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| Mycorrhiza-bacteria interactions and functioning in boreal forest soils |
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Dr. Robin Sen
Division of General Microbiology, Department of Biosciences, P.O. Box 56, FIN-00014 University of Helsinki, Finland
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Boreal forest trees are highly dependent on root symbiotic ectomycorrhizal (ECM) fungi for growth in low pH, lignin-rich podzolic soils. In Scots pine seedling microcosm studies, extensive mycorrhizal fungal colonization of the constituent upper humus (O) and underlying mineral (E and B) horizons was visualised and horizon-specific ECM identified following ITS-RFLP/sequence phylogenetics. Soil N and P mobilisation and uptake was confirmed in analyses of mycorrhizosphere compartment-specific gene and/or enzyme expression and linked to organic acid production. Bacterial biofilms in O horizon mycorrhizospheres were shown to be mainly comprised of forest soil specific gram -ve bacteria, Bacillus spp. and Crenarchaea based on carbon profiling and 16S sequence phylogenetics. Preferential organic acid utilisation by bacteria in the external mycorrhizosphere highlights mycorrhizal fungal involvement in weathering of E and B horizon minerals. Mycorrhizospheres developed in the O horizon did not host Pseudomonas fluorescens which was further confirmed in the rapid loss of marker gene tagged P. fluorescens introduced into mycorrhizal silver birch root systems. However, Scots pine mycorrhizospheres developed on petroleum contaminated soils do support catabolic plasmid harbouring fluorescent pseudomonads and increased hydrocarbon oxidation activity. Based on these data it is hypothesised that, in functional terms, the mycorrhizosphere behaves as an external rumen/gut of the host tree.
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REFERENCES
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TIMONEN, S., TAMMI, H., & SEN, R. (1997).
Characterisation of the host genotype and fungal diversity in Scots pine ectomycorrhiza from natural humus microcosms using isozyme and PCR-RFLP analyses.
New Phytologist 135: 313-323.
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NURMIAHO-LASSILA, E. L. TIMONEN, S., HAAHTELA, K. & SEN, R. (1997).
Bacterial colonization patterns of intact Pinus sylvestris mycorrhizospheres in dry pine forest soil: an electron microscopy study.
Canadian Journal of Microbiology 43: 1017-1035.
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TIMONEN, S & SEN, R. (1998).
Heterogeneity of fungal and plant enzyme expression in intact Scots pine-Suillus bovinus and -Paxillus involutus mycorrhizospheres developed in natural forest humus.
New Phytologist 138: 355-366.
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TIMONEN, S., JØRGENSEN, K., HAAHTELA. K. & SEN, R. (1998).
Bacterial community structure at defined locations of the Pinus sylvestris-Suillus bovinus and -Paxillus involutus mycorrhizospheres in dry forest humus and nursery peat.
Canadian Journal of Microbiology 44: 499-513.
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SARAND, I., TIMONEN, S., NURMIAHO-LASSILA, E-L., KOIVULA, T., HAAHTELA, K. ROMANTSCHUK, M & SEN, R. (1998).
Microbial biofilms and degradative catabolic plasmid harbouring fluorescent pseudomonads in Scots pine mycorrhizospheres developed on petroleum contaminated soil.
FEMS Microbiology Ecology 27: 115-126.
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SARAND, I., TIMONEN, S., KOIVULA, T., PELTOLA, R., HAAHTELA, K, SEN, R. & ROMANTSCHUK, M. (1999).
Tolerance and biodegradation of m-toluate by Scots pine, a mycorrhizal fungus and fluorescent pseudomonads individually and under associative conditions.
Journal of Applied Microbiology 86: 817-826.
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