Mutualistic, root-inhabiting fungi of orchids identification and functional types

Abstract

Orchids depend on fungi for germination and protocorm development and maintain mycor- rhizas in their adult forms. Mycorrhizal fungi are therefore a driving force in orchid evolution and speciation. However, reliable data on fungal identities are crucial. Information from molecular phylo- genetics and transmission electron microscopy congruently revealed distinct fungal groups as orchid mycobionts. Sebacinales Group B, Tulasnellales, and Ceratobasidiales were found associated with ter- restrial orchids in open grasslands and arbuscular mycorrhizal forests and with epiphytes, whereas Sebacinales Group A, Telephorales, Russulales, some Euagaricales, and Tuberales form mycorrhizas with terrestrial orchids in ectomycorrhizal forests. Enzyme and isotope analyses revealed that the for- mer obtain carbon from rotten organic material to nourish the protocorm, whereas the latter take car- bon from ectomycorrhizas of woody plants to supply nutrients to protocorms and adult heterotrophic or mixotrophic plants. Mycobionts of terrestrial orchids are of narrower host range than previously expected. Te few investigations on mycobionts of epiphytic orchids indicate sharing of hosts. DNA- based fungal identities of mycobionts from tropical terrestrial and epiphytic orchids, host range, and inclusion of data from several ecological parameters are still needed to determine whether association strategies differ between epiphytic and terrestrial orchids or between temperate and tropical habitats.