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

Previous studies of nitrogen-assimilating enzymes in ectomycorrhizal a ssociations of the Pinaceae (Norway spruce, Douglas fir) and the Fagac eae (beech) have suggested that the host plant may regulate the expres sion of some fungal enzymes. To improve our understanding of the regul ation of nitrogen-assimilating enzymes in ectomycorrhizal plants, we h ave extended this work to a third plant family, the Myrtaceae. Glutama te dehydrogenases, specific either for NAD (NAD-GDH) or NADP (NADP-GDH ), and aspartate aminotransferase (AAT) were investigated by enzyme as says and electrophoretic patterns on polyacrylamide gels. Non-mycorrhi zal roots of four species of Eucalyptus (E. diversicolor, E. globulus, E. nitens and E. regnans) were characterized by a high activity of NA D-GDH, while only NADP-GDH was found in Hebeloma westraliense in pure culture. In associations of these eucalyptus with Hebeloma westraliens e, both NAD- and NADP-GDHs were detected. By contrast, NAD- and NADP-G DHs found in the free-living mycelium of Laccaria laccata were not det ected in the associated ectomycorrhizas. In ectomycorrhizas of five sp ecies of eucalypts (E. diversicolor, E. globulus, E. grandis, E. regna ns and E. urophylla) associated with either Laccaria laccata, Sclerode rma verrucosum or Pisolithus tinctorius, fungal AAT, which was very ac tive in free-living mycelia, was not detected in the symbiotic tissues . In contrast, the two root AAT isoenzymes remained active in the myco rrhizas and were even stimulated. These results suggest that fungal ge ne expression is moderated by the host plant but also depends on the f ungal associate. Factors which may contribute to the observed differen ces in enzyme activity between Hebeloma and Laccaria are discussed.