plant surface microbiology.pdf

21 Carbohydrates and Nitrogen: Nutrients and
Signals in Ectomycorrhizas
Uwe Nehls
1 Introduction
Due to plant litter, forest soil is rich in complex carbohydrates (e.g., cellulose
and lignin). Nevertheless, these carbohydrates are only slowly degraded by
specialized microorganisms and thus forest soils are rather poor in readily
cleavable carbohydrates that are necessary for the growth of the majority of
microbes including ectomycorrhizal fungi.
Basidiomycetes are able to transfer nutrients and metabolites over long distances.
Exploring a rich source of readily utilizable carbohydrates would thus
favor the colonization of other soil areas, too. The association with fine roots
of woody plants forming ectomycorrhizas is a way that secures exclusive
access to such a rich carbohydrate source for ectomycorrhizal fungi.
Organic compounds contained in root exudates are candidates for the carbon
transfer from the host to the mycorrhizal fungus. Low-molecular-weight
root exudates comprise soluble sugars, carboxylic acids and amino acids
(Marschner 1995; Smith and Read 1997; Hampp and Schaeffer 1999). The best
growth of ectomycorrhizal fungi (ECM) fungi occurs on the hexoses glucose,
fructose, and mannose. Sucrose, which is the preferred transport sugar in
most host plants cannot be used by ECM investigated so far (e.g., Salzer and
Hager 1991), Laccaria bicolor being possibly an exception (Tagu et al. 2000).
Even if plant-derived hexoses are most important for ectomycorrhizal
fungi, there is ample evidence that soil carbon sources are also intensively
used. Among these are starch, dextrins, glucans, oligosaccharides or sugar
alcohols (Palmer and Hacskaylo 1970; Cao and Crawford 1993; Berredjem et
al. 1998), proteins (Abuzinadah and Read 1986), or even cellulose or lignin
(Norkrans 1950; Trojanowski et al. 1984; Taber and Taber 1987; Haselwandter
et al. 1990).
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004