The diversity of fungi associated with endemic and endangered orchids of Southern Africa

Abstract

Orchids form unique symbiotic relationships and are adapted to growing in extreme environments. The growth and survival of orchids are aided by soil microbes such as bacteria, actinomycetes and fungi, particularly mycorrhizal fungi. This orchid-mycorrhiza interaction is important because mycorrhizal fungi provide nutrients for seed germination, seedling establishment and growth of mature orchids. Without their mycorrhizal partners, orchid propagation in nature is often unsuccessful. Therefore, understanding the taxonomy, distribution and habitat specificity of these fungi is crucial for propagating orchids. Terrestrial orchids worldwide are going extinct due to the rapid rate of urban development, encroachment of invasive plant species and overcollection. In Southern Africa, populations of most endemic orchids are declining at an alarming rate. Therefore, potential solutions such as orchid translocation to protected habitats and in vitro seed germination are urgently required to combat the rapid decline of orchid populations. However, knowledge of the diversity, distribution, and specificity of the mycorrhizal symbionts of orchids in South Africa is sparse. Therefore, the overall aim of the research presented in this dissertation was to report on the diversity of orchid mycorrhizae associated with the South African endemic orchids in the genera Habenaria and Brachycorythis.

Chapter one: This chapter of the dissertation is a literature review detailing the general background knowledge available on orchid-mycorrhizal associations. The main focus of the review is to highlight the importance of mycorrhizae for the establishment and conservation of orchid populations in nature. Furthermore, this chapter gives an overview of the current knowledge of orchid-mycorrhizal associations.

Chapter two: This chapter investigated the mycorrhizal diversity present in the rhizosphere soil of a critically endangered orchid, Brachycorythis conica subsp. transvaalensis (Albertina Sisulu Orchid). For this purpose, the fungal diversity of the rhizosphere soil was compared to that in soil from a location where no orchids were previously recorded. Using high throughput sequencing, the fungal diversity present in both soil types was effectively compared. This study showed significant overlap in the fungal diversity in each of the soil samples. Moreover, the orchid rhizosphere soil contained more potential mycorrhizal fungal species.

Chapter three: This chapter focused on comparing the orchid-mycorrhizal relationships of two endemic terrestrial orchids in the genus Habenaria. Namely, the Habenaria barbertoni which is an endangered orchid and the Habenaria epipactidea which is more abundant. Microscopy, fungal isolations, molecular cloning, and high-throughput sequencing were used to identify potential mycorrhizal associates in the roots and tubers of each orchid. Culturedependent techniques provided limited information on the mycorrhizal symbionts of these orchids as only two potential mycorrhizal fungi were isolated. With culture-independent DNA based techniques, numerous potential mycorrhizal fungi from the Ceratobasidiaceae, Tulasnellaceae, Serendipitaceae, Pezizaceae, Hoehnelomycetaceae and Omphalotaceae were detected. This study showed significant differences in mycorrhizal diversity between the two orchid species.

Conclusion: In the research presented in this dissertation the mycorrhizal fungi associated with orchids in the genus Habenaria were successfully recorded and the potential orchid mycorrhizal associates of the Brachycorythis conica subsp. transvaalensis were identified. Some of the fungi detected could not be identified and thus are potential new mycorrhizal species. Information about the presence and identity of orchid mycorrhizal fungi at different locations can help in choosing sites that harbour fungi that are compatible with orchid species. In future, this data can be used for translocating orchids to conservation sites as well as developing and implementing other suitable conservation measures for orchids in South Africa. This study also contributed to the current knowledge of the types of orchid mycorrhizal fungi that occur in South African soils.