Identification of New Pathogenicity Genes in Magnaporthe Oryzae through the Construction of an Agrobacterium Tumefacines-Mediated Insertion Mutant Library
An understanding of plant pathogen-host interactions is essential to design efficient strategies to control disease in crops. Magnaporthe oryzae, an ascomyceteous fungus and causal agent of rice blast disease, is a model organism to study host-microbe interactions. The overall aim of this dissertation project was to identify genes involved in pathogenicity through the construction and characterization of a random insertional mutagenesis library. In order to saturate the genome with DNA inserts, a collection of >54,000 insertion lines of the M. oryzae strain 70-15 was generated via two transformation methods, PEG/CaCl2 (polyethylene glycol)-mediated protoplast transformation and Agrobacterium tumefaciens-mediated transformation. The first part of this dissertation describes the optimization of both transformation approaches, compares their efficiency and provides a description of the high-throughput processing and phenotypic analysis of the insertion lines. An in vitro appressorium assay of 12,000 T-DNA insertion strains allowed the identification of 135 lines that were classified as morphologically or functionally different than wild-type. Rice infection assays demonstrated that 112 of these strains exhibited defects in pathogenicity.The second part of this dissertation project analyzed the T-DNA integration patterns in a subset of pathogenicity mutants. This section aimed to identify the disrupted genes via recovery of M. oryzae sequences adjacent to the sites of T-DNA insertion. Genomic mapping of 61 T-DNA insertions in pathogenicity mutants via rescuing M. oryzae chromosomal T-DNA flanking sequences using inverse PCR resulted in the identification of 22 conserved hypothetical genes with predicted function, 11 predicted open reading frames without a GenBank significant match, two unannotated regions of the genome assembly and seven intergenic regions. The final part of this dissertation describes the characterization of a M. oryzae pathogenicity mutant that contains a T-DNA insertion in the upstream region of two divergently transcribed genes that encode the vacuolar type-ATPase subunit c`` and the general transcription factor TFIIA subunit γ. Genetic complementation demonstrated the insertion of the T-DNA in the promoter region of the general transcription factor TFIIA subunit γ is responsible for observed defects in conidiation, appressorium morphogenesis, and appressorium function. This is the first report relating the function of TFIIA subunit γ to pathogenicity
