Isotopic and molecular evidence for saprotrophic Marasmiaceae mycobionts in rhizomes of Gastrodia sesamoides

Gastrodia sesamoides, a common obligate mycoheterotrophic orchid species found in eastern Australia relies on a soil fungus to provide a source of carbon nutrition. The identity of this fungus is not known although in other studies of Gastrodia species a number of mycobionts have been suggested including Fomes and Mycena. In this study the fungal community of rhizomes of G. sesamoides has been identified via fungal ITS-DNA PCR amplification, cloning and sequencing. Although a number of fungi were identified by this approach the most common fungal ITS DNA within the orchid were saprotrophic members of the Marasmiaceae (Campanella and Marasmius spp.). Analysis of the natural carbon and nitrogen stable isotope abundances of stems of G. sesamoides showed an enrichment in 13C and low levels of 15N. These data suggest that G. sesamoides obtains its carbon parasitically from free-living saprotrophic fungi and not from an ectomycorrhizal fungal partner of a photosynthetic plant, as is common for other obligate mycoheterotrophic orchid species.