EXPRESSION OF GLUTAMATE-DEHYDROGENASE AND ASPARTATE-AMINOTRANSFERASE IN EUCALYPT ECTOMYCORRHIZAS

Previous studies of nitrogen-assimilating enzymes in ectomycorrhizal associations of the Pinaceae (Norway spruce, Douglas fir) and the Fagaceae (beech) have suggested that the host plant may regulate the expression of some fungal enzymes. To improve our understanding of the regulation of nitrogen-assimilating enzymes in ectomycorrhizal plants, we have extended this work to a third plant family, the Myrtaceae. Glutamate dehydrogenases, specific either for NAD (NAD-GDH) or NADP (NADP-GDH), and aspartate aminotransferase (AAT) were investigated by enzyme assays and electrophoretic patterns on polyacrylamide gels. Non-mycorrhizal roots of four species of Eucalyptus (E. diversicolor, E. globulus, E. nitens and E. regnans) were characterized by a high activity of NAD-GDH, while only NADP-GDH was found in Hebeloma westraliense in pure culture. In associations of these eucalyptus with Hebeloma westraliense, both NAD- and NADP-GDHs were detected. By contrast, NAD- and NADP-GDHs found in the free-living mycelium of Laccaria laccata were not detected in the associated ectomycorrhizas. In ectomycorrhizas of five species of eucalypts (E. diversicolor, E. globulus, E. grandis, E. regnans and E. urophylla) associated with either Laccaria laccata, Scleroderma verrucosum or Pisolithus tinctorius, fungal AAT, which was very active in free-living mycelia, was not detected in the symbiotic tissues. In contrast, the two root AAT isoenzymes remained active in the mycorrhizas and were even stimulated. These results suggest that fungal gene expression is moderated by the host plant but also depends on the fungal associate. Factors which may contribute to the observed differences in enzyme activity between Hebeloma and Laccaria are discussed.