Nitrogen metabolism in the ectomycorrhizal fungus Hebeloma crustuliniforme

Abstract

SUMMARYWith glucose as the carbon source, the ectomycorrhizal basidiomycete Hebeloma crustuliniforme (Bull. ex. Amans) Quel. utilizes nitrate, ammonium and amino acids as sources of nitrogen. Inoculated as a suspension of fine hyphae in such liquid media, the fungus expands rapidly, exhibiting 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 appears to be carried out primarily via the NADPH‐glutamate dehydrogenase (GDH) pathway. Its NADPH‐GDH levels in exponentially growing mycelia are several times greater than the apparent nitrogen assimilation rates of the mycelia. Its levels of glutamine synthetase activity in comparison are very small during the exponential period of growth. The greatest 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 for the NADH‐GDH. Aspartate aminotransferase and alanine aminotransferase in H. crustuliniforme are highly active during the exponential period of growth, and their concentrations are increased further in the presence of amino acids as sole nitrogen sources, suggesting a key role for these enzymes in interactions between pathways of amino acid metabolism and carbon metabolism in the fungus.