|M.Sc Student||Igbaria Aeid|
|Subject||The MAP Kinase Signaling Network of Cochliobolus|
heterostrophus: Modulation of Gene Expression and
Role in Virulence and Stress Responses
|Department||Department of Biology||Supervisor||Professor Benjamin Horwitz|
|Full Thesis text|
Filamentous fungal pathogens of plants, like other eukaryotic microbes, respond to a variety of signals from the environment. These signals may originate from the host plant, or they may include purely physical or chemical stimuli from the environment.
Conserved fungal mitogen-activated protein kinase (MAPK) cascades are required for development, conveying signals to alter gene expression. Pathogenicity MAP kinases, related to yeast FUS3/KSS1, are essential for virulence in many fungal species, including Cochliobolus heterostrophus, a necrotrophic pathogen causing Southern corn leaf blight. In this study, phenotypes conferred by loss of function mutations in two MAPK genes, HOG1 and MPS1 were characterized, and also compared with the third MAPK, CHK1, which was studied previously. Mutants in CHK1 and MPS1 have similar phenotypes regarding autolytic appearance, light pigmentation, and dramatic reduction in virulence and conidiation. The similar phenotypes of mps1 and chk1 mutants are also reflected by joint regulation of some genes, for example a hydrophobin and a cytochrome P450. Unlike chk1, mps1 mutants are female-fertile and form normal-looking appressoria. C. heterostrophus hog1 mutants have smaller appressoria and cause decreased disease symptoms on maize leaves. HOG1 mediates resistance to hyperosmotic and, to a lesser extent, oxidative stresses through induction of genes involved in glycerol biosynthesis and redox regulation, including glycerol-3-phosphate phosphatase, transaldolase, and a monosaccharide transporter. Only HOG1, but not MPS1 or CHK1, was rapidly phosphorylated in response to osmotic stress. Mutants in MPS1 were even more resistant than wild type to hyperosmotic and oxidative stresses; loss of MPS1 in hog1 mutants actually improved resistance to stress. In summary, all three MAP kinases contribute to the regulation of central developmental functions under normal and stress conditions, and full virulence of the fungus cannot be achieved without appropriate input from all three pathways.