Genera in the tribe Pyroleae (subfamily Monotropoideae, family Ericaceae) occur as understory plants in northern temperate zones where some form major components of ecosystems. Most have been poorly studied in terms of their association with symbiotic fungi. In this study, colonization patterns of mycorrhizal roots of five members of the Pyroleae (Pyrola asarifolia Michx., Pyrola chlorantha Sw., Orthilia secunda (L.) House, Chimaphila umbellata (L.) W. Bart., Moneses uniflora (L.) Gray) were explored. Root samples were processed for light, fluorescence, and laser scanning confocal, scanning electron, and transmission electron microscopy, as well as for immunocytochemistry. Roots of all species had enlarged epidermal cells containing hyphal complexes, Hartig nets confined to the epidermis, and mantles. Epidermal cells were penetrated by hyphae originating from the Hartig net at more than one site either along the inner tangential wall or radial walls. The outer tangential wall of epidermal cells of all species, except M. uniflora, was thicker than radial and inner tangential walls and consisted of two layers, the outer containing nonesterified pectins that were labeled with JIM 5 antibodies. Radial walls and inner tangential walls did not label, but cortical cell walls did. Intracellular hyphal complexes developed initially around centrally positioned, enlarged epidermal cell nuclei and, through branching, occupied most of the cell volume. Senescence and degradation of the complexes followed. The fungal species in these symbiotic associations may be important functionally in nutrient exchange, as well as in contributing to broader linkages with other hosts in these plant communities. [ABSTRACT FROM AUTHOR]