ASSIMILATION OF INORGANIC NITROGEN IN ECTOMYCORRHIZAS

Pathways of inorganic nitrogen assimilation are very often different in higher plants and fungi and this raises the intriguing problem of their coexistence and interactions in ectomycorrhizas. Results based on amino acid accumulation, enzyme assays, electrophoretic patterns on gels and immunogold labelling have consistently shown that beach ectomycorrhizas 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 spruce and Douglas-fir ectomycorrhizas, fungal NADP-GDH was found to be involved in ammonium assimilation, jointly with the GS enzyme. Enzyme repression seems to be due to the host plant but is moderated by the structure of the ectomycorrhizas and especially by the degree of association of the fungus with root tissues, as found in Eucalypt ectomycorrhizas. In all the ectomycorrhizas investigated, fungal aspartate aminotransferase (AAT) was found to be repressed. This suggests that, in addition to the net flux of nitrogen translocated from the fungus to the plant, some amino acids may be translocated from the plant to the fungus. On the basis of the way the enzymes are distributed between fungi and roots, ammonium assimilation in ectomycorrhizas can be divided into three pathways which have implications for the exchange of nitrogen and carbon at the symbiotic interface.