Endophytic Fungi Associated with Australian Orchids

Abstract

Fungal endophytes of orchids Australia is rich in orchid flora with over 1000 native species currently recorded. A significant proportion of Australia's terrestrial orchids are critically endangered, endangered or threatened. Threats to many orchid species include habitat destruction, degradation and fragmentation from increased urbanisation, overgrazing, altered fire regimes and unfortunately, excessive collecting by orchid fanciers. Conservation efforts for Australian orchids include both ex situ and in situ approaches. Ex situ efforts involve the growth of orchid species under horticultural conditions and long term storage of plant and associated fungal material in laboratories and herbaria. In situ approaches include re-establishing plants in the wild and protection of current populations through management initiatives. All orchids depend on fungi for their nutritional needs. As the seeds of orchid are minute and contain very few stored reserves, fungal colonisation is essential for further growth and development following germination (Smith and Read 1997). During plant colonisation fungal hyphae penetrate cells and form elaborate coiled structures known as pelotons (see Figure 1). A peloton is the site of nutrient exchange between plant and fungus and it is via these structures that young orchids receive sugars and inorganic substances (eg. P, N) necessary for further growth. Because the fungi grow within plant cells they are called endophytes. Mature photosynthetic orchids remain colonised by fungi and supply their endophytes with sugars but continue to receive inorganic nutrients. A number of orchid species (such as Dipodium spp.), the so-called myco-heterotrophic orchids, completely lack photosynthetic capacity and are heavily dependent on a fungal partner to provide both sugars and inorganic nutrients throughout their lifetime. Fungal endophytes have now been investigated in a large number of orchid species from around the world. The traditional approach to identify the fungal endophytes of orchids has been to isolate pelotons from orchid tissues and to maintain fungal colonies in pure culture. The fungi were then identified on the basis of anatomy and morphology including such features as nucleus number, hyphal cross wall structure and spore dimensions (eg. Perkins et al. 1995). Worldwide, the fungi involved with orchids are almost all members of the phylum Basidiomycota group, however many do not produce sexual spores, and are consequently assigned to the form genus Rhizoctonia (Rasmussen 2002). Form genera are used in fungi when the sexual spores that are essential in the classification of fungi are not produced. The form genus Rhizoctonia produces septate hyphae in culture, but there are few other morphological characters to distinguish different species. The presence of Rhizoctonia in orchids is intriguing as fungi in this group are usually renowned as serious pathogens of many agriculturally important plant species. In recent years, analysis of myco-heterotrophic orchid species have shown non-Rhizoctonia fungi can also colonise orchids. These are mostly higher basidiomycete genera such as Thelephora, Russula and Coprinus. In addition, various species of Rhizoctonia have been matched up to their sexual stages, which occur in genera such as Thanatephorus and Ceratobasidium. The sexual stage has been induced by altering the culture conditions, often over long periods of growth and with the addition of soil to the cultures, such as by Warcup (1985). Recently, DNA sequence data has also been used to connect Rhizoctonia cultures to sexual stages (Bougoure et al. 2005)