Nutritional and genetic diversity in orchid mycorrhizal fungi from Caladenia species

Mehra, S 2014, Nutritional and genetic diversity in orchid mycorrhizal fungi from Caladenia species, Doctor of Philosophy (PhD), Applied Science, RMIT University.


Document type: Thesis
Collection: Theses

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Title Nutritional and genetic diversity in orchid mycorrhizal fungi from Caladenia species
Author(s) Mehra, S
Year 2014
Abstract Orchids depend on symbiotic associations with mycorrhizal fungi for seed germination and nutrition. Complex orchid-fungus relationships are continuously subject to transfers of organic and inorganic nutrients during their life stages, though these vary quantitatively from seed to mature plant. This research explored the possible sources of carbon, nitrogen and phosphorus nutrition in the genus Caladenia through its OMF. Spider orchids (Caladenia subgenus Calonema) have a large proportion of threatened species and represent a significant challenge in conservation. Knowledge of what resources from the environment are made available by the OMF to Caladenia species is lacking in the literature and is necessary in order to conserve them. This research examined the potential sources of nutrition available for orchid mycorrrhizal fungi in the wild, including the associated plants on site as potential partners in a tripartite symbiosis, and typical litter as sources of saprophytic nutrition for the OMF. Differences among OMF in patterns of utilization of purified C, N and P sources were investigated, showing that these were different in effective and ineffective mycobionts isolated from single plants and found reasons for and the consequences of the high diversity in OMF in orchids.

Using molecular techniques (PCR and sequencing), OMF from two species of Caladenia were traced on to other plants at two sites in Victoria but OMF were not widely distributed on the associated vegetation and so must rely more on saprotrophy. The ability of OMF from four traditional groups of Caladenia to grow on a wide range of insoluble and soluble C sources available in natural ecosystems was greater in OMF from common than endangered orchids and dichotomised in their ability to use sucrose, the most common energy source in orchids. This is the first time this common assumption has been examined critically. Further, even effective OMF from the endangered C. fulva varied in their ability to grow on a wide range of C, N and P sources. The OMF isolated from a single plant of C. fulva differed in seed germination and further growth of the orchid.

This research was also focused on determining why OMF isolates from single orchids typically show so much diversity. This study showed for the first time that orchid seedlings inoculated with a single isolate produced multiple OMF genotypes in few months. Furthermore, OMF across eight species from the three sub-tribes of orchids, using molecular methods such as RAPDs, microsatellites (ISSR), ITS sequencing and real-time PCR showed this genetic polymorphism among re-isolated pelotons. It is suggested that this is a result of rapid fungal mutation inside the orchid pelotons, in a short interval of time, under axenic conditions suggesting rapid mutation rather than invasion by multiple genetically diverse fungi.

These findings have thus elucidated the nutrition of orchid mycorrhizal fungi (OMF) of both endangered and common orchids and found reasons for and consequences of the high diversity in OMF. The research has therefore provided new insights into the nutritional physiology, genetics and ecology of OMF from species of native terrestrial orchids from Australia.

Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Applied Science
Keyword(s) Caladenia
OMF (Orchid mycorrhizal fungi)
saprotrophic
mycorrhiza
polymorphism
endophyte
symbiotic
mixotrophy
mutation
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