Nitric oxide: a chemical effector of pathogenesis in magnaporthe oryzae

Thesis

Research detailed in this thesis investigated the generation of Nitric Oxide (NO) and its role in the pathogenesis of the rice blast fungus Magnaporthe oryzae. Two putative nitric oxide synthase genes and single copy nitrate and nitrite reductase genes were cloned as potential sources of NO in M. oryzae. Single and double gene disrupted mutants were generated and their phenotypes assessed.

Detection of NO is problematic. Herein, a fluorescent plate reader assay was developed, exploiting the NO sensitive dye DAR-4M AM and the NO scavenger PTIO, to compare wildtype NO generation with the mutant strains. All strains were assessed for infection-related development on an artificial surface inductive to appressorium formation and maturation in the wildtype strain. Appressorium formation in the presence of PTIO and the NO donor DETANONOate was recorded for all strains on this surface. The pathogenicity of the wildtype and mutant strains were assessed, in terms of their ability to infect rice and barley plants. Finally, the capacity of each strain to metabolise nitrogen was evaluated to confirm the disruption of the nitrate and nitrite reductase genes.

Collectively, the data demonstrate that the plate reader assay provides robust evidence for the generation of NO in M. oryzae. However, none of the various mutant strains showed a reduction in NO emission during germling morphogenesis. However, they exhibited significantly different infection-related development on an inductive artificial surface as compared with the wildtype strain. Moreover, exogenous application of PTIO to the wildtype strain provided evidence for NO and its involvement in germination and appressorium development. No significant differences in the infection of rice and barley leaves were observed between the wildtype and mutant strains, indicating their disrupted genes are dispensable for pathogenesis. The nitrate and nitrite reductase genes were found to be essential for nitrate assimilation.

In summary, this work provides the most robust evidence for the generation of NO in fungi to-date, but the molecular mechanism underpinning the generation of NO in M. oryzae remains elusive.

Authors: Johnson, J

• Publication date: 2011
• DOI: 10.5287/ora-de8zxqbqj
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Nitric Oxide; Magnaporthe oryzae
• Subjects: Cell Biology (see also Plant sciences)