AMMONIUM ASSIMILATION BY SPRUCE HEBELOMA SP ECTOMYCORRHIZAS

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 N-15 incorporation into free amino acids were measured together with the effect of specific enzyme inhibitors. Exposure of detached ectomycorrhizas to ((NH4)-N-15)2SO4 showed that the greatest flow of N-15 enters into the amido group of glutamine. Label was also detected in glutamic acid, alanine and gamma-aminobutyric acid. Large amounts of alanine and glutamate accumulated in response to the addition of methionine sulfoximine (MSX) together with a decrease in N-15 incorporation into both amido- and amino-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.