The amounts of cyclic AMP (cAMP), fructose-2,6-bisphosphate (F26BP),
trehalose and glycogen were
determined in cell suspension cultures of the ectomycorrhiza-forming fungus
Amanita muscaria (L. ex Fr.)
Hooker. For the assay of cAMP a protocol was developed that enabled the
detection of as little as 50 fmol
of this secondary messenger by an enzyme-linked immuno assay (EIA). Values
varied from <1 and up to 5 pmol cAMP mg−1
d. wt according to the age of the fungal culture. Typically, a transient
increase
in cAMP
occurred after c. 4 d of culture of the fungus on glucose-containing
medium. This increase (up to 100%) was
followed by the start of the logarithmic growth phase, and by a more
persistent increase in F26BP. In parallel,
glucose in the medium started to decrease, whilst the amounts of
fungal carbohydrates, especially the disaccharide
trehalose, increased. From these data we assume that a high initial rate
of
glucose uptake caused an increase in the
fungal pools of storage carbohydrates and, via activation of an adenylate
cyclase,
of cAMP. According to data
reported for yeast cells this should enhance the formation of F26BP by
phosphorylation of relevant enzymes. In
animal and yeast cells an increase in the concentration of F26BP stimulates
glycolysis by activation of the ATP-dependent phosphofructokinase (PFK).
A. muscaria also possesses an F26BP activated PFK and, under conditions
of symbiosis, host-derived carbohydrates are supplied mainly in the form
of glucose. The implications of these
findings to the regulation of carbohydrate metabolism of symbiotic plant
root/fungus structures (ectomycorrhiza) are discussed.