Chlorophyllous and Achlorophyllous Specimens of Epipactis microphylla (Neottieae, Orchidaceae) Are Associated with Ectomycorrhizal Septomycetes, including Truffles

Abstract

Mycoheterotrophic species (i.e., achlorophyllous plants obtaining carbon from their mycorrhizal fungi) arose many times in evolution of the Neottieae, an orchid tribe growing in forests. Moreover, chlorophyllous Neottieae species show naturally occurring achlorophyllous individuals. We investigated the fungal associates of such a member of the Neottieae, Epipactis microphylla, to understand whether their mycorrhizal fungi predispose the Neottieae to mycoheterotrophy. Root symbionts were identified by sequencing the fungal ITS of 18 individuals from three orchid populations, including achlorophyllous and young, subterranean individuals. No rhizoctonias (the usual orchid symbionts) were recovered, but 78% of investigated root pieces were colonized by Tuber spp. Other Pezizales and some Basidiomycetes were also found. Using electron microscopy, we demonstrated for the first time that ascomycetes, especially truffles, form typical orchid mycorrhizae. All identified fungi (but one) belonged to taxa forming ectomycorrhizae on tree roots, and four of them were even shown to colonize surrounding trees. This is reminiscent of mycoheterotrophic orchid species that also associate with ectomycorrhizal fungi, although with higher specificity. Subterranean and achlorophyllous E. microphylla individuals thus likely rely on tree photosynthates, and a partial mycoheterotrophy in individuals plants can be predicted. We hypothesize that replacement of rhizoctonias by ectomycorrhizal symbionts in Neottieae entails a predisposition to achlorophylly.

Further advances in orchid mycorrhizal research

“…Helaxectris spp., Epipactis spp., Dipodium spp. (Taylor et al 2004;Selosse et al 2004;Dearnaley and Le Brocque 2006) and further study (eg. mycobiont identification; stable C and N isotope ratios, CO 2 exchanges) is required of rare chlorophyllous species to confirm physiological status (ie.…”

“…This evidence has taken two forms. Identical fungal ITS sequences in orchid roots and ECM of surrounding trees indicate epiparasitic interactions, although fulfilment of Koch's postulates, remain (Taylor and Bruns 1997;Selosse et al 2002a;Selosse et al 2004;Bidartondo et al 2004;Girlanda et al 2006;Abadie et al 2006). In the second form of experiment, stable isotope ratios of carbon and nitrogen within orchids match those of local ECM fungi (Gebauer and Meyer 2003;Trudell et al 2003;Bidartondo et al 2004;Whitridge and Southworth 2005;Julou et al 2005;Abadie et al 2006) indicating common pools of nutrients.…”

“…Recent evidence shows that a third orchid nutritional mode exists -mixotrophy (Julou et al 2005). Such orchids are photosynthetic at the adult stage but augment their carbon requirements via mycorrhizal fungi (Gebauer and Meyer 2003;Bidartondo et al 2004;Selosse et al 2004;Julou et al 2005). Mixotrophic orchids may be an evolutionary step between photosynthetic and MH orchids (Julou et al 2005).…”

“…In agreement with Rasmussen (2002) the majority of orchid mycobionts are basidiomycetes but a striking exception has been the fungal partners of Epipactis. Selosse et al (2004) analysed the fungal ITS regions of colonised roots of chlorophyllous and achlorophyllous individuals of E. microphylla (Ehrh.) Swartz over three French sites.…”

Promoting role of an endophyte on the growth and contents of kinsenosides and flavonoids of Anoectochilus formosanus Hayata, a rare and threatened medicinal Orchidaceae plant

“…Third, under the promoting role of the endophyte, the decomposition of starch grains may provide the carbon required for the growth of the host plant A. formosanus itself. In addition, all the abovementioned reasons may simultaneously occur because nutrient exchange in photosynthetic orchids is highly complex (Selosse et al, 2004;Cameron et al, 2006). Plants secrete both high-and low-molecular weight compounds from their roots, whose exudates function not only as nutrients for soil microbes but also as signal molecules in plant-microbe interactions (Sugiyama and Yazaki, 2012).…”