טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentRonen Mordechai
SubjectThe Encounter of Cochliobolus Heterostrophus with its
Host: The Role of Oxidative Systems in
Necrotrophic Infection by a Plant
Pathogen
DepartmentDepartment of Biology
Supervisor Professor Benjamin Horwitz
Full Thesis textFull thesis text - English Version


Abstract

Reactive oxygen species (ROS) are synthesized by specific NADPH oxidases (Nox), enzymes inserted in the plasma membrane. Nox enzymes use cytoplasmic NADPH to produce superoxide. ROS can serve both defense and differentiation signaling roles in animals and plants. Fungal NADPH oxidases have a structure very similar to the human gp91phox. Specific isoforms of fungal Nox have been reported to be required for various physiological processes and cellular differentiation events, including development of sexual fruiting bodies, ascospore germination, and hyphal growth in both mutualistic and antagonistic plant-fungal interactions. We identified three NADPH oxidase homologues in the necrotrophic filamentous fungus Cochliobolus heterostrophus. ChNoxA has a developmental role as the mutant presents reduced pigmentation accompanied by decreased sporulation and is female sterile, but still can induce maize infection. ChNoxB and ChNoxC show no distinctive phenotypes but ChNoxB is non virulent. Both ChNoxAB, ChNoxABC, and the mutant in the regulatory complex subunit ChNoxR present all ChNoxA and ChNoxB phenotypes. We exposed germinating fungal spores to maize leaf extract as an axenic bioassay mimicking the plant signals the fungi is exposed to when it encounters its host, and observed distinctive induction in secreted superoxide production, a mechanism that might exploit the plant’s hypersensitive response which is initialized by an oxidative burst and lead to programmed cell death favoring the necrotrophic fungus pathogen. Superoxide production was impaired in noxABC and ChNoxR mutants. We show plant extract contain phenolic compounds (an effective antioxidants), and that both plant extract and phenolic exposure result in reduced cytoplasm state, but still act as ChAP1 (a redox regulated transcription factor) nuclear retention signals, strengthen the notion that necrotrophic fungi are not exposed to an oxidative stress during infection. We show Nox derived ROS act as an internal signal and are required for important antioxidant gene to be properly expressed.