title: Growing evidence for facultative biotrophy in saprotrophic fungi: data from microcosm tests with 201 species of wood-decay basidiomycetes.
abstract: Ectomycorrhizal (ECM) symbioses have evolved a minimum of 78 times independently from saprotrophic lineages, indicating the potential for functional overlap between ECM and saprotrophic fungi. ECM fungi have the capacity to decompose organic matter, and although there is increasing evidence that some saprotrophic fungi exhibit the capacity to enter into facultative biotrophic relationships with plant roots without causing disease symptoms, this subject is still not well studied. In order to determine the extent of biotrophic capacity in saprotrophic wood-decay fungi and which systems may be useful models, we investigated the colonization of conifer seedling roots in vitro using an array of 201 basidiomycete wood-decay fungi. Microtome sectioning, differential staining and fluorescence microscopy were used to visualize patterns of root colonization in microcosm systems containing Picea abies or Pinus sylvestris seedlings and each saprotrophic fungus. Thirty-four (16.9%) of the tested fungal species colonized the roots of at least one tree species. Two fungal species showed formation of a mantle and one showed Hartig net-like structures. These features suggest the possibility of an active functional symbiosis between fungus and plant. The data indicate that the capacity for facultative biotrophic relationships in free-living saprotrophic basidiomycetes may be greater than previously supposed.
authors.affiliation: Department of Biology, Stanford University, 371 Serra Mall, Stanford, CA, 94305, USA.
authors.forename: Gabriel R
authors.identifier: http://orcid.org/0000-0003-3676-0821
authors.initials: GR
authors.lastname: Smith
authors.affiliation: Department of Forest Mycology and Plant Pathology, Uppsala BioCenter, Swedish University of Agricultural Sciences, PO Box 7026, Uppsala, SE-75007, Sweden.
authors.forename: Roger D
authors.initials: RD
authors.lastname: Finlay
authors.affiliation: Department of Forest Mycology and Plant Pathology, Uppsala BioCenter, Swedish University of Agricultural Sciences, PO Box 7026, Uppsala, SE-75007, Sweden.
authors.forename: Jan
authors.initials: J
authors.lastname: Stenlid
authors.affiliation: Department of Forest Mycology and Plant Pathology, Uppsala BioCenter, Swedish University of Agricultural Sciences, PO Box 7026, Uppsala, SE-75007, Sweden.
authors.forename: Rimvydas
authors.initials: R
authors.lastname: Vasaitis
authors.affiliation: Department of Forest Mycology and Plant Pathology, Uppsala BioCenter, Swedish University of Agricultural Sciences, PO Box 7026, Uppsala, SE-75007, Sweden.
authors.forename: Audrius
authors.initials: A
authors.lastname: Menkis
mesh_terms: D001487:Basidiomycota / Q000235:genetics / Q000502:physiology*; D025301:Hyphae / Q000254:growth & development; D038821:Mycorrhizae / Q000502:physiology; D028222:Picea / Q000382:microbiology*; D041605:Pinus sylvestris / Q000382:microbiology*; D018517:Plant Roots / Q000382:microbiology*; D036226:Seedlings / Q000382:microbiology; D013559:Symbiosis; D014934:Wood / Q000382:microbiology*
keywords: 
  Picea abies
  ; 
  Pinus sylvestris
  ; biotrophy-saprotrophy continuum; ectomycorrhizal (ECM) evolution; facultative biotrophy; plant-fungus interaction; saprotrophic fungi; symbiosis