Abstract
Showy lady's slipper (Cypripedium reginae Walter, Orchidaceae) and black ash (Fraxinus nigra Marshall, Oleaceae) often co-occur in close proximity in fens in western Newfoundland, Canada. Metabarcoding of DNA extracted from root samples of both species following surface sterilization, and others without surface sterilization was used to determine if there were shared fungal endophytes in the roots of both species that could form a common mycorrhizal network between them. A wide variety of fungi were recovered from primers amplifying the nuclear ribosomal internal transcribed spacer region (ITS2). Sixty-six fungal sequences were shared by surface-sterilized roots of both orchid and ash, among them arbuscular mycorrhizal fungi (Claroideoglomus, Dominikia, Glomus and Rhizophagus), ectomycorrhizal fungi (Inocybe and Tomentella), the broad-host root endophyte Cadophora orchidicola, along with root pathogens (Dactylonectria, Ilyonectria, Pyricularia, and Xylomyces) and fungi of unknown function. There appear to be multiple fungi that could form a common mycorrhizal network between C. reginae and F. nigra, which might explain their frequent co-occurrence. Transfer of nutrients or carbon between the orchid and ash via one or more of the shared fungal endophytes remains to be demonstrated.
Highlights
Orchids (Orchidaceae) are famous for their dependence on fungi
The tiny “dust seeds” of most orchids need to be penetrated, colonized and fed by a compatible fungus to initiate germination, and most orchids remain nutritionally dependent on their associated fungus while the seedling grows into a protocorm, a nutritional mode known as mycoheterotrophy (Burgeff, 1959; Rasmussen and Rasmussen, 2009)
Samples were labeled with plant name (F. nigra or C. reginae), and presence or absence of the other plant growing within 15 m, yielding four comparison groups: (a) orchids with no ash nearby (ONA), (b) orchids with ash nearby (AO-Orch), (c) ash with no orchids nearby (ANO), and (d) ash with orchids nearby (AO-Ash)
Summary
Fungi Connecting Orchid and Ash. While achlorophyllous plants, including orchids, need an external source of energy throughout their life, the extent of mixotrophy, or partial nutritional dependence on fungi, in chlorophyllous orchids has been little investigated. Substantial evidence has accumulated to show that several Orchidaceae species that are chlorophyllous and photosynthetic as adults continue to obtain at least a part of their carbon energy from a shared, or common mycorrhizal network; Temperate orchid genera in which this has been documented include Cephalanthera, Epipactis (Bidartondo et al, 2004), Goodyera (Voronina et al, 2018), Listera (Gebauer and Meyer, 2003), Ophrys (Girlanda et al, 2011), and Rhizanthella (Warcup, 1985). Arbuscular mycorrhizal fungi (AMF) have been found associated with the roots of the Mediterranean grassland orchids Anacamptis and Ophrys (Voyron et al, 2017), and two temperate North American species of the lady’s slipper orchids, Cypripedium californicum and C. parviflorum (Shefferson et al, 2005). The role of AMF in orchid nutrition has not been experimentally determined
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