Seedling mycorrhiza: a discussion of origin and evolution in Orchidaceae

Abstract

Recent phylogenetic analyses confirm the monophyly of Orchidaceae as sister group to the remainder of Asparagales, and identify the sequence of early branching lineages in Orchidaceae. Orchid seedling mycorrhiza (OSM) involving rhizoctonious fungi is distributed widely in all subfamilies, including the first branching ones, and its status as a founding event is thus supported. OSM is recognized as one element in the character syndrome that distinguishes orchid biology, and we argue that OSM was the first to evolve. We also discuss the possible evolutionary origins of OSM in Asparagales. The prevalent mycobionts suggest a derivation from a pathogenic relationship, and sister group comparison offers little support for derivation from other mycorrhizal relationships. A combination of in situ sowings and molecular identification of seedling mycobionts has established that a broad range of fungi besides rhizoctonious mycelia are presently involved in OSM, presumably evolving secondarily and often in parallel in different orchid clades. Structural features and internal patterns of mycobiont behaviour appear to have remained largely the same, implying that OSM needs only minor physiological adjustment to accommodate new mycobionts. Such modifications will have involved checkpoints for recognition/rejection and the formation/ breakdown of pelotons. These physiological mechanisms are so far largely unknown. The trophic versatility of the mycobionts and the apparently easy shifts could be a main factor in the ecological adaptability of orchids and proliferation of the family.

Comparative seed germination and seedling development of the ghost orchid,Dendrophylax lindenii(Orchidaceae), and molecular identification of its mycorrhizal fungus from South Florida

“…and Taeniophyllum obtusum ( Irawati, 2009 ). Our verification of Ceratobasidium from D. lindenii in South Florida lends support for Yukawa’s view (2009) that members of the Ceratobasidiaceae are linked to orchids in the Vandeae tribe, especially the Angraecinae subtribe, by a ‘phylogenetic signal’ ( Yukawa et al , 2009 ; Rasmussen and Rasmussen, 2014 ). Furthermore, Chomicki et al (2014) found mycorrhizal structures (pelotons) in various states of digestion in adult ghost orchid roots, and a specialized mechanism for restricting fungal growth in the lower cortex without reducing photosynthesis in the upper root layer, thereby maximizing carbon gain.…”

“…Ceratobasidiaceae members are known to promote seed germination in a wide range of orchid subfamilies, specifically Apostasiodeae, Vanilloideae, Epidendroideae, Orchidoideae ( Dressler, 1993 ; Rasmussen et al , 2015 ) and Vandeae ( Irawati, 2009 ; Rasmussen and Rasmussen, 2014 ). Currently, there is only one published study on Thanatephorus (Ceratobasidiaceae) germination promotion capacity in seeds of Taeniophyllum obtusum , also a leafless orchid species ( Irawati, 2009 ).…”

Fine‐scale spatial distribution of orchid mycorrhizal fungi in the soil of host‐rich grasslands

“…The identity of OrM symbionts of adult plants largely depends on the identity and habitat of the orchid host (Dearnaley et al, 2012). In particular, photosynthetic orchids in sunny grassland habitats mainly associate, both as seedlings and as adult plants, with fungi in the 'rhizoctonia' complex sensu lato (Smith & Read, 2008;Dearnaley et al, 2012;Rasmussen & Rasmussen, 2014), a polyphyletic assemblage encompassing Agaricomycetes belonging to the Serendipitaceae (Sebacinales; Weiß et al, 2016), Ceratobasidiaceae and Tulasnellaceae (Roberts, 1999;Taylor et al, 2002;Weiß et al, 2004). The Tulasnellaceae, in particular, are the most frequently found OrM fungi in both temperate and tropical regions (Dearnaley et al, 2012).…”

Availability of orchid mycorrhizal fungi on roadside trees in a tropical urban landscape

“…By forming obligate relationships with compatible fungi, orchid hosts gain carbon, nutrients, and water at one or more life history stages 13,27 . The majority of orchids associate with fungal taxa belonging to the polyphyletic/“rhizoctonia” group of Basidiomycetes, specifically Ceratobasidiaceae, Serendipitaceae (Sebacinales), and Tulasnellaceae 13,28,29 . Identifying specific OMF may be advantageous for orchid conservation—e.g., orchid propagation, ex situ seeding, seedling translocation—as contrasting fungal taxa may differ in nutrient uptake efficiency or stimulation of germination 30–33 .…”