Specialization in mycorrhiza of orchids across temperate Australian terrestrial orchid genera

A project undertaken at the Royal Tasmanian Botanic Gardens, and supervised by Nigel Swarts

Collaborators

Magali Wright, NRM South Tasmania; Ryan Phillips, Australian National University; Noushka Reiter, Royal Botanic Gardens Melbourne

Summary

Australia has one of the most threatened terrestrial orchid floras with over 165 species listed on the EPBC act. The unique diversity in the Australian Orchidaceae and subsequent high levels of threat make studies into their biology and ecology a high priority from both evolutionary and conservation perspectives. Orchids rely on mycorrhizal associations to facilitate seed germination and growth. This is an obligate interaction for orchids since their minute dust-like seeds do not contain sufficient stored nutrition for development of the embryo. The orchid-mycorrhizal association is essential to facilitate uptake of soil nutrient and water to the germinating seed and subsequent developing plant. In temperate terrestrial orchids, the association extends to adulthood, where mature plants retain mycorrhizal associations to varying degrees of infection and nutritional dependency.

In this study we investigated the diversity and specialisation of the mycorrhizal association of Australia's temperate terrestrial orchid flora. We isolated mycorrhizal fungi from mature orchid plants and grew them in culture for DNA sequencing.  ITS sequencing data was used to determine fungal diversity among three key families including Sebacinaceae, Tulasnellaceae, and Ceratobasidiaceae that are known to associate with Australian terrestrial orchids. With an analysis including over 1000 DNA sequences, we found that high specificity for one or very few closely related mycorrhizal species was common among the majority of orchids sampled. Across the three fungal families we found a common trend where often closely related orchids would associate with a particular mycorrhizal species or a narrow group of closely related mycorrhizal species. Geographical distribution varied widely with some mycorrhiza spanning the continent with others occurring in highly localised areas. This research has important implication for sourcing fungi for propagation trials for reintroduction which is now becoming a high priority for many rare and threatened orchid species.

Funding for this project was provided by the Hermon Slade Foundation (HSF 13-02) and the Australian Orchid Foundation (275-10).

Publication

Phillips, R.D., Barrett, M.D., Dalziel, E.L., Dixon, K.W. and Swarts, N.D. (2016). Geographical range and host breadth of Sebacina orchid mycorrhizal fungi associating with Caladenia in south-western Australia. Botanical Journal of the Linnean Society, 182, 140-151.

Figure 1. Caleana major

Figure 2. Culture of Pterostylis rubenachii on nutrient agar media

Figure 3. Germination of Caladenia seedlings on oatmeal agar