plant surface microbiology.pdf

18 Mycorrhizal Development and Cytoskeleton 303
is possible that the antibody used for IIF microscopical detection of MTs in
Pinus is not able to recognize the MTs of cortical cells although it visualizes
well MTs in meristem and vascular tissue. The MT cytoskeleton of cortical
cells of Pinus could also be more sensitive to the processing of the samples
for IIF microscopy than the MTs in meristem and vascular tissue. Interestingly,
it has not been possible to visualize MFs in the cortical cells of short
roots although plenty of MFs are seen in vascular tissue, where they were
already visualized in the early days of the study of MFs in plant cells (Pesacreta
et al. 1982).
The structure of cell wall and cytoskeleton in differentiating cortical cells
in short roots of pine is of special interest, since this is the region of the short
root in which the ectomycorrhizal fungus invades and establishes the Hartig
net. When root morphogenesis and ectomycorrhiza formation in Scots pine
was studied (Niini et al. 1996; Tarkka et al. 1998), a group of polypeptides with
molecular weight slightly above 43 kDa were observed to be short root-specific.
By using peptide sequencing, it was shown that the polypeptides represented
a group of peroxidases. By reverse genetics a full-length cDNA of one
of the peroxidases, Psyp1, was cloned and sequenced (Tarkka et al. 2001). The
signal sequence suggests that Psyp1 is secreted and could be involved in cell
wall formation. In ectomycorrhiza Psyp1 expression is downregulated, which
agrees with the idea that the growth of the fungal hyphae in the intercellular
space might inhibit the cortical cell wall differentiation (Niini 1998). There
may be signalling or linkages between adjacent plant cells that can regulate
the organization of their cytoskeletal structures. This exchange of information
might be mediated through plasmodesmata, or alternatively, through the
intervening cell wall (Canut et al. 1998; Overall et al. 2001).
Fig. 1. Microtubule cytoskeleton visualized with indirect immunofluorescence technique
with a-tubulin antibody and viewed with laser scanning confocal microscopy in
Pinus sylvestris short root (A) and ectomycorrhiza with Suillus bovinus (B). A In cortex
only few microtubules are distinguished in the cortical cells with numerous round amyloplasts.
In stele microtubules with mainly transverse orientation are abundant in elongating
cells differentiating to vascular tissue. Strong vertical bands represent wall thickenings
in a xylem cell. B Microtubules are hardly seen in pine cortical cells, but they are
clearly distinguished as long tracks in hyphae forming the fungal sheath and penetrating
into the root cortex. A,B Bars 20 mm. C–E Cytoskeletal elements in Suillus bovinus
hyphae visualized with rhodamine-phalloidin staining of actin (C, D) and indirect
immunofluorescence microscopy with a-tubulin antibody (E). C A strong actin signal at
hyphal tip, D an actin ring at the site of the future septum. E Microtubule tracks in a
hyphal branch. C–E Bars 10 mm