Evidence for novel and specialized mycorrhizal parasitism: the orchid Gastrodia confusa gains carbon from saprotrophic Mycena

Abstract

We investigated the physiological ecology of the Asian non-photosynthetic orchid Gastrodia confusa. We revealed its mycorrhizal partners by using molecular identification and identified its ultimate nutritional source by analysing carbon and nitrogen natural stable isotope abundances. Molecular identification using internal transcribed spacer and large subunit nrDNA sequences showed that G. confusa associates with several species of litter- and wood-decomposer Mycena fungi. The carbon and nitrogen isotope signatures of G. confusa were analysed together with photosynthetic plant reference samples and samples of the ectomycorrhizal epiparasite Monotropa uniflora. We found that G. confusa was highly enriched in (13)C but not greatly in (15)N, while M. uniflora was highly enriched in both (13)C and (15)N. The (13)C and (15)N signatures of G. confusa were the closest to those of the fruit bodies of saprotrophic fungi. Our results demonstrate for the first time using molecular and mass-spectrometric approaches that myco-heterotrophic plants gain carbon through parasitism of wood or litter decaying fungi. Furthermore, we demonstrate that, several otherwise free-living non-mycorrhizal, Mycena can be mycorrhizal partners of orchids.

Figure 1

Phylogenetic relationships of the mycorrhizal fungi of G. confusa (black circles) based on LSU ribosomal DNA sequences of Mycenaceae available in GenBank. GenBank accessions are shown in parentheses. The numbers shown to the right of black circles indicate the fungal ITS sequence type shown in electronic supplementary material C. Phylogenetic analysis was conducted using NJ with 1000 bootstrap replicates (values of more than 60% are near branches).

Figure 2

Mean (±1 s.d.) δ¹³C and δ¹⁵N values in flower stalks of two myco-heterotrophic species (G. confusa and M. uniflora), leaves of five autotrophic species (open circles), sporocarps of saprotrophic fungi (SF, open squares), bamboo litter (BL, open triangles), decayed bamboo stems (DS, upward-pointing open triangle) and in soil samples collected from (a) a dense bamboo forest (G. confusa) and (b) its neighbouring evergreen broadleaf forest, (M. uniflora, filled triangle). Plant species: Di, Damnacanthus indicus; Gp, Gynostemma pentaphyllum; Pk, Piper kadsura; Tha, Thelypteris acuminata; Tra, Trachelospermum asiaticum.