Modifications of symbionts during ectomycorrhiza formation what do we know and where do we go?

Abstract

Ectomycorrhizas are subterranean organs resulting from the alteration in root structure by soil-inhabiting symbiotic fungi. Hyphae of the mycobiont have to contact the root surface, become attached to the root, and subsequently enter the root by growing between epidermal cells (and in some species, cortical cells) to form the Hartig net. A chemotropic stimulus might be involved in early hypha-root contact and recognition-adhesion may involve a polysaccharide-lectin interaction, but further research is needed to confirm this. Fungal hyphae adhering to the root surface change their mode of growth from apical, extension growth to a loss of this pattern resulting in a multi-branched mycelium. A similar change in pattern of branching occurs as hyphae form the Hartig net. In both cases, a change in the cytoskeleton might precede the change in branching. The ingress of hyphae between epidermal cells in angiosperm roots triggers radial rather than axial elongation of these cells; a reorientation of the cytoskeleton and subsequently the cellulose microfibrils is hypothesized to be involved in this process. Wall changes in root cells contiguous to Hartig net hyphae also occur, and these might facilitate nutrient exchange between the symbionts.