Members of the Orchidales produce dust seeds so lacking in nutrient and carbon reserves that they are dependent upon appropriate fungal partners to supply the essential additional carbon and nutrients required for successful establishment in nature. These plants are thus initially mycoheterotrophic before they normally go on to develop green leaves and become autotrophic as adults. However, at least 210 species of orchid remain fully mycoheterotrophic into adulthood. The metabolic pathways involved in the nitrogen and carbon fluxes from fungus-to -plant in mycoheteratrophic are still poorly understood, but result in characteristic enrichment in 13C and 13N. In this thesis in vitro studies on agar are employed to investigate the effects of different N sources, with and without sugar or cellulose carbon sources on the heterotrophic growth of mycorrhizal orchid seedlings and metabolite pools in the plants and their fungal partners. It is hypothesised that glutamine, a central metabolite in assimilation of mineral by basidiomycete fungi, passes from fungus-to -plant as in traditional models of N assimilation by ectomycorrhizal plants. A range of nitrogen and carbon sources including isotopically labelled 14C and 13C-glucose together with 15N labelled N sources including nitrate, ammonium, ammonium nitrate, glutamine, glycine and arginine were supplied to orchid mycorrhizal fungi in pure culture and to mycorrhizal seedlings of Goodyera repens or Dactylorhiza fuchsii. In three compartment Petri dishes in which the seedlings were grown with their fungal partners colonising the two other compartments, simultaneous additions of DC and 15N tracers provided compelling evidence of fungal assimilatory pathways providing co-transport of C and N into heterotrophic seedlings of G. repens. Nitrate was a better N source than ammonium or amino acids for the plants, and metabolomic analysis showed strong up-regulation of relative abundance of glutamine in the fastest growing plants. The findings presented in this thesis provides strong circumstantial evidence that glutamine serves as a major combined N and C source transported from fungus-to-plant in heterotrophic orchid seedlings.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604555 |
| Date | January 2012 |
| Creators | Abualghaith, Abdulaziz |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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