Ectomycorrhiza Development: A Molecular Perspective

Abstract

The symbiosis between trees and soilborne ectomycorrhizal fungi results in an intimate relationship between the plant and its symbiotic partner. It provides several benefits to both the host plant and the fungal associate. The prospecting and absorbing activities of the extraradical hyphae are committed to responding to the metabolic needs of the plant. On the other hand, the fungal hyphae within the root are protected from competition with other soil microbes and, therefore, are preferential users of the plant photoassimilates. The development of ectomycorrhiza involves the differentiation of structurally specialized fungal tissues and interfaces between the symbionts (Massicotte et al. 1987; Kottke and Oberwinkler 1989; Bonfante-Fasolo and Perotto 1992) and a highly coordinated metabolic interplay (Harley and Smith 1983; Smith and Smith 1990; Martin and Hilbert 1991; Martin and Botton 1993). Morphogenesis of ectomycorrhiza involves substantial reorganization of cell populations and results from the expression of plant and fungal developmental programs. A complex set of signals presumably triggers morphogenetic and physiological changes, so that a permanent and highly efficient symbiotic structure evolves. An understanding of the molecular communication that underlies the temporal and spatial control of genes involved in symbiosis development is now within reach, as more sophisticated techniques of molecular and genetic analysis are applied to mycorrhizal interactions. However, in no instances are the physiological functions for mycorrhiza-specific gene products known, or the mechanisms regulating their expression understood. Consequently, one of the current areas of active research of developmental biology of mycorrhiza is to determine the nature of the mechanisms that control activation and expression of these developmentally critical genes that are indispensable for the orderly progression of morphogenesis. Here we review recent results shedding light on the nature of the molecular signals and mechanisms that determine or modulate the development of ectomycorrhiza.