Selosse, Marc-Andre , Roy, Melanie , Martos, Florent .
How some forest plants survive in shade by eating their mycorrhizal fungi.
In the typical mycorrhizal symbiosis, which links soil fungi with the roots of ~90% of plant species, fungi exploit soil mineral nutrients in return for plant produced carbon. In contrast, mycoheterotrophy, in which plants instead receive carbon from their associated fungi, has been found in several forest-understorey achlorophyllous plants from various families, including orchids. In temperate ecosystems, orchids have highly specific association to narrow fungal taxa that, in turn, form mycorrhizae with surrounding green plants. Isotopic methods support that the surrounding plants are the ultimate carbon source for the orchid-fungi symbiosis. When we investigated tropical mycoheterotrophic orchids, we found a more flexible picture: some orchids were not specific, such as Aphyllorchis spp. from Thailand; some associated with normally saprobic fungi (Mycena, Gymnopus, Resinicium), such as Wullschlaegelia and Gastrodia spp. from Carribean and Mascarene Islands, with evidence that decaying plant material is their ultimate carbon source. Based on this, we revisited the current understanding of evolution of mycoheterotrophic clades and their adaptation to tropical versus temperate conditions. Moreover, green forest-understorey plants, phylogenetically close to mycoheterotrophs, were recently shown to be mixotrophic, i.e. they receive carbon from both their fungi and photosynthesis. Phylogenetic analyses suggest that in orchids and pyroloids (Ericaceae), mixotrophy preceded the evolution of mycoheterotrophy. In some mixotrophic Cephalanthera and Epipactis spp., achlorophyllous plants (AP) can even be found rarely in natural populations. To understand AP rarity, we investigated in situ AP morphology, physiology (gas exchanges) and fitness over several years, in several European populations. We showed that impaired carbon budget and bad regulation of water exchanges due to vestigial leaves and stomata, made AP unfit. Thus, we propose, a contrario, that co-evolution of several traits is required for successful transition to mycoheterotrophy, explaining why this transition remains rare in mixotrophic lineages.
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1 - Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, UMR 5175, Equipe Interactions Biotiques , 1919 Route de Mende, Montpellier, 34000, France
2 - Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, UMR 5175, Equipe Interactions Biotiques , 1919 Route de Mende, Montpellier, 34000, France
3 - UMR C53 Peuplements végétaux et Bioagresseurs en milieu tropical, Université de La Réunion, 15 Avenue René Cassin, BP 7151, Saint-Denis, 97715, France
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Wasatch B/Cliff Lodge - Level C
Date: Tuesday, July 28th, 2009
Time: 9:30 AM