ASSIMILATION OF INORGANIC NITROGEN IN ECT OMYCORRHIZAS

Pathways of inorganic nitrogen assimilation are very often different i n higher plants and fungi and this raises the intriguing problem of th eir coexistence and interactions in ectomycorrhizas. Results based on amino acid accumulation, enzyme assays, electrophoretic patterns on ge ls and immunogold labelling have consistently shown that beach ectomyc orrhizas are similar to higher plants in that glutamine synthetase (GS ) and glutamate synthase (GOGAT) are operative, while fungal glutamate dehydrogenese (NADP-GDH) is strongly repressed. By contrast, in spruc e and Douglas-fir ectomycorrhizas, fungal NADP-GDH was found to be inv olved in ammonium assimilation, jointly with the GS enzyme. Enzyme rep ression seems to be due to the host plant but is moderated by the stru cture of the ectomycorrhizas and especially by the degree of associati on of the fungus with root tissues, as found in Eucalypt ectomycorrhiz as. In all the ectomycorrhizas investigated, fungal aspartate aminotra nsferase (AAT) was found to be repressed. This suggests that, in addit ion to the net flux of nitrogen translocated from the fungus to the pl ant, some amino acids may be translocated from the plant to the fungus . On the basis of the way the enzymes are distributed between fungi an d roots, ammonium assimilation in ectomycorrhizas can be divided into three pathways which have implications for the exchange of nitrogen an d carbon at the symbiotic interface.