plant surface microbiology.pdf

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Ingrid Kottke
Picea mariana Mill. B.S.P. (Thomson et al. 1989). The superficial layer of the
fungal wall may contain hydrophobins, cysteine-rich proteins, self-assembling
at the wall/air interface (Wösten et al. 1994; Wessels 1997; Wösten and
Vocht 2000). Hydrophobins were localized using antibodies in mycorrhizas
formed by P. tinctorius and E. globulus and in mycorrhizas of Tricholoma terreum
(Schaeff.) Quél. with the compatible host Pinus sylvestris L. (Mankel et
al. 2000; Tagu et al. 2001). The cuticle-like layer may thus be considered as the
hydrophobic surface appropriate for hyphal attachment by hydrophobins.
Attachment to the tips of root hairs was observed (Kottke 1997), but
occurred only within a defined, susceptible zone (Thomson et al. 1989).
Hyphae may also attach to the surface of root cap cells (Bonfante et al. 1998).
This kind of attachment differs from that to the short root surface. Staining
for cysteine-rich proteins was found to be negative (Kottke 1997). Attachment
was neither followed by enlargement of hyphae or lobed ramification, nor by
any digesting process (Thomson et al. 1989; Kottke 1997). Instead, thickening
of fungal wall has been observed and the appearance of b-1,3-glucans in the
root cell wall was shown (Bonfante et al. 1998).
No attachment to the surface of long roots was found. Hyphae grow along
the long roots in acropetal direction without apparent changes (Fig. 7 c).
6 Digestion of the Suberin Layer and the Cell Wall of the
Root Cap
The cuticle-like layer covers all the cell junctions of short roots (Figs. 5a, 6).
The hyphae, therefore, must penetrate the suberin layer and the wall of the
moribund root cap cell when establishing the Hartig net. Vesicles, probably
containing a cutinase-like enzyme were frequently observed in hyphae dissolving
the suberin (Fig. 8a). The hyphae split away the suberized root cap cell
wall and proliferate below, on top of the surface of the cortical cell (Fig. 8b, c,
d). This process may explain why finally, when the hyphal sheath covers the
rootlet, the cuticle-like layer is no longer found. The suberin layer became
integrated into the hyphal sheath (Fig. 8d).
The hyphae digest the suberin layer locally and disrupt the root cap cell
wall, but do not attack the wall of the live cortical cell (Fig. 8a, b). While the
enzyme activity remains to be proven in situ, there are many indications for a
controlled cell wall hydrolyzing activity of ECM fungi (for review, see Cairney
and Burke 1994). ECM fungi digest cell wall material, including the suberin
layer of the moribund root cap, but not material of live cells during mycorrhiza
formation (Chilvers 1968; Piché et al. 1983b; Kottke and Oberwinkler
1986). A strict spatial and temporal regulation of enzyme activity has, thus, to
be expected when the hyphae contact alive cells.