טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentOfir Degani
SubjectG Protein and MAPK Pathways in the Maize Pathogen
Cochliobolus heterostrophus: Signaling for Gene
Expression, Development and Virulence
DepartmentDepartment of Biology
Supervisor Full Professor Horwitz Benjamin


Abstract

G-protein-linked pathways evolved to allow responses to extracellular agonists in eukaryotic cells.  In Cochliobolus heterostrophus, Ga (CGA1) and Gb (CGB1) subunit genes play essential roles in development and virulence. Nevertheless, CGA1 mutants can cause normal lesions on plants, unlike other fungal pathogens. We therefore identified host physiology states that may alter this phenotype. Δcga1 mutants were indeed less virulent on several maize varieties under most conditions, but not all, as virulence was nearly normal on detached senescing leaves. The role of the G protein subunits was also examined with the help of a CGA1 and CGB1 double mutant strain that was deficient in development and virulence. Epistatic relationships were found between the two genes in pigmentation, surface hydrophobicity, and resistance to different stresses. Furthermore, new, unique traits of this mutant suggest crosstalk between the MAPK and the G-protein pathways. We identified 4 new hydrophobins in the genome of this fungus and explored their expression profile in a series of signaling mutants. A transgenic strain expressing a activated allele, cga1Q204L was made as an additional tool for this purpose. We also examined the involvement of G protein subunits and MAPK in controlling the expression of two distinct target gene groups, extracellular enzymes and key enzymes that belong to biosynthetic pathways that might help confer resistance to the host defenses. The cga1 strains had normal ability to grow on different carbon sources while the MAPK null mutant was impaired. Comparison of the expression profile of three key biosynthetic pathway enzymes in the WT strains and in the signaling mutants, revealed specific patterns in the different mutants. These genes are highly expressed during the infection and may be associated with the fungal response to the host.