Characterization of defense responses in the Arabidopsis thaliana mutant enhanced disease resistance 1

Plants must be able to defend against a variety of diseases and nearly every cell must contain the machinery necessary for disease resistance. A mutant in Arabidopsis thaliana, enhanced disease resistance 1, confers enhanced resistance to bacterial and fungal pathogens. To better understand how edr1-mediated resistance occurs, I performed microarray experiments using wild type and edr1 plants inoculated with the fungal pathogen, Golovinomyces cichoracearum. Strikingly, inoculated edr1 mutants had increased expression of known and putative defense-associated genes. Additionally, many of these genes are transcription factors and there is enrichment for their binding sites in promoters of genes identified as being upregulated in edr1. The EDR1 protein is present in the nucleus in a degraded form, suggesting that EDR1 can be localized to the same organelle as transcription factors. Analysis of Gene Ontology annotations revealed the timing of gene expression in edr1 compared to wild type-genes associated with defense, reactive oxygen species (ROS), and phosphorelay were induced early, and that the defense and ROS genes has sustained higher expression. The genes identified as having increased expression in edr1 compared to wild type are also expressed more highly in plants that have been exposed to other pathogens as well as in a constitutive defense mutant. The results I obtained from these experiments demonstrate that EDR1 is specifically negatively regulating defense responses to G. cichoracearum, which had not been known before. I also attempted to identify potential substrates of EDR1 by yeast two-hybrid assays. Two potential interacting proteins were identified and one was chosen for further experiments. AtMYC2 is a transcription factor that is induced in response to different plant hormones. The edr1 mutant is more sensitive to ABA, consistent with a role in negative regulation of AtMYC2. Unfortunately, no other data supporting the biological relevance of this interaction was found. The yeast two-hybrid result supports the observation that EDR1 is regulating transcription factors. My dissertation research has demonstrated that EDR1 negatively regulates disease resistance through repression of transcription factors and other signaling pathways.