plant surface microbiology.pdf

21 Carbohydrates and Nitrogen: Nutrients and Signals in Ectomycorrhiza 385
dependent repression of amino acid transporter gene expression (indicated
by AmAAP1), together with posttranslational events (e.g., increased degradation
of plasma membrane transport proteins) that are described for yeasts
(Springael and Andre 1998), could thus result in a highly reduced fungal
capacity for re-uptake of amino acids at the plant/fungus interface. In combination
with efflux mechanisms (e.g., nitrogen leakage), this would thus result
in a net export of nitrogen.
13 Carbohydrate and Nitrogen-Dependent Regulation of
Fungal Gene Expression
Carbohydrates as well as nitrogen are essential components of biological molecules
(e.g., amino acids or nucleotides), and obviously have a great impact on
fungal gene expression (e.g., Gonzales et al. 1997).
With regard to carbon and nitrogen nutrition, four different patterns of
regulation have been observed in A. muscaria. The amino acid importer gene
AmAAP1 is only regulated by nitrogen nutrition, while the hexose transporters
AmMst1 and AmMst2 (Nehls et al. 1998) are only regulated by carbohydrate
nutrition. On the other hand, AmProt1 (protease; Nehls et al. 2001b)
and AmTPS1 (trehalose-6-phosphate synthase) are regulated by both nitrogen
as well as carbon nutrition. Nevertheless, the impact of carbon and nitrogen
nutrition differs significantly for both genes. While AmProt1 is mainly
regulated by nitrogen, AmTPS1 is mainly regulated by carbon availability.
Comparable gene expression patterns have been described for fungi (Gonzales
et al. 1997) as well as plants (Coruzzi and Zhou 2001), revealing a universal
and phylogenetically old regulation strategy.
14 Conclusions
Since large EST projects of ectomycorrhizal model systems are currently
under progress (Tagu and Martin 1995; Johansson et al. 2000; Voiblet et al.
2001; Wipf et al. 2003), macro- and micro-array hybridization will enable an
overview of the general impact of carbon and nitrogen nutrition on gene
expression for different ectomycorrhizal fungi.
Present data suggest that carbon- and nitrogen-dependent gene repression
in ectomycorrhizal fungi is presumably similar to that of saprophytic
ascomycetes (yeast, Neurospora). Ascomycotic model organisms could thus
help to develop working models for ectomycorrhizal function (e.g., nitrogen
uptake from soil and release at the plant/fungus interface; see Fig. 3) that
could be investigated in turn in an ectomycorrhizal model system. In addition,
differences, e.g., in carbon-dependent gene regulation for an ectomycorrhizal
fungus (A. muscaria) and saprophytic ascomycetes (yeast, Neurospora)