plant surface microbiology.pdf

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Marjatta Raudaskoski, Mika Tarkka and Sara Niini
4.2 Fungal Hyphae
An extensive MT cytoskeleton with strictly axial orientation is observed in the
long polarized apical cells of ectomycorrhiza-forming and other filamentous
fungi (Salo et al. 1989; Raudaskoski et al. 1991; Niini and Raudaskoski 1998;
Raudaskoski et al. 2001). In the dikaryotic hyphae of these fungi the longitudinally
running MTs (Fig. 1E) appear to keep the two nuclei with different
mating type genes close to each other by forming a cage of crossing MTs
around the nuclear pair (Runeberg et al. 1986; Salo et al. 1989).Actin is visualized
as a cap in the hyphal tips (Fig. 1C), as small plaques along the apical cell
and as a ring (Fig. 1D) at the site where the cross wall will be formed (Salo et
al. 1989; Raudaskoski et al. 1991; Gorfer et al. 2001).
Comparison of the structure/organization of actin cytoskeleton in the taxonomically
closely related slow-growing ectomycorrhiza-forming S. bovinus
and fast-growing wood-decaying S. commune reveals several differences. In
the slow-growing hyphae of S. bovinus the actin cap is more extensive than in
S. commune, and the cap can be easily visualized with fluorochrome-labeled
phalloidin that binds only to filamentous actin (Barak et al. 1980). In contrast,
the visualization of the actin cap at the hyphal tips of S. commune succeeds
only with an actin antibody, which visualizes actin monomers in addition to
actin filaments. These differences suggest that the structure of MFs is more
stable and their number is higher at the hyphal tip in the slow-growing ectomycorrhizal
than in the fast-growing wood-decay fungus. In the hyphae of S.
bovinus, it is also possible to distinguish occasionally actin filaments (Gorfer
et al. 2001) comparable to those seen at a specific growth phase in budding
yeast cells of S. cerevisiae (Kilmartin and Adams 1984).Actin filaments are not
observed in the hyphae of fast-growing filamentous fungi, such as S. commune
(Runeberg et al. 1986; Raudaskoski et al. 1991) or Neurospora crassa
(Heath et al. 2000). These observations suggest that there probably are some
basic differences in the structure of the actin cytoskeleton between slowgrowing
and fast-growing filamentous fungi. The question whether these differences
are associated with the ability of S. bovinus to form ectomycorrhiza
with P. sylvestris root cells and whether these specific features occur in all
ectomycorrhiza-forming fungi has to be answered the in the future.
The use of drugs against polymerized tubulin and actin has given insights
into their roles in hyphal growth. The depolymerization of the MT cytoskeleton
with an anti-MT drug leads to strong branching of the hyphae in ectomycorrhizal
fungi such as Amanita muscaria, Hebeloma cylindrosporum, Paxillus
involutus, and S. bovinus (Niini and Raudaskoski 1993). In contrast, the
depolymerization of actin filaments from the hyphae of S. bovinus with
cytochalasin D leads to swelling of the hyphal tip cells and loss of the polarized
growth pattern (Niini 1998; Raudaskoski et al. 2001). Nonpolarized
growth and strong branching of hyphae are also observed when an ectomycorrhizal
fungus is associated with the plant root cells (Kottke and Oberwin-