Using interactions of Arabidopsis and the pathogenic oomycete Hyaloperonospora parasitica (Hp) I studied transcriptional reprogramming associated with plant immune responses. I focused on a cluster of, Arabidopsis genes that exhibited in microarray experiments a strong and coordinated L&barbelow;ate/sustained U&barbelow;p-regulation in R&barbelow;esponse to Hp&barbelow; (LURPs). Insertional mutants of individual LURP genes showed partially reduced resistance to Hp, suggesting their concerted activity and coordinated up-regulation is required for full defense activation. AtWRKY70, a LURP member encoding a WRKY transcription factor, is a key control point for this defense mechanism.

I initiated multiple approaches to uncover regulatory mechanisms coordinating LURP expression. Using reporter gene assays and electrophoretic mobility shift assays I have isolated promoter regions of two LURP genes containing candidate cis-elements likely to contribute to their co-regulation. A 39-bp region of the LURP1 promoter that mediates reporter expression in response to defense related treatments contains a W-box motif that interacts with nuclear Arabidopsis factors in vitro. This sequence is strongly enriched in promoters of the PR1 regulon, a cluster of genes co-regulated defense genes that overlaps with the LURP cluster. In the promoter of CaBP22 , a LURP member that represents the average LURP expression profile, I identified two regions that mediate defense specific GUS reporter activity. Each region contains a copy of a novel inverted repeat sequence that interacts with nuclear proteins in vitro producing a constitutive shift that is enhanced after defense stimulation. This sequence is enriched in promoters from the LURP cluster, suggesting a possible regulatory role for this motif for coordinating LURP expression.

Using a transgenic Arabidopsis line containing a minimal Hp-responsive region of the CaBP22 promoter fused to GUS, I have established a reliable and specific high-throughput screening system for plant defense elicitors that allowed me to identify 114 candidate molecules. One representative, 3,5-dichloroanthranilic acid (DCA), induced resistance to Hp and Pseudomonas syringae, a plant bacterial pathogen, without exhibiting direct antibiotic activity. Genetic analyses indicated that DCA elicits the plants natural defense response downstream of the defense hormone salicylic acid by activating two distinct branches of the defense signaling network.