UBC Theses and Dissertations

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UBC Theses and Dissertations

Lipid and acetate metabolism influence host colonization by the fungal plant pathogen Ustilago maydis Lambie, Scott Cameron


Ustilago maydis is an obligate fungal pathogen of maize that causes disease known as the common smut of corn. Haploids with compatible mating loci fuse to form a dikaryotic cell type that is filamentous and pathogenic; invasion of the host by this cell type leads to the formation of tumors which contain diploid teliospores. Colonization of the plant presents numerous challenges for U. maydis because the host environment may be limited in nutrients and plant defense responses lead to the creation of toxic molecules such as reactive oxygen species (ROS). Understanding the mechanisms employed by U. maydis to overcome such obstacles is necessary to develop strategies to fight smut disease and protect crops from disease caused by fungal plant pathogens. This study focused on the characterization of a group of genes encoding phospholipases (PLs), enzymes which have been implicated in virulence, morphogenesis and nutrient acquisition of a number of pathogenic fungi. This work included a characterization of one PL, Lip2, which may function to repair ROS-induced damage of membranes that occurs during host colonization. Mutants defective in lip2 were less virulent in maize seedlings, showed sensitivity to H₂O₂ and the drugs chloroquine and quinacrine, and were resistant to ionic stress. A transcriptional profile of a Δlip2 mutant suggested that Lip2 contributes to stress responses and carbon metabolism. Furthermore, lipidomic profiles of the Δlip2 mutant revealed changes in lipid composition that may be linked to the mutant phenotypes. Other aspects of central carbon metabolism were also explored including the utilization of acetate as a carbon source by U. maydis and the role of an ATP-citrate lyase, Acl1, in pathogenesis and fungal development. Acetate had a repressive effect on mating and filamentation, and promoted reduced growth and virulence compared to favorable carbon sources such as glucose. Furthermore, Acl1 was shown to be essential to cause disease and for growth on glucose. Overall, this study reveals potential mechanisms employed by U. maydis during plant colonization to resist the defense response. It also highlights the utility U. maydis as a model system to understand the metabolic and nutritional aspects of fungal phytopathogenesis.

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