| For many potential disease causing pathogens, plants have evolved specific recognition mechanisms that condition rapid defense responses. The best understood example of specific recognition leading to defense responses is that triggered by plant resistance (R) genes. R genes are highly polymorphic and have evolved to recognize, either directly or indirectly, the protein products of specific pathogen avirulence (avr) genes. This R-avr interaction is classically referred to as the gene-for-gene response and leads to several hallmark responses such as the production of reactive oxygen intermediates and antimicrobrial toxins, large cellular influxes of Ca++, and the hypersensitive response (HR), a type of programmed cell death. To further investigate R gene signaling in Arabidopsis, several lines of experimentation were initiated in the laboratory of Dr. Jeffery Dangl. We describe the cloning and characterization of AtTIP49a, an evolutionarily conserved, negative regulator of disease resistance. Additionally, we demonstrate that AtTIP49a is essential for sporophyte and female gametophyte development. These experiments illustrate the importance of sensitized genetic assays for the investigation of Arabidopsis defense pathways. We utilize EMS mutagenesis to characterize RPP8 mediated resistance to the Peronospora parasitica (Pp) isolate Emco5. Numerous mutant alleles of RPP8 are described, including mutations that define an important cis-regulatory element necessary for the expression of RPP8 at the RNA level. RPP8 mediated resistance was previously demonstrated to signal redundantly through several pathways. We show that the recently identified SGT1b gene is not essential for RPP8 mediated resistance and, conversely, we demonstrate a requirement for the RAR1 gene in halting Pp Emco5 growth. These experiments further illustrate the complexity of RPP8 signaling. Finally we describe the exciting discovery that LSD1, a negative regulator of oxidative stress-induced HR-like cell death, interacts with two Arabidopsis homologs of the mammalian CCAAT box binding factor genes. This data demonstrates the power of combining yeast two-hybrid analysis and phage display in the search for protein-protein interactions. We propose a detailed model for further investigation of the role of CCAAT box binding proteins in Arabidopsis cell death. |
