Wide geographical and ecological distribution of nitrogen and carbon gains from fungi in pyroloids and monotropoids (Ericaceae) and in orchids

* Stable isotope abundance analyses recently revealed that some European green orchids and pyroloids (Ericaceae) are partially myco-heterotrophic, exploiting mycorrhizal fungi for organic carbon and nitrogen. Here we investigate related species to assess their nutritional mode across various forest and climate types in Germany and California. * C- and N-isotope signatures of five green pyroloids, three green orchids and several obligate myco-heterotrophic species (including the putatively fully myco-heterotrophic Pyrola aphylla) were analysed to quantify the green plants' nutrient gain from their fungal partners and to investigate the constancy of enrichment in (13)C and (15)N of fully myco-heterotrophic plants from diverse taxa and locations relative to neighbouring autotrophic plants. * All green pyroloid and one orchid species showed significant (15)N enrichment, confirming incorporation of fungi-derived N compounds while heterotrophic C gain was detected only under low irradiance in Orthilia secunda. Pyrola aphylla had an isotope signature equivalent to those of fully myco-heterotrophic plants. * It is demonstrated that primarily N gain from mycorrhizal fungi occurred in all taxonomic groups investigated across a wide range of geographical and ecological contexts. The (13)C and (15)N enrichment of obligate myco-heterotrophic plants relative to accompanying autotrophic plants turned out as a fairly constant parameter.     

Deception above, deception below: linking pollination and mycorrhizal biology of orchids

Several key characteristics of the species-rich orchid familyare due to its symbiotic relationships with pollinators andmycorrhizal fungi. The majority of species are insect pollinatedand show strong adaptations for outcrossing, such as pollinationby food- and sexual-deception, and all orchids are reliant onmycorrhizal fungi for successful seedling establishment. Recentstudies of orchid pollination biology have shed light on thebarriers to reproductive isolation important to diversificationin different groups of deceptive orchids. Molecular identificationof orchid mycorrhizal fungi has revealed high fungal specificityin orchids that obtain organic nutrients from fungi as adults.Both pollinator and fungal specificity have been proposed asdrivers of orchid diversification. Recent findings in orchidpollination and mycorrhizal biology are reviewed and it is shownthat both associations are likely to affect orchid distributionand population structure. Integrating studies of these symbioseswill shed light on the unparalleled diversification of the orchidfamily. Key words: Mutualism, myco-heterotrophy, pollinator limitation, speciation Received 5 October 2007; Revised 12 December 2007 Accepted 21 December 2007     

The ectomycorrhizal specialist orchid Corallorhiza trifida is a partial myco-heterotroph

The leafless, circumboreal orchid Corallorhiza trifida is often assumed to be fully myco-heterotrophic despite contrary evidence concerning its ability to photosynthesize. Here, its level of myco-heterotrophy is assessed by analysing the natural abundance of the stable nitrogen and carbon isotopes (15)N and (13)C, respectively. The mycorrhizal associates and chlorophyll contents of C. trifida were investigated and the C and N isotope signatures of nine C. trifida individuals from Central Europe were compared with those of neighbouring obligate autotrophic and myco-heterotrophic reference plants. The results show that C. trifida only gains c. 52 +/- 5% of its total nitrogen and 77 +/- 10% of the carbon derived from fungi even though it has been shown to specialize on one specific complex of ectomycorrhizal fungi similar to fully myco-heterotrophic orchids. Concurrently, compared with other Corallorhiza species, C. trifida contains a remarkable amount of chlorophyll. Since C. trifida is able to supply significant proportions of its nitrogen and carbon demands through the same processes as autotrophic plants, this species should be referred to as partially myco-heterotrophic.     

Mycorrhizal acquisition of inorganic phosphorus by the green-leaved terrestrial orchid Goodyera repens

BACKGROUND AND AIMS: Mycorrhizal fungi play a vital role in providing a carbon subsidy to support the germination and establishment of orchids from tiny seeds, but their roles in adult orchids have not been adequately characterized. Recent evidence that carbon is supplied by Goodyera repens to its fungal partner in return for nitrogen has established the mutualistic nature of the symbiosis in this orchid. In this paper the role of the fungus in the capture and transfer of inorganic phosphorus (P) to the orchid is unequivocally demonstrated for the first time. METHODS: Mycorrhiza-mediated uptake of phosphorus in G. repens was investigated using spatially separated, two-dimensional agar-based microcosms. RESULTS: External mycelium growing from this green orchid is shown to be effective in assimilating and transporting the radiotracer (33)P orthophosphate into the plant. After 7 d of exposure, over 10 % of the P supplied was transported over a diffusion barrier by the fungus and to the plants, more than half of this to the shoots. CONCLUSIONS: Goodyera repens can obtain significant amounts of P from its mycorrhizal partner. These results provide further support for the view that mycorrhizal associations in some adult green orchids are mutualistic.     

Fungal specificity bottlenecks during orchid germination and development

Fungus-subsidized growth through the seedling stage is the most critical feature of the life history for the thousands of mycorrhizal plant species that propagate by means of 'dust seeds.' We investigated the extent of specificity towards fungi shown by orchids in the genera Cephalanthera and Epipactis at three stages of their life cycle: (i) initiation of germination, (ii) during seedling development, and (iii) in the mature photosynthetic plant. It is known that in the mature phase, plants of these genera can be mycorrhizal with a number of fungi that are simultaneously ectomycorrhizal with the roots of neighbouring forest trees. The extent to which earlier developmental stages use the same or a distinctive suite of fungi was unclear. To address this question, a total of 1500 packets containing orchid seeds were buried for up to 3 years in diverse European forest sites which either supported or lacked populations of helleborine orchids. After harvest, the fungi associated with the three developmental stages, and with tree roots, were identified via cultivation-independent molecular methods. While our results show that most fungal symbionts are ectomycorrhizal, differences were observed between orchids in the representation of fungi at the three life stages. In Cephalanthera damasonium and C. longifolia , the fungi detected in seedlings were only a subset of the wider range seen in germinating seeds and mature plants. In Epipactis atrorubens , the fungi detected were similar at all three life stages, but different fungal lineages produced a difference in seedling germination performance. Our results demonstrate that there can be a narrow checkpoint for mycorrhizal range during seedling growth relative to the more promiscuous germination and mature stages of these plants' life cycle.