Genetic analysis of susceptibility of Arabidopsis to downy mildew infection

The mechanisms by which host plants contribute to their colonization by hemibiotrophic and biotrophic fungal and oomycete pathogens is a poorly understand aspect of molecular plant pathology. The aim of this study was to identify Arabidopsis genes controlling cellular processes that influence susceptibility to the biotrophic oomycete Hyaloperonospora parasitica. Chapter 2 describes a forward genetic approach to identify Arabidopsis mutants that show loss of compatibility to H. parasitica. Six of these so called downy mildew resistant (dmr) mutants were characterized in more detail. dmr3 and dmr4 show resistance to a broad range of pathogens whereas dmr1, dmr2, dmr5 and dmr6 show a type of resistance more specific to H. parasitica. Chapter 3 shows that constitutive high expression of defense related genes in the dmr5 mutant contributes to resistance to H. parasitica. Surprisingly, it was found that dmr5 mutants were mutated in RPM1, a resistance gene of the NBS-LRR class that confers resistance to Pseudomonas syringae bacteria carrying the avrB or avrRPM1 genes. RPM1 function in dmr5 mutants is largely, but not completely, lost. Genetic data indicate that dmr5 encodes a mild autoactive form of RPM1. Further studies on the mutant RPM1 protein could provide more insight to R protein function. Chapter 4 describes the map based cloning of DMR6 that encodes a putative 2OG-Fe(II) oxygenase. DMR6 is strongly induced in infected host cells during both compatible and incompatible interactions with H. parasitica. DMR6 is also highly induced after application of BTH and is constitutively high expressed in broad range disease resistance mutants. Absence of a functional DMR6 gene results in resistance to H. parasitica, that is accompanied with high expression of several defense-related genes including DMR6 itself. Together, the obtained data indicate that DMR6 is a negative regulator of defense. Chapter 5 describes the identification of a group of Arabidopsis genes that is induced during compatible interactions but not during incompatible interactions. Most of these so called compatible-specific genes (CS-genes) are also induced during various abiotic stress conditions such as cold, dehydration, high salt and high light, but also in response to ABA. The activation of CS-genes can either reflect a physiological response on pathogen infection or be triggered by pathogen effector proteins. As the CS-genes are not induced as part of a defense response they provide a first glimpse into non-defense related processes during compatible interactions.