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Metabolism and infection in the stagonospora nodorum-wheat pathosystem

Waters, Ormonde Dominick (2008) Metabolism and infection in the stagonospora nodorum-wheat pathosystem. PhD thesis, Murdoch University.

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Stagonospora nodorum is a necrotrophic fungal pathogen, and the causal agent of stagonospora nodorum blotch of wheat. Despite the economic importance of this disease, the molecular basis of the pathosystem is poorly understood. The aim of this study was to investigate the interaction between metabolism and infection in this pathosystem, with particular reference to the metabolism of mannitol.

In common with many fungi, the main metabolite produced by S. nodorum is the acyclic hexitol mannitol. Among the previously suggested roles for this compound is a role in pathogenicity. The metabolism of mannitol has been hypothesised as occurring in a cycle involving the enzymes mannitol 2-dehydrogenase (Mdh1) and mannitol 1-phosphate 5 dehydrogenase (Mpd1). A strain was created harbouring disruption constructs for both of these genes. The double mutant was unable to synthesise or catabolise mannitol, and was unable to sporulate. Addition of exogenous mannitol completely restored in vitro sporulation, and partially restored in planta sporulation. This demonstrated an essential role for mannitol in asexual sporulation. This is the first demonstrated role for this compound.

A 13C NMR study of the wild type strain, the mdh1 and mpd1 single mutants, and mpd1mdh1 double mutant was undertaken to investigate carbon utilisation and cycling. Disruption of Mpd1 significantly altered the metabolite profile with the mpd1 mutants producing trehalose and glycerol in place of mannitol. Labelling patterns in the double mutant showed that scrambling of label can be explained by the triosephosphate isomerase triangle and pentose phosphate pathway. This suggests the contribution of mannitol to label scrambling has been overstated in previous studies.

The evidence did not support the metabolism of mannitol in S. nodorum as occurring in a cycle, but rather as two separate pathways.

A GC-MS analysis of diseased and non-diseased tissue from infected leaves, compared to non-infected and mock-inoculated leaves, could not detect any metabolites associated with a systemic host reaction to pathogen attack.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Oliver, Richard and Solomon, Peter
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