MYCORRHIZAS - INTEGRATED DEVELOPMENT BETWEEN ROOTS AND FUNGI

Most vascular plants have coevolved with root inhabiting symbiotic fungi and the resulting structures, mycorrhizas, are the result of the close association and a subsequent modification of both symbionts. In vesicular-arbuscular mycorrhizas, the mycobiont undergoes pronounced alterations in morphogenesis involving appressorium formation, arbuscule development and, in some genera, differentiation of vesicles. The triggers for these modifications are unknown. Changes in the roots primarily involve subcellular modifications. In ectomycorrhizas, hyphae of the mycobiont undergo dramatic changes in growth form as they contact the root surface and as they penetrate intercellularly. The underlying mechanism controlling the change in hyphal branching may involve the cytoskeleton, but to date there is little evidence for this. Ericoid mycorrhizas are specialized on the hair roots of members of some families in the Ericales. Considerable information has been obtained on the role of the mucilaginous sheath surrounding those roots in the colonization process. Appressorium formation precedes hyphal penetration of epidermal cells and subsequent coil formation. In orchids little is known about the initial contact between fungal hyphae and either protocorms or roots. In both structures, there is some evidence that entry may be via epidermal outgrowths (hairs). Hyphae develop intracellularly and are surrounded by host plasma membrane. Ectendomycorrhizas, arbutoid mycorrhizas and monotropoid mycorrhizas are of limited occurrence and have been studied less extensively.