NH assimilation in the ectomycorrhizal basidiomycete Laccaria bicolor (Maire) Orton, a N-NMR study.

Nuclear magnetic resonance spectroscopy was used to monitor NH assimilation and amino acid biosynthesis in the ectomycorrhizal basidiomycete Laccaria bicolor (Maire) Orton. (strain S238). In mycelium growing rapidly on NH , [amido- N]glutamine was the major N-labelled species. When N-labelled mycelium was transferred into medium containing NH , the resonance for [amino- N]glutamine decreased with a half-life of about 3.0 h, whereas the resonance for [amino- N]glutamine remained unchanged. Such behaviour is consistent with glutamine synthetase (GS) being the major route of NH assimilation. However, the higher accumulation of [ N]alanine observed in the presence of the GS inhibitor, methionine sulfoximine, indicated that a part of the glutamate pool was formed by the glutamate dehydrogenase (GDH) pathway. When the mycelium was in stationary phase (i.e. low extracellular NH ), the intramolecular N labelling of glutamine suggested that the GDH and GS pathways were simultaneously assimilating NH . The N supply and the growth stage, therefore, influence the expression of the activities of GDH and GS. The current isotopic data identify other fates of absorbed N: glutamate decarboxylation gives rise to γ-aminobutyrate; transamination between glutamate and pyruvate yields alanine; and arginine accumulates. It is concluded that GS is the main pathway of primary assimilation of NH in L. bicolor, but GDH may also contribute significantly to this process.