Activities and localization of aspartate aminotransferases (AAT) were determined in spruce and beech ectomycorrhizas, as well as in nonmycorrhizal roots and fungi in pure culture. Preliminary data emphasized the importance of AAT in amino acid synthesis; thus, aspartate derivatives (asparagine, citrulline, and arginine) accounted for about 63 of the total nitrogen of the free amino acid pool. Specific activities were in the same range for both symbiotic tissues and free-living partners; however, electrophoretic patterns revealed that the two host isoforms were detected in ectomycorrhizal tissues, whereas the single fungal isoform was absent in the associations investigated. Dissection of the symbiotic tissues into three parts (vascular cylinder, cortical region, and mycelial layer) confirmed these results: the cortical region, including both host and fungal cells, contained only the two host isoforms. Results from immunological tests and immunocytochemical labelling using antibodies raised against fungal AAT were also consistent with a strong suppression of the fungal AAT accumulation in the association. Implications of fungal AAT repression in the symbiotic tissues are discussed.
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