STUDY OF ENZYMES INVOLVED IN NITROGEN-MET ABOLISM OF DOUGLAS LACCARIA-LACCATA ECTOMYCORRHIZAS

The ectomycorrhizal fungus Laccaria laccata was found to contain two d ifferent glutamate dehydrogenases (GDH);one is dependent on NADP as co factor whereas the other utilizes NAD. The NADP-GDH exhibited a fairly good affinity for ammonium (Km : 5 mM) and NAD-GDH exhibited a high a ffinity for glutamate (Km : 0,24 mM) which is consistent with an anabo lic role for the former enzyme and a catabolic role for the latter. Bo th fungal and Douglas-fir root glutamine synthetases (GS) were found t o have a very high affinity for ammonium (Km : 24 and 28 mu M), sugges ting that these enzymes can play a key role in the assimilation of amm onium into amino acids. The NAD-GDH found in the free-living Laccaria laccata was suppressed in Douglas-fir ectomycorrhizas, while the funga l NADP-GDH and GS remained active in symbiotic tissues. Immunogold lab elling 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 expr essed as two isoforms in Laccaria laccata cultivated in pure culture w as not detected in Douglas ectomycorrhizas, which is in agreement with previous data indicating a constant repression of this fungal enzyme in ectomycorrhizal associations.