Compared to saprophytic interactions, a horizontal gene transfer was believed to be more likely in mycorrhizal symbiosis due to special anatomical features of the plant/fungus interface. However, even under optimized conditions (selective advantages of transgenic fungi after a horizontal gene transfer), no indication for a gene transfer from trees to ectomycorrhizal fungi were observed, neither under laboratory nor under field conditions. Thus, in contrast to the phytopathogenic fungus P. brassicae that takes up host plant DNA during each infection cycle (Bryngelsson et al. 1988), uptake of plant host DNA that exceed a particular size (necessary for the transfer of information) must be a rather rare event in ectomycorrhizal symbiosis.
One reason for this might be the presence of plant cell walls that are (in contrast to phytopathogenic interactions) not degraded but modulated and filled up with electron-dense material in ectomycorrhizal symbiosis (Kottke and Oberwinkler 1987). Furthermore, as shown for other plants, genomic DNA is presumably degraded during the senescence of root cells in aged
ECM. Together with the plant cell wall, that avoids DNA transfer due to its limited pore size, DNA fragmentation during senescence could prevent the transfer of genetic information from trees to the ectomycorrhizal fungi.
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