upregulated during defense responses. My research shows that RPP8 alleles from the Columbia and Landsberg erecta ecotypes are upregulated by H. parasitica and the defense signaling molecule salicylic acid, suggesting a potential feedback loop. RPP8-Ler is also systemically upregulated after infection of the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Additionally, RPP8-Ler expression is increased during wounding and heat stress. I also examined the role of regulatory cis elements in the RPP8 promoter. Three W-boxes are essential for basal and inducible RPP8 expression, and are required for resistance to Emco5. The X-box, a unique cis element in the RPP8 promoter, is essential for strong basal expression and wound-induced upregulation, and affects spatial expression of RPP8-Ler. However, the X-box is not required for RPP8-Ler upregulation during pathogen or SA treatment. R genes may be induced as part of global defense responses, which could prime the host for more effective pathogen recognition. / Master of Science