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      Iron and reactive oxygen in wheat-pathogen interactions

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      Greenshields_PhD_thesis.pdf (1.974Mb)
      Date
      2007
      Author
      Greenshields, David Lewis
      Type
      Thesis
      Degree Level
      Doctoral
      Metadata
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      Abstract
      Iron is an essential component of various proteins and pigments for both plants and pathogenic fungi. However, redox cycling between the ferric and ferrous forms of iron can also catalyse the production of dangerous free radicals and iron homeostasis is therefore tightly regulated. During pathogen attack, plants quickly produce large amounts of reactive oxygen species at the site of attempted pathogen ingress. This so-called oxidative burst has received considerable attention, but no single enzyme has been shown to account for the phenomenon. Using inductively coupled plasma mass spectrometry and histochemistry, I show that iron is secreted to the apoplast of the diploid wheat Triticum monococcum during attack by the powdery mildew fungus Blumeria graminis f.sp. tritici. This iron accumulates at cell wall appositions synthesised de novo beneath sites of pathogen attack. I further show, using histochemistry and pharmaceutical inhibitors, that this apoplastic iron accumulation is required for production of H2O2 in the oxidative burst. To understand the impact of this massive change in iron homeostasis on gene transcription, I employ a 187 gene targeted macroarray platform and establish that iron overload induces the expression of iron homeostasis-related genes and defence-related genes through iron itself and iron-mediated H2O2 production, respectively. To illustrate how the plant is able to withstand the negative effects of its own oxidative defences, I characterise a novel quinone redox cycle, and show that simultaneous induction of a protective quinone reductase isoform and downregulation of reactive oxygen-producing quinone reductase isoform prevents the spread of reactive oxygen during pathogen attack. Finally, in an effort to understand the impact of iron on fungal pathogenicity, I investigate iron uptake in the head blight pathogen, Fusarium graminearum. Fungi use at least two separate systems to take up iron, one based on enzymatic iron reduction and the other based on the synthesis and secretion of small iron chelators termed siderophores. Using mutants disrupted in either of two modes of iron uptake, I establish that siderophore production is essential for full F. graminearum virulence on wheat. This thesis exposes iron as an important component of both plant defence and fungal virulence.
      Degree
      Doctor of Philosophy (Ph.D.)
      Department
      Biology
      Program
      Biology
      Supervisor
      Wei, Yangdou; Selvaraj, Gopalan
      Copyright Date
      2007
      URI
      http://hdl.handle.net/10388/etd-07232007-105608
      Subject
      plant defense
      resistance
      fungal virulence
      pathogenicity
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      • Graduate Theses and Dissertations
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