Blended colonies of the ectomycorrhizal fungus, Laccaria bicolor (R. Mre.) Orton, grew axenically as a suspension of fine hyphae in a defined buffered medium with glucose (doubling time 1-2 d) but not acetate as the carbon source, and either ammonium or nitrate as nitrogen sources. A number of amino acids were found to be excellent nitrogen sources for this basidiomycete, but were less effective as sources of carbon. During post-exponential growth in medium containing inorganic nitrogen, the fungal symbiont released amino acids. L. bicolor has the enzymatic potential to assimilate ammonium by the activities of glutamine synthetase, NADH-glutamate dehydrogenase and NADPH-glutamate dehydrogenase. It also contains highly active aspartate and alanine aminotransferases. The activities of glutamine synthetase, NADPH-glutamate dehydrogenase and aspartate aminotransferase were greater in the presence of nitrate than in the presence of ammonium and declined as the culture aged, suggesting a biosynthetic role for these enzymes. In contrast, the activities of NADH-glutamate dehydrogenase and alanine aminotransferase increased during post-exponential growth, and also in cultures growing on amino acids as a carbon source, suggesting a catabolic role for these enzymes.
Keywords: Ectomycorrhizal fungi; Laccaria bicolor.; enzyme regulation; nitrogen nutrition.