ABSTRACT
Within the rhizosphere, which hosts a large and diverse community of prokaryotic and eukaryotic microbes that compete and interact with each other and with plant roots, mycorrhizal fungi are almost ubiquitous. The ectomycorrhizal hyphae and the root tips form a novel composite organ, so-called mycorrhiza, which is the site of nutrient and carbon transfer between the two symbionts. This association allows terrestrial plants to grow efficiently in suboptimal environments [1]. Among the various types of mycorrhizal symbioses, the endomycorrhizal, ectomycorrhizal, and ericoid associations are found on most annual and perennial plants (probably >90%). About two-thirds of these plants are symbiotic with arbuscular mycorrhizal glomalean fungi [2]. Ericoid mycorrhizas are ecologically important, but mainly restricted to heathlands [3]. While a relatively small number of plants, ~8000, form ectomycorrhiza, their global importance is amplified by their wide occupancy of terrestrial ecosystems. Trees of Betulaceae, Cistaceae, Dipterocarpaceae, Fagaceae, Pinaceae, Myrtaceae, Salicaceae, and several tribes
FIGURE 1 The ectomycorrhizal symbiosis. (A) A seedling of Douglas fir (Pseudotsuga menziesii) colonized by the ectomycorrhizal basidiomycete Laccaria bicolor. The fungal mycelium has developed ectomycorrhizas on the root system and has produced a fruiting body above ground. (Photograph courtesy of P.Frey-Klett.) (B) Transverse section of a Eucalyptus/Pisolithus ectomycorrhiza showing the extramatrical hyphae (EM), the mantle (M); the fungal hyphae have begun to penetrate between the epidermal cells (E) of the root cortex (C) to form the Hartig net (HN). Epidermal cells (E) are radially enlarged. CC, central cylinder. (Photograph courtesy of B.Dell.)
in Fabaceae are ectomycorrhizal plants (Fig. 1A), dominating boreal, temperate, Mediterranean, and some subtropical forest ecosystems [1]. Within days after their emergence in the upper 10 cm of the soil profiles (e.g., organic humus and mor layer), most of the short roots of these ectomycorrhizal shrubs and trees are colonized by ectomycorrhizal fungi, and in most cases symbiotic colonization is close to 100% [4]. The fungal mycelium and the root tips form a novel composite organ, so-called ectomycorrhiza, which is the site of nutrient and carbon transfer between the two symbionts [5,6].
