STRUCTURAL ASPECTS OF ECTOMYCORRHIZA OF PINUS-PINASTER (AIT) SOL FORMED BY AN IAA-OVERPRODUCER MUTANT OF HEBELOMA-CYLINDROSPORUM ROMAGNESI

Mycorrhizas formed on Pinus pinaster by an IAA-overproducer mutant str ain (h1 FIR 4 h1 F1 331) of Hebeloma cylindrosporum are described. The wild strain (h1) was used as reference. The wild type mycorrhiza had a thin mantle and a uniseriate Hartig net, which reached the second la yer of cortical cells, as generally described. Conversely, the mutant type mycorrhiza had special features: a large mantle and a highly deve loped Hartig net (up to seven layers of hyphae in width) which reached the endodermis. Even when the Hartig net was formed of several layers of hyphae, the cortical cells remained alive, indicating that the fun gus may aid survival of the host cells. Intracellular hyphae surrounde d by invaginated host plasmalemma and by interfacial material, were fr equently seen in cortical cells. In this case, both fungal and host ce lls were alive. Even if the mutant strain is more invasive, the IAA ov erproduction does not seem to modify its symbiotic status. It is possi ble that IAA plays a role in cell wall loosening, allowing the fungus to overcome barriers which normally prevent the invasion of living cor tical cells.