Patterns of ectomycorrhizal host-fungus specificity in the Pacific Northwest Public Deposited

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  • Results from approximately 400 fungus-host pure culture inoculations indicate that specificity of ectomycorrhizal associations is a complex phenomenon and cannot be based solely on field observations of sporocarp-host associations. Of the numerous sporocarp-host specific fungi tested, most formed ectomycorrhizae with one or more unexpected, non-associated hosts. These results conclusively demonstrate that ectomycorrhizal fungi which produce sporocarps only with a specific host species or genus can still form mycorrhizae with other "non-associated" hosts. The ability to form ectomycorrhizae with various hosts is termed "ectomycorrhizal host potential". Some fungi, however, showed superior mycorrhizal development on their particular hosts over other non-associated hosts, indicating further specialization in those associations. A large group of fungi known for diverse sporocarp-host associations showed wide ectomycorrhizal host potential by forming abundant, well developed ectomycorrhizae with all or most hosts. It's suggested that these fungi may share similar compatibility or recognition factors common to many ectomycorrhizal hosts thus allowing for diverse host associations. A spectrum from mycorrhizal generalists to specialists was seen among the hosts in their ability to form mycorrhizae with diverse fungi. The ericaceous hosts Arctostaphylos uva-ursi and Arbutus menziesii were broadly receptive towards the fungi, forming mycorrhizae with 25 of the 28 tested. This included most of the fungi which produce sporocarps only in association with specific conifers. At the opposite extreme, five Alnus spp. showed marked specialization and restrictiveness towards their fungal partners. Only three fungi consistently formed mycorrhizae with the Alnus spp.; numerous fungi which formed mycorrhizae with all other hosts were unable to do so with Alnus. The seven conifer hosts showed intermediate mycorrhiza forming abilities among the test fungi and less defined patterns when compared to each other. Few infrageneric differences were seen among the three Pinus spp. Douglas-fir and western larch showed similar response towards potential fungus associates; most fungal symbionts that fruit only with the one formed ectomycorrhizae with the other. Twenty-three Rhizopogon species differed strongly in ectomycorrhizal host potential among Douglas-fir, western hemlock, and lodgepole pine. Comparisons within the sections of Rhizopogon, however, revealed great similarities in both ectomycorrhizal appearance and host potential. Relationships of species both within and between sections were apparent. Detailed culture studies likewise supported the infrageneric classifications and supported transfer of some species to other sections and elevation of all sections to subgeneric rank. Rhizopogon specialization via sporocarp-host specificity, limited ectomycorrhizal host potential, and incompatibility with some hosts are suggested as major contributors to the speciation and diversification of the genus in the Pacific Northwest. Numerous host-fungus combinations showed indications of incompatibility. This occured most often when sporocarp-host specific fungi were inoculated onto non-associated hosts. Disruption of the cortex by the invading fungus, collapse of cortical cells, and lignificationas indicated by intense safranin staining of the cortical cells were the most common indicators of incompatibility. These host reactions suggest a type of phenolic defense mechanism or hypersensitive response as displayed in many plant-pathogen interactions. The need for future research on the mechanisms and factors that determine ectomycorrhizal host-fungus compatibility and specificity is emphasized.
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