Changing partners in the dark: isotopic and molecular evidence of ectomycorrhizal liaisons between forest orchids and trees.
In the mycorrhizal symbiosis, plants exchange photosynthates for mineral nutrients acquired by fungi from the soil. This mutualistic arrangement has been subverted by hundreds of mycorrhizal plant species that lack the ability to photosynthesize. The most numerous examples of this behaviour are found in the largest plant family, the Orchidaceae. Although these non-photosynthetic orchid species are known to be highly specialized exploiters of the ectomycorrhizal symbiosis, photosynthetic orchids are thought to use free-living saprophytic, or pathogenic, fungal lineages. However, we present evidence that putatively photosynthetic orchids from five species which grow in the understorey of forests: (i) form mycorrhizas with ectomycorrhizal fungi of forest trees; and (ii) have stable isotope signatures indicating distinctive pathways for nitrogen and carbon acquisition approaching those of non-photosynthetic orchids that associate with ectomycorrhizal fungi of forest trees. These findings represent a major shift in our understanding of both orchid ecology and evolution because they explain how orchids can thrive in low-irradiance niches and they show that a shift to exploiting ectomycorrhizal fungi precedes viable losses of photosynthetic ability in orchid lineages.
Bidartondo
Martin I
MI
Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA. m.bidartondo@kew.org
Burghardt
Bastian
B
Gebauer
Gerhard
G
Bruns
Thomas D
TD
Read
David J
DJ
D000704:Analysis of Variance; D001483:Base Sequence; D002247:Carbon Isotopes; D005658:Fungi / Q000235:genetics* / Q000502:physiology; D005858:Germany; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D038821:Mycorrhizae / Q000502:physiology*; D009587:Nitrogen Isotopes; D009841:Oligonucleotides; D029595:Orchidaceae / Q000502:physiology*; D010788:Photosynthesis / Q000502:physiology*; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D013559:Symbiosis; D014197:Trees
D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D009841:Oligonucleotides