Dynamics of symbiotic establishment between an IAA-overproducing mutant ofthe ectomycorrhizal fungus Hebeloma cylindrosporum and Pinus pinaster

To clarify the early steps of symbiotic establishment, we studied the dynam ics of Pinus pinaster (Ait.) Sol. tap root colonization and mycorrhiza form ation by an IAA-overproducing mutant of the ectomycorrhizal fungus Hebeloma cylindrosporum Romagnesi and by the corresponding wild type strain. Differ ences between wild type and mutant strains were quantitative rather than qu alitative and were detected two days after inoculation. Both fungal strains established a typical Hartig net when they colonized the tap roots. Conseq uently, colonized tap roots exhibited features of a true mycorrhiza and fun gal colonization enhanced plant growth. Fungal colonization and Hartig net formation were more rapid with the mutant than with the wild type. Coloniza tion, especially with the mutant strain, increased rhizogenesis and the pro duction of mycorrhizas. The mutant formed a hypertrophic Hartig net in tap roots and mycorrhizal short roots and we obtained evidence that the process of short root transformation into mycorrhiza started before their emergenc e from the tap root. Hyphae of the Hartig net in the tap root penetrated th e cortex of young lateral roots at the beginning of their elongation, after the endodermis layer broke under the pressure of the elongating lateral ro ot. Colonization was inhibited when triiodobenzoic acid was added to the cu lture medium, providing circumstantial evidence that auxin is involved in m ycorrhiza formation.