A structural study of the interaction between the ectomycorrhizal fungus Pisolithus tinctorius and Pinus strobus roots

Abstract

Stages in ectomycorrhizal development between the fungus P. tinctorius (Pers.) Coker et Couch and short roots of P. strobus L. were followed in growth pouches. Short roots from preinoculation through Hartig net formation were processed for light and electron microscopy. Fungal hyphae approaching the surface of roots have Thiery-positive substances in their modified walls and in lomasomes, indicating the possibility of extracellular polysaccharide secretion. Hyphae grow between and beneath the flattened, tannin-filled superficial root cells and subsequently into the intercellular region of the cortex. A Hartig net several hyphae wide is formed, isolating the cortical cells from each other. Plasmodesmata were not observed in these cortical cells. Middle lamella material is always present around the intercellular hyphae, suggesting limited digestion by the hyphae. Cortical cell cytoplasm becomes necrotic in regions of mature Hartig net formation.