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The ability to understand the interactions between plants and the variety of pathogens they encounter on a daily basis is an important area of research. In the following work presented in this dissertation, I sought to better understand the mechanisms that Turnip crinkle virus (TCV) employs to elude the defense responses of the host plant Arabidopsis thaliana. It was previously determined that TCV coat protein (CP) interacts with a transcription factor, TIP, within the about 10 amino acid region near the N-terminus of the CP called the R-domain. When this interaction was disrupted by making single amino acid substitutions through the R-domain region, I observed a marked variation in symptom severity and alterations in both basal and resistance gene mediated responses. To further explore the effect of the TCV CP-TIP interaction on virus invasiveness and the plant defense systems, I analyzed virus accumulation and defense gene expression from the susceptible (Col-0) and resistant (Di-17) lines throughout a time course of infection. I discovered that the wildtype TCV (wtTCV) had a transient replicative advantage over CP mutants that were not able to bind TIP. This effect occurred within the first 4 to 6 days of infection. Research reported here demonstrates that the ability of wtTCV to bind TIP causes a suppression of the basal defense response that facilitates viral invasion of the systemic leaves in the susceptible ecotype Col-0.
Further experiments confirmed that TIP-CP binding also had an effect on the R-gene meditated defense conditioned by the HRT (Hypersensitive Response to TCV) gene in the resistant line Dijon-17. This was demonstrated by monitoring virus accumulation and symptom development between wtTCV and several CP mutants with altered TIP binding ability. I demonstrate that expression of the TIP gene in the presence or absence of the R-gene HRT altered development of disease symptoms and systemic spread of the virus.
A primary outcome of the research reported in this dissertation is the demonstration that the interaction of TCV CP with the TIP transcription factor modulates both major defense layers of the plant immune system. These are the basal defense layers referred to as Pathogen Triggered Immunity (PTI) defense and the development of systemic acquired resistance modulated by R-gene mediated defense referred to as the Effector Triggered Immunity (ETI) defense.
Advisor: T. Jack Morris