plant surface microbiology.pdf

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Marjatta Raudaskoski, Mika Tarkka and Sara Niini
the central vacuole becomes fragmented (Bonfante and Perotto 1995). These
processes could also be controlled by rearrangements of MTs or MFs, or by
both cytoskeletal elements.
3.2 Fungal Hyphae
In the research of cytoskeleton in endomycorrhiza, the structure and function
of fungal MTs and MFs have gained less attention than those of plant cells. In
the IIF microscopical investigation of nonsymbiotic hyphae of Glomus
mossae, the MTs were visualized with tubulin and MFs with actin antibodies
in the multinucleate hyphae originating from germinated spores (Åström et
al. 1994). MTs extended in the cortical and central parts of the hyphae to the
extreme hyphal tip, continued from the main hypha into a branch and the
position of nuclei appeared to follow the MT tracks. Long MFs were also visualized
in the hyphae (Åström et al. 1994). MTs and MFs could be involved in
the intra- and intercellular nuclear movements and cytoplasmic streaming,
respectively, recorded in living nonsymbiotic hyphae of different endomycorrhizal
fungi (Bago et al. 1998; Giovannetti et al. 1999). The presence of both
MTs and MFs in nonsymbiotic hyphae suggests that these structures could
also play a significant role in hyphal morphogenesis associated with the formation
of endomycorrhiza, such as the differentiation of the appressorium at
the root surface at the beginning of the symbiosis and the formation of vesicular
and arbuscular structures in the plant cell after the establishment of the
symbiosis.
4 Organization of Cytoskeleton in Ectomycorrhiza
4.1 Root Cells
The effect of ectomycorrhiza formation on the plant cell cytoskeleton is more
difficult to investigate than that of endomycorrhiza or orchid mycorrhiza. In
the ectomycorrhizal symbiosis, the fungal hyphae grow between the cortical
cells of the host plant, forming a hyphal network for nutrient exchange called
the Hartig net. The plant cells of the Hartig net have thick cell walls and accumulations
of secondary metabolites such as phenols and starch. The thick cell
walls inhibit rapid penetration of fixatives necessary for preservation of
cytoskeletal elements, which is seen as lack of MTs or MFs from the published
ultrastructural studies of ectomycorrhiza. Autofluorescence of secondary
metabolites hampers the recording of cytoskeletal structures when they have
been preserved during fixation. In spite of these difficulties, some knowledge
of cytoskeletal structure has been obtained in Pinus sylvestris–Suillus bovinus
ectomycorrhiza (Timonen et al. 1993; Niini and Raudaskoski 1998).