The ectomycorrhizal Laccaria bicolor, cultivated axenically
in
modified Pachlewski's medium, can use a broad spectrum of nitrogen
and carbon sources. The fungus exhibited greater growth in the presence
of
ammonium or nitrate than with ammonium nitrate. With
the latter salt, the drop in pH of the culture medium indicated that
ammonium was taken up preferentially to nitrate. L. bicolor grew
more poorly on amino acids such as glycine, alanine, aspartate and glutamate,
the
latter three being very poor carbon sources. By
contrast, glycine was used as nitrogen and carbon sources. Glucose and
maltose
in mixture, with a 20[ratio ]5 ratio, were the most
effective carbohydrates for promoting growth, followed by starch, dextrins,
maltose, glucose and sorbitol. L. bicolor failed to grow in
the presence of sucrose and galactose.
Isocitrate dehydrogenase (IDH), a key enzyme linking carbon and nitrogen
metabolism, is present in L. bicolor as NAD- and
NADP-dependent proteins. Both enzymes were stimulated in response to
nitrogen starvation and appeared to operate in close
association with the glutamine synthetase and NADP-dependent glutamate
dehydrogenase of the fungus, indicating that 2-oxoglutarate produced by
IDHs
is probably utilized in the assimilation of inorganic nitrogen. The highest
specific activities of NAD
and NADP-IDH coincided in most cases with the rapid growth periods of the
fungus, although in slow growing conditions obtained
by addition of glycine or sorbitol to the culture media, only NADP-IDH
activity increased in conjunction with growth. In addition,
activity of the NADP-IDH was constantly higher than that of NAD-IDH.
These results are indicative that the NADP-dependent
enzyme plays a substantial biosynthetic role, possibly by an
additional production of reduced nucleotides.