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Title: Molecular variation ofBrassica downy mildeweffector genes
Author: Meitz, Julia
ISNI:       0000 0004 2669 5680
Awarding Body: The University of Warwick
Current Institution: University of Warwick
Date of Award: 2008
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Hyaloperonospora parasitica, phylum Oomycota, is a biotrophic plant pathogen that causes downy mildew on its cruciferous host plants. Naturally-infected plants show disease resistance visible as a hypersensitive response (HR).. This response results from the interaction of pathogen effectors with receptors and signalling molecules in the plant cell, causing localised necrosis killing the infected host cell. H parasitica isolates that are host-specific on the model plant Arabidopsis have been used to identify such pathogen effector genes. In this work I attempted to use this information to identify similar genes in isolates host specific to the crop plant Brassica oleracea. Potential effector molecules, encoding amino-terminal signal peptides, produced by Arabidopsis isolates ofH parasitica were sequenced in up to . 16 isolates and analysed for selection pressure, using population genetic methods. Highly variable genes were categorised as candidate virulence factors that might be involved in molecular interactions with the host detection machinery. Since they exhibit high amino acid variability, they are likely to be under positive selection pressure for change. The most diverse gene (ATR13) was cloned and expressed in planta to test for HR. This group of genes were then used to determine if they were under similar levels of selection in B. oleracea isolates of H parasitica. To search for the homologues in this related H parasitica subspecies a genomic library and an expressed sequence tag (EST) library were produced, as direct methods of gene isolation and were generally unsuccessful. The two main strategies employed were the use of microsyntenies to localise effector regions in the genome and a random sequencing approach to search for genes that encode signal peptides and effector motifs, such as the predicted targeting motifs RXLR and DEER, which are important to the delivery of oomycete effectors into host cells. While it was possible to find the regions of ATR13 and ATRl in the genome, it was not possible to find the orthologues of these avirulence factors. Using the EST approach many new candidate effectors were identified. It is clear from the data produced in this project that although the H parasitica isolates from B. oleracea and Arabidopsis both infect Brassicacae, they are very distantly related. So far no highly variable genes were found in the crop pathogen, although many interesting new candidates for secreted virulence factors were identified and work on a potential locus of avirulence factors has been initiated.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available