Morphological analysis of early contacts between pine roots and two ectomycorrhizal Suillus strains

Selection of ectomycorrhizal strains for application in forestry is mostly based on the evaluation of symbiotic performance in small-scale experiments. Two Suillus collinitus strains isolated from a Mediterranean and an alpine area were inoculated onto two pine tree species (Pinus pinea and P. nigra ssp. laricio var. corsicana) typical of these two environments. The early events during contact between the cell surfaces of plant and fungal partners were analysed morphologically using ultrastructural and immunocytochemical techniques. All four plant-fungus combinations led to a similar degree of mycorrhizal infection and to a similar colonization pattern. The first contact of fungal hyphae with root cap cells usually involved breakdown of the outermost electron-opaque layer of the plant cell walls. Hyphae further developed between this layer and the underlying wall strata. Ultrastructural observations revealed that S. collinitus strain J3-15-24, isolated from a Mediterranean area, induced a defence reaction in the roots of P. nigra, which grows typically in alpine areas. These observations suggest functional differences between the two fungal strains in their mycorrhizal capabilities.