Ammonium assimilation by spruce—Hebeloma sp. ectomycorrhizas

summary In order to investigate the relative contribution of glutamine synthetase and NADP‐glutamate dehydrogenase to the assimilation of ammonium (NH4+) by spruce ectomycorrhizas, changes in free amino acid content and kinetics of 15N incorporation into free amino acids were measured together with the effect of specific enzyme inhibitors. Exposure of detached ectomycorrhizas to (15NH4) 2SO4 showed that the greatest flow of 15N enters into the amido group of glutamine. Label was also detected in glutamic acid, alanine and γ‐aminobutyric acid. Large amounts of alanine and glutamate accumulated in response to the addition of methionine sulfoximine (MSX) together with a decrease in 15N incorporation into both amido‐ and ammo‐nitrogen of glutamine. These results are consistent with a major role of glutamate dehydrogenase and glutamine synthetase in nitrogen assimilation in the symbiosis and do not suggest any significant role for glutamate synthase in the synthesis of glutamate. A large accumulation of unlabelled asparagine in response to MSX and albizziine inhibition suggests the occurrence of an unlabelled NH4+ pool in the host plant. The transfer of nitrogen compounds between the fungal cells and the host tissues is discussed.