Date of Award

12-1-2012

Degree Name

Doctor of Philosophy

Department

Molecular Biology, Microbiology and Biochemistry

First Advisor

Fakhoury, Ahmad

Abstract

Fusarium virguliforme is a soil-borne pathogen that causes sudden death syndrome (SDS) disease in soybean. SDS is one of the most significant diseases of soybean in the United States. Fungal infection results in root and crown rot as well as SDS typical foliar symptoms including chlorosis, necrosis and complete defoliation. The use of soybean cultivars tolerant to SDS is still the most effective way to overcome the disease. On the other hand, the fungal isolates are known to have varied levels of aggressiveness on soybean indicated by the field and greenhouse experiments. Understanding the pathogen and its defense mechanism is the first step in exploring the pathogen-plant interaction. Therefore, the primary aim of this research was to elucidate the mechanism behind F. virguliforme response to soybean defense mechanisms. We further attempted to identify chromosome length polymorphism among F. virguliforme isolates and characterize the possible relationship to their level of aggressiveness. In order to fulfill the first objective, a series of differentially expressed genes were identified in F. virguliforme in the presence of soybean phytoalexin, glyceollin. The Fvgrx2 gene, a Saccharomyces cerevisiae grx2homologue, was selected for further analysis. This study demonstrates for the first time the identification and characterization of dithiol glutaredoxin gene in F. virguliforme . The role of FvGRX2 in the fungal defense to phytoalexin, glyceollin and induced oxidative burst was also investigated by generating anFvΔgrx2 knockout. In order to establish a link between the fungal karyotype and the level of fungal aggressiveness, the chromosome length polymorphism (CLP) was assessed for twenty-two F. virguliforme isolates exhibiting different levels of aggressiveness on soybean. The findings are instrumental in identifying novel pathogenicity such as the ones involved in phytotoxin production, fungicide resistance and aggressiveness.

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