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Abstract Detail

Symbioses: Plant, Animal, and Microbe Interactions

Selosse, Marc-André [1], Gonneau, Cédric [1], Bellino, Alessandro [2], Jersakova, Jana [3].

Plants eating mycorrhizal fungi: the evolution from mixotrophy to mycoheterotrophy.

The evolution of land plants provided repeated emergences of mycoheterotrophy, where secondary achlorophyllous plants exploit carbon from their mycorrhizal fungi. This condition, suggested to be an adaptation to forest environments where little light is available, recently made strong achievements due to two tools: fungal molecular barcoding allowed identification of the (often uncultivable) mycorrhizal fungi; natural isotopic abundances supported which fungal guild was giving carbon to the mycoheterotrophic plants. Temperate and Mediterranean species, mainly orchids and Montropoideae (Ericaceae), have specific basidiomycetous mycorrhizal partners that usually form mycorrhizae with surrounding trees (ectomycorrhizal fungi). Intermediate evolutionarily steps in evolution to mycoheterotrophy were found to exist, where the plant is still green and photosynthetic, but partly uses carbon from its fungal associates. This strategy, called mixotrophy, is now well described for green temperate orchids and Montropoideae species associated to ectomycorrhizal basidiomycetes or sometimes ascomycetes. Phylogenies support that mixotrophy pre-dispose to evolution of mycoheterotrophy. In some mixotrophic Cephalanthera and Epipactis spp. (orchids), the rare survival of achlorophyllous plants (albinos) further supports their use of fungal carbon. Although albinos are candidate to be the first step in a transition to full mycoheterotrophy, our investigations on their nutrition, survival and seed production demonstrate major physiological dysfunctions, resulting in 1000x-reduced fitness of albinos as compared to green conspecifics. A more detailed analysis was carried out to understand C allocations from mycoheterotrophic and photosynthetic carbon in various organs of mixotrophic perennial orchids. We used (1) spontaneous isotopic differences between these two sources (high 13C enrichment in fungal biomass versus low 13C enrichment in C3 photosynthates), and (2) field experiments reducing access to light or to fungi. An original temporal and spatial optimization of resource use was found: while underground growth and survival was supported by fungal carbon, photosynthesis was mainly used for seed production, but not vegetative reserves. This entails a fatal dependence on photosynthesis, so that mycoheterotrophy cannot arise suddenly from simple mutational drift in mixotrophs. While some complementary adaptations may pave the way to a successful mycoheterotrophy, this may be rare in evolution of mixotrophs. As a result, mixotrophy turns out to be evolutionarily metastable, and this may act to limit the C load of mixotrophs on mycorrhizal associations.

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1 - Museum National d'Histoire Naturelle, Institut de Systématique et d'Evolution de la Biodiversite, CP50 - 45 rue Buffon , Paris, 75005, France
2 - Università degli studi di Salerno, Dipartimento di chimica e biologia, Via giovanni Paolo II 132, Fisciano, 84084, Italy
3 - University of South Bohemia, Faculty of Science, Branišovská 31, Ceske Budejovice, 31062, Czech Republic

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Presentation Type: Oral Paper:Papers for Topics
Session: 36
Location: Salon 5/The Shaw Conference Centre
Date: Tuesday, July 28th, 2015
Time: 9:00 AM
Number: 36005
Abstract ID:476
Candidate for Awards:None

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