plant surface microbiology.pdf

13 The Surface of Ectomycorrhizal Roots and the
Interaction with Ectomycorrhizal Fungi
Ingrid Kottke
1 Introduction
Most of the trees in the temperate and alpine regions live in symbiosis with
root fungi forming ectomycorrhizas. Ectomycorrhizas (ECM) display a very
specified cellular organization.A fungal sheath covers the root surface and the
hyphae invade intercellularly between the root cortical cells establishing the
so-called Hartig net. The hyphal sheath is formed in a species-specific manner
(Agerer 1998), but the architecture of the Hartig net is similar in all the
ectomycorrhizas, independent of plant and fungal species (Blasius et al. 1986,
Kottke and Oberwinkler 1987, 1989). Establishing the Hartig net, hyphal
growth undergoes important changes. The hyphae invade as multi-branched,
fan-like lobes in intimate juxtaposition, starting at the root surface and finally
covering the root cortical cells in a dense mono-layer (Fig. 1; Jacobs et al. 1989;
Brunner and Scheidegger 1992; Kottke et al. 1996).
The Hartig net structure is only established in so-called short roots, a special
root type of the ectomycorrhiza-forming plants (Marks and Foster 1973;
Wong et al. 1990). It was hypothesized that the surface of these rootlets might
trigger the attachment of hyphae and the change of their growth characters
(Jacobs et al. 1989; Brunner and Scheidegger 1992; Kottke 1997; Bonfante et
al. 1998). Cysteine-rich, moderately hydrophobic proteins (“hydrophobins”)
in the walls of the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker
& Couch were shown to be highly expressed in the early stage of mycorrhiza
formation and were considered to attach the hyphae to the root surface of
Eucalyptus globulus ssp. bicostata Kirkp. (Tagu et al. 1996, 2000, 2001; Martin
et al. 1999). A hydrophobic root surface was, therefore postulated and a cuticle-like
layer on the surface of short roots may be the substrate for adhesion
of ectomycorrhizal fungi (Kottke 1997). Recent ultrastructural studies comparing
long and short roots have supported this hypothesis by revealing the
origin of the cuticle-like layer on short, but not on long roots. Differences
were also detected in the amounts of methyl-esterified pectins in the cortical
cell walls of both the root types. Furthermore, when establishing the Har-
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004