STUDY OF ENZYMES INVOLVED IN NITROGEN-METABOLISM OF DOUGLAS LACCARIA-LACCATA ECTOMYCORRHIZAS

The ectomycorrhizal fungus Laccaria laccata was found to contain two different glutamate dehydrogenases (GDH);one is dependent on NADP as cofactor whereas the other utilizes NAD. The NADP-GDH exhibited a fairly good affinity for ammonium (Km : 5 mM) and NAD-GDH exhibited a high affinity for glutamate (Km : 0,24 mM) which is consistent with an anabolic role for the former enzyme and a catabolic role for the latter. Both fungal and Douglas-fir root glutamine synthetases (GS) were found to have a very high affinity for ammonium (Km : 24 and 28 mu M), suggesting that these enzymes can play a key role in the assimilation of ammonium into amino acids. The NAD-GDH found in the free-living Laccaria laccata was suppressed in Douglas-fir ectomycorrhizas, while the fungal NADP-GDH and GS remained active in symbiotic tissues. Immunogold labelling also indicated the presence of both enzymes which were found to be located in the cytosol of the fungal cells and evenly distributed in the ectomycorrhizas. Aspartate aminotransferase (AAT), clearly expressed as two isoforms in Laccaria laccata cultivated in pure culture was not detected in Douglas ectomycorrhizas, which is in agreement with previous data indicating a constant repression of this fungal enzyme in ectomycorrhizal associations.