Concept
Ectomycorrhizal extramatrical mycelium (also known as extraradical mycelium) is the collection of filamentous fungal hyphae emanating from ectomycorrhizas. It may be composed of fine, hydrophilic hypha which branches frequently to explore and exploit the soil matrix or may aggregate to form rhizomorphs; highly differentiated, hydrophobic, enduring, transport structures. Apart from mycorrhizas, extramatrical mycelium is the primary vegetative body of ectomycorrhizal fungi. It is the location of mineral acquisition, enzyme production, and a key means of colonizing new root tips. Extramatrical mycelium facilitates the movement of carbon into the rhizosphere, moves carbon and nutrients between hosts and is an important food source for invertebrates. The mycelial growth pattern, extent of biomass accumulation, and the presence or absence of rhizomorphs are used to classify fungi by exploration type. Agerer first proposed the designation of exploration types in 2001, and the concept has since been widely employed in studies of ectomycorrhizal ecology. Four exploration types are commonly recognized: Contact, Short-distance, Medium-distance and Long-distance. Contact exploration types possess a predominantly smooth mantle and lack rhizomorphs with ectomycorrhizas in close contact with the surrounding substrate. Short-distance exploration types also lack rhizomorphs but the mantle is surrounded by frequent projections of hyphae, which emanate a short distance into the surrounding substrate. Most ectomycorrhizal ascomycetes are included in this group. Medium-distance exploration types are further divided into three subtypes defined by the growth range and differentiation of its rhizomorphs. Medium-distance Fringe form interconnected hyphal networks with rhizomorphs that divide and fuse repeatedly. Medium-distance Mat types form dense hyphal mats which aggregate into a homogeneous mass. Finally, the Medium-distance Smooth sub-type has rhizomorphs with smooth mantles and margins.
