Mixotrophy of Platanthera minor, an orchid associated with ectomycorrhiza-forming Ceratobasidiaceae fungi

Authors

  • Takahiro Yagame,

    1. Fungus/Mushroom Resource and Research Center, Faculty of Agriculture, Tottori University 4-101 Koyama-Minami, Tottori, 680-8553, Japan
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  • Takamichi Orihara,

    1. Kanagawa Prefectural Museum of Natural History 499 Iryuda, Odawara, Kanagawa 250-0031, Japan
    2. The United Graduate School of Agricultural Sciences, Tottori University, 4-101 Koyama-cho-minami, Tottori 680-8553, Japan
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  • Marc-André Selosse,

    1. Centre d’ Ecologie Fonctionnelle et Evolutive, 1919 Route de Mende, F–34293 Montpellier Cedex 5, France
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  • Masahide Yamato,

    1. Fungus/Mushroom Resource and Research Center, Faculty of Agriculture, Tottori University 4-101 Koyama-Minami, Tottori, 680-8553, Japan
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  • Koji Iwase

    1. Department of Natural and Environmental Science Teikyo University of Science 2525 Yatsusawa, Uenohara 409-0193, Japan
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Author for correspondence:
Takahiro Yagame
Tel: +81 857 31 5485
Email: t_yagame@muses.tottori-u.ac.jp

Summary

  • We investigated the fungal symbionts and carbon nutrition of a Japanese forest photosynthetic orchid, Platanthera minor, whose ecology suggests a mixotrophic syndrome, that is, a mycorrhizal association with ectomycorrhiza (ECM)-forming fungi and partial exploitation of fungal carbon.
  • We performed molecular identification of symbionts by PCR amplifications of the fungal ribosomal DNA on hyphal coils extracted from P. minor roots. We tested for a 13C and 15N enrichment characteristic of mixotrophic plants. We also tested the ectomycorrhizal abilities of orchid symbionts using a new protocol of direct inoculation of hyphal coils onto roots of Pinus densiflora seedlings.
  • In phylogenetic analyses, most isolated fungi were close to ECM-forming Ceratobasidiaceae clades previously detected from a few fully heterotrophic orchids or environmental ectomycorrhiza surveys. The direct inoculation of fungal coils of these fungi resulted in ectomycorrhiza formation on P. densiflora seedlings. Stable isotope analyses indicated mixotrophic nutrition of P. minor, with fungal carbon contributing from 50% to 65%.
  • This is the first evidence of photosynthetic orchids associated with ectomycorrhizal Ceratobasidiaceae taxa, confirming the evolution of mixotrophy in the Orchideae orchid tribe, and of ectomycorrhizal abilities in the Ceratobasidiaceae. Our new ectomycorrhiza formation technique may enhance the study of unculturable orchid mycorrhizal fungi.

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