NITROGEN-METABOLISM IN THE ECTOMYCORRHIZAL FUNGUS HEBELOMA CRUSTULINIFORME

With glucose as the carbon source, the ectomycorrhizal basidiomycete H ebeloma crustuliniforme (Bull. ex. Amans) Quel. utilizes nitrate, ammo nium and amino acids as sources of nitrogen. Inoculated as a suspensio n of fine hyphae in such liquid media, the fungus expands rapidly, exh ibiting a distinct exponential phase with a doubling time of 1-2 d and a maximum content of mycelial protein during the first 7 d of growth. Its growth when amino acids serve as sole carbon and nitrogen source is, however, less rapid, and requires several days of induction under these conditions. In H. crustuliniforme, assimilation of ammonium appe ars to be carried out primarily via the NADPH-glutamate dehydrogenase (GDH) pathway. Its NADPH-GDH levels in exponentially growing mycelia a re several times greater than the apparent nitrogen assimilation rates of the mycelia. Its levels of glutamine synthetase activity in compar ison are very small during the exponential period of growth. The great est levels of NADH-GDH are found in glucose-grown cultures during the post-exponential period, and in glucose-free amino acid-grown cultures during the active period of growth. This suggests a catabolic role fo r the NADH-GDH. Aspartate aminotransferase and alanine aminotransferas e in H. crustuliniforme are highly active during the exponential perio d of growth, and their concentrations are increased further in the pre sence of amino acids as sole nitrogen sources, suggesting a key role f or these enzymes in interactions between pathways of amino acid metabo lism and carbon metabolism in this fungus.