¹⁵N and ¹³C natural abundance of two mycoheterotrophic and a putative partially mycoheterotrophic species associated with arbuscular mycorrhizal fungi

Abstract

Summary In contrast to mycoheterotrophs that associate with ectomycorrhizal and saprotrophic fungi, we know little about the ecophysiology of arbuscular mycorrhizal mycoheterotrophs. Here, we identify the mycorrhizal fungi of two unrelated mycoheterotrophs and one putative partial mycoheterotroph that form arbuscular mycorrhizas, and analyse their carbon (C) and nitrogen (N) isotope signatures. We used molecular methods to identify the mycorrhizal fungi of Dictyostega orobanchoides, Burmannia capitata (Burmanniaceae) and Voyria aphylla (Gentianaceae). Their C and N sources were investigated by analysing their stable isotope natural abundances (δ13C and δ15N). In addition, four putative partially mycoheterotrophic Burmannia species were grown ex situ. We found that both mycoheterotrophs and a green Burmannia species are associated with nonoverlapping Glomeromycota fungi. The investigated mycoheterotrophs are significantly more enriched in 13C than co‐occurring autotrophic plants but lack significant 15N enrichment. The green Burmannia species is not significantly enriched in 13C and 15N compared with surrounding plants and can grow fully autotrophically under controlled conditions. Our results suggest that mycoheterotrophic Burmanniaceae and Gentianaceae are able to exploit arbuscular mycorrhizal fungi. Green relatives of mycoheterotrophic Burmanniaceae from high‐light grassland sites also associate with arbuscular mycorrhizal fungi but we found no evidence that they receive detectable amounts of C from fungi.

Carbon and Nitrogen Metabolism in Mycorrhizal Networks and Mycoheterotrophic Plants of Tropical Forests: A Stable Isotope Analysis

“…2). Although barcoding on a single, highly polymorphic locus such as ITS has some limitations (Sanders and Croll, 2010), our data are congruent with published evidence, based on ribosomal DNA coding sequences, that different AMF clades cooccur in Voyria species (Bidartondo et al, 2002;Merckx et al, 2010), and that several Glomus species occur in Burmanniaceae (Merckx and Bidartondo, 2008;Merckx et al, 2010). Fungal specificity is often relaxed in tropical MH species (Martos et al, 2009;Roy et al, 2009), although the absence of fungal overlap between coexisting AMF-MH at CE2 is congruent with some partner preference.…”

“…An exciting perspective is that some green species related to AMF-MH may be partially MH Cameron and Bolin, 2010;Merckx et al, 2010). Since AMF-MH are similar in 15 N and 13 C to their fungal sources, which are also similar to their own food source, isotopic enrichment is not a general fingerprint for MH nutrition.…”

“…This may not apply to open forest or grassland ecosystems where all leaves receive similar light levels, entailing similar 13 C abundances, and where higher frequency of C 4 plants (Bonal et al, 2000;Lü ttge, 2008) complicates the use of 13 C abundances. Published investigations on candidate partially MH (=mixotrophic) plants associated with AMF showed that five green Burmannia species from French Guiana grassland (Merckx et al, 2010) and two Gentianaceae species from North American hardwood forest (Cameron and Bolin, 2010) were not enriched in 13 C, or in 15 N (except for one Gentianaceae), as compared with surrounding ground plants. Given the higher light level in these ecosystems, this finding does not necessarily disprove partially MH nutrition in itself, and, indeed, Merckx et al (2010) conclude compellingly that the ability of the Burmannia species to grow in pots, isolated from any mycelial network, demonstrates the possibility of full autotrophy.…”

“…We currently have only two isotopic studies on AMF-MH, all from South American rain forests (Merckx et al, 2010; this study); more broadly, research on tropical mycorrhizal symbioses should be encouraged (Alexander and Selosse, 2009). More direct assessments of the fractionation at the interface between AMF and AMF-MH or autotrophic hosts are required, including analyses of transferred carbohydrates.…”

“…Allen and Allen (1990) and Nakano et al (1999) analyzed 13 C abundances in spores of AMF associated with herbs: The 13 C abundance followed that of their C 3 or C 4 hosts, with some depletion (22‰ to 24‰ in d 13 C). Isotopic abundances were recently measured in two tropical rainforest AMF-MH, Burmannia capitata (Burmanniaceae) and Voyria aphylla (Gentianaceae): They showed higher 13 C abundance (+5.5‰ in d N abundance compared with understory green plants (Merckx et al, 2010). Enrichment in 13 C for AMF-MH is paradoxical, since depletion is expected in AMF (Allen and Allen, 1990;Nakano et al, 1999).…”