Mycorrhizas of tropical mountain forests

Ochid mycorrhizae

We mainly investigated two aspects of orchid mycorrhizas: 

New mycobionts of orchids

Mycorrhizas with Tulasnellales and Sebacinales

New mycobionts of orchids  –  a puzzling finding from Reserva Biológica San Francisco (RBSF)

We discovered a new group of orchid mycobionts associated with diverse epiphytic and terrestrial members of Higher Epidendroideae, the most derived orchid group [88], see Fig. 1.  We identified these fungi by a unique combination of ultrastructural characters (Bauer et al. 2006) and by molecular phylogeny. The fungi belong to Atractiellomycetes in the “rust” fungal lineage (Pucciniomycotina) and are thus the evolutionary most basal mycobionts in Basidiomycota (red arrow, Fig. 1).

The finding may challenge Yakawa et al. (2009) who claimed to have found the earliest switch in Monocotyledons from Glomeromycota (arbuscular mycorrhiza AM) to Basidiomycota (orchid mycorrhiza OM). However, the linkage of basal Atractiellomycetes to most derived Orchidaceae may also be a recent switch opening the discussion for opportunistic behaviour of orchids in respect to fungal associations [88].

Fig. 1: Worldwide, only few groups of fungi operate as orchid mycobionts. Tulasnella, Ceratobasidium and Sebacina are the most common ones (blue arrows) and are also associated with the most basal group of orchids, the Apostasioideae (green arrows; Yakawa et al. 2009). Findings from Old World partial or fully heterotrophic orchids in diverse subfamilies indicate additional, independent switches to saprophytic or ectomycorrhizal fungi (yellow arrows; Taylor et al. 2004, Martos et al. 2009, Roy et al. 2009; 84). All these orchid mycobionts belong to Agaricomycotina, the mushrooms, but the new orchid mycobionts are hosted in Pucciniomycotina, the rust fungi. enlarge figure (pdf-file, 0.1 MB).

References

Bauer R, Begerow D, Sampaio J, Weiß M & Oberwinkler F. 2006 The simple-septate basidiomycetes: a synopsis. Mycol. Progr. 5, 41-66.

Martos F, Dulormne M, Pailler T, Bonfante P, Faccio A, Fournel J, Dubois MP, Selosse MA. 2009. Independent recruitment of saprotrophic fungi as mycorrhizal partners by tropical achlorophyllous orchids. New Phytol. 184: 668-681.

Roy M, Watthana S, Stier A,  Richard F,  Vessabutr S, Selosse M-A. 2009 Mycoheterotrophic orchids from Thailand tropical dipterocarpacean forests associate with a broad diversity of ectomycorrhizal fungi. BMC Biology 7: 51.

Taylor DL, Bruns TD, Hodges SA. 2004. Evidence for mycorrhizal races in a cheating orchid. Proc. R. Soc. London B 271: 35-43.

Yukawa T, Ogura-Tsujita Y, Shefferson RP, Yokoyama J. 2009. Mycorrhizal diversity in Apostasia (Orchidaceae) indicates the origin and evolution of orchid mycorrhiza. Am. J. Bot. 96: 1997–2009.

Epiphytic orchids form mycorrhizas with Tulasnellales and Sebacinales

For the first time a small group of epiphytic orchids occurring in high numbers in the tropical mountain rain forest of Southern Ecuador was studied by combining light microscopy (Fig. 2), TEM (Fig. 3), and molecular phylogeny (Fig. 4).

Fig. 2: Hyphae of mycorrhizal fungus, stained by methyl blue, in the cortical cells of an epiphytic orchid root. Image: I. Kottke.

Fig. 3: Electron micrograph displaying doliporus with continuous, dish shaped parenthesomes and slime in the cell walls as typical for Tulasnella species. Image: I. Kottke.

Tulasnellales, a group of Basidiomycota related to the cantharelle clade, are the main mycorrhizal colonizers, but Sebacinales were also found [82, 68, 74]. Surprisingly, the Sebacinales associated with the Andean ericads and the epiphytic orchids are distinct, and the Tulasnellales are different in orchids from other regions.

Fig. 4: Molecular phylogenetic placement of Tulasnella sequences from mycorrhizas of the epiphytic, pleurothallid orchids [Stelis, Pleurothallis] sampled in the cloud forest of Southern Ecuador [courtesy of JP Suarez]. enlarge figure (pdf-file, 0.2 MB).

Next research project: 
Structure of the Hartig net and establishment of ectomycorrhizas of temerate forests

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