Date on Master's Thesis/Doctoral Dissertation

8-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Biology

Committee Chair

Perlin, Michael H.

Subject

Fungi--Physiology; Fungi--Development

Abstract

Dimorphism is a highly conserved process in fungi in which a transition between a unicellular, yeast-like growth form and either a unicellular or multicellular, filamentous growth form occurs in response to several different environmental cues. The phytopathogenic fungus Usti/ago maydis undergoes the dimorphic transition from a yeast-like saprobic growth form to a unicellular, hyphal growth form in response to successful mating and subsequent host cues. In addition, U maydis can also undergo haploid filament formation in response to several environmental cues including low ammonium conditions and growth in lipids or acid pH. On solid media deficient in ammonium (SLAD), U maydis produces a filamentous colony morphology, while in liquid low ammonium media the cells do not form filaments. The p21-activated protein kinases (P AKs) play a substantial role in regulating the dimorphic transition in fungi. Many activities of the P AKs are relayed through the mitogen-activated protein kinase (MAPK) pathway. The PAK-like Ste20 homologue, Smul, is required for a normal response to pheromone via up-regulation of pheromone expression, potentially through the MAPK pathway. Disrupting smul reduced this upregulation, with the effect more pronounced in the a2 mating background. Our experiments suggest that Smul also plays a role regulating cell length and the filamentous response on solid SLAD media. Yeast two hybrid analysis identified the conserved protein-arginine methyltransferase, Hs17, as a potential interacter of Smul. Hsl7 regulates cell length and the filamentous response to solid SLAD, yet in an opposite fashion to Smul. Interestingly, simultaneous disruption of hsl7 and overexpression of smulleads to a hyper-filamentous response on solid SLAD. In addition, the double mutant strain also forms filaments in liquid SLAD, while neither single mutant nor wild type strains display this phenotype. A similar filamentous response in both solid and liquid SLAD was also observed in strains lacking another PAK-like protein kinase involved in cytokinesis and polar growth, Cla4. This was not observed in a strain deleted for the activator of Cla4, the Rho-like GTPase Racl. My data suggest that Hsl7 may regulate cell cycle progression, while both Smul and Cla4 appear to be necessary for the filamentous response in U maydis.

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