plant surface microbiology.pdf

21 Carbohydrates and Nitrogen: Nutrients and Signals in Ectomycorrhiza 379
response to the C-flux through the enzyme. Whether hexokinase phosphorylation
or its association with other proteins is responsible for signal generation
is still a mater of debate, but it is rather likely that the conformational
changes of the enzyme during its enzymatic activity are sensed and not the
generated hexose phosphates per se. The signaling pathways involve the activation/deactivation
of the SNF1 protein complex that is presumably mediated
by phosphorylation/dephosphorylation (AMPK kinase and REG1/GLC7
phosphatase complex, respectively; Lesage et al. 1996; Johnston 1999). The
sugar-dependent gene repression is mediated by a DNA-binding protein like
MIG1 (yeast) or CREA (Aspergillus, Neurospora; Felenbok and Kelly 1996),
acting as repressors.
While in the pure fungal culture AmMst1 was induced and AmPAL
repressed by elevated hexose concentrations, both genes were strongly
expressed in entire mycorrhizas (Nehls et al. 1998, 1999a). It could thus be
concluded that in mycorrhizas the sugar-dependent regulation of both genes
is either modified by developmental events, or different in the sheath and Hartig
net hyphae. To address this question, ectomycorrhizas were dissected and
gene expression was investigated separately for hyphae of the fungal sheath
and the Hartig net (Nehls et al. 2001a). Similar to low external hexose concentrations
in pure fungal culture, AmMst1 was expressed only at the basal level
in hyphae of the fungal sheath. In contrast, AmPAL revealed a high transcript
level in this fungal structure. For Hartig net hyphae the opposite expression
pattern was observed. As for hyphae in pure culture in the presence of high
external hexose concentrations, the transcript level of AmMst1 was sixfold
enhanced while the expression of AmPAL was only barely detectable.
Owing to the opposite regulation of both genes in hyphae of fungal sheath
and Hartig net that resembles the hexose-dependent expression of these
genes in pure culture, different hexose concentrations in the apoplast of the
fungus/plant interface (hexose concentration >2 mM) and the apoplast of the
fungal sheath (hexose concentration 2 mM) could be assumed in
the Hartig net that would trigger the observed hexose-dependent fungal gene
expression. Fructose withdrawal from the apoplast presumably takes place
mainly within the innermost one or two layers of the fungal sheath since fructose
uptake by A. muscaria hyphae is rather efficient when the glucose concentration
is