Abstract
Interactions of plants with pathogenic microorganisms are influenced by multiple biotic and abiotic factors. The host immune system can detect pathogens and restrict their growth and development, but also non-immunity related host factors and processes can contribute to pathogenesis by attracting pathogens, stimulating their development and providing nutrition. We use
... read more
the model plant species Arabidopsis thaliana and its biotrophic pathogen Hyaloperonospora arabidopsidis (Hpa), causing downy mildew, to study the role of immunity and non-immunity related host processes in plant disease susceptibility. The work described in this thesis was aimed at identifying Arabidopsis genes that alter susceptibility to the downy mildew pathogen. In Chapters 2, 3 and 4, results of studies on susceptibility of natural Arabidopsis accessions to Hpa are presented. The Arabidopsis line C24 is resistant to all downy mildew isolates tested so far, however, genetic mechanisms of this broad-spectrum resistance (BSR) were unknown. Using segregation analysis and quantitative trait loci mapping, we found that BSR is multigenic and mediated by different combinations of isolate-specific resistance loci, some of which confer only partial resistance. In Chapter 3, quantitative resistance of C24 to the Hpa isolate Waco9 was studied in more details. Backcross mapping facilitated by whole-genome sequencing revealed two major loci, which interact to confer strong quantitative resistance to Hpa. To identify Arabidopsis genes which contribute to susceptibility to downy mildew in a non-isolate specific manner, we performed association mapping in the core Arabidopsis HapMap population of susceptibility to a mixture of four Hpa isolates (Chapter 4). Natural variation at three loci, CYTOKININ RESPONSE FACTOR 1 (CRF1), STOMATAL CARPENTER 1 (SCAP1), and the unknown gene At5g53750, was associated with susceptibility to Hpa. Analysis of mutants and silencing lines indicated that SCAP1 and CRF2, the close homolog of CRF1, could play a role in the Arabidopsis-downy mildew interaction. However, functions of these genes in Arabidopsis susceptibility remain unknown. The downy mildew pathogen forms specialized infectious structures called haustoria, which penetrate plant cell wall but remain separated from host plasma membrane. To gain insight the molecular events occurring specifically in the Hpa-infected cells of Arabidopsis, we used an N-glycotagging approach coupled to label-free quantitative proteomics (described in Chapter 5). We found 18 candidate complex N-glycoproteins of Arabidopsis associated with downy mildew infection. Analysis of mutants in the corresponding genes suggested that Arabidopsis PLASMODESMATA GERMIN-LIKE PROTEIN 1 (PDGLP1) and the subtilase ATSBT3.5 are involved in regulating the host susceptibility to Hpa. Functions of the identified novel putative players in the interaction between Arabidopsis and downy mildew are largely unknown, but their further characterization might point to new processes contributing to plant susceptibility to (hemi-)biotrophic pathogens
show less