Introduction to Fungi, Third Edition

IMPORTANCE OF HOMOBASIDIOMYCETES
527
Fig19.10 Ectomycorrhiza on beech roots collected from humus. (a) General view. Note the stunted growth and dichotomous
branching of the lateral roots which are entirely covered by the mantle. (b) Transverse section through such a mycorrhizal root tip.
A mantle several hyphae thick has formed a sheath around the epidermis. Branching hyphae grow between the outer cortical cells
(arrows) to form the Hartig net. (a) kindly provided by A.E. Ashford.
(chanterelle) are found under both broad-leaved
and coniferous trees.
The latter two species together with
Tricholoma matsutake are the three most valuable
mycorrhizal Homobasidiomycetes in commercial
terms, with a combined annual crop value of
over US$ 2 billion (Hall et al., 2003). Considerable
efforts have been made to develop methods for
their cultivation, but success has been limited.
Whilst it is possible to cultivate mycelium of
these species in vitro on agar media and to
inoculate seedlings of host trees, the crops of
fruit bodies after outplanting have been disappointing
or altogether absent (Wang & Hall,
2004). The situation is more encouraging for the
ectomycorrhizal ascomycete truffles Tuber melanosporum
and T. magnatum (see p. 423), but even
with these species the establishment of commercial
plantations has failed to reverse the decline
in overall harvests due to overcollection and
destruction of natural habitats.
Fortunately, there are several ectomycorrhizal
species which, although inedible, can be
used as reliable inoculum to promote the
growth of trees in challenging environmental
situations. Probably the most successful group
is the gasteromycete genus Pisolothus, which is
used extensively in afforestation programmes
(see p. 581).
19.3.2 Degradation of wood
The complex architecture of wood is summarized
in Fig. 19.11. The middle lamella separating
adjacent cells consists of a-(1,4)-linked galacturonic
acid polymers collectively called pectins, in
which the carboxylic acid group may be derivatized
by methylation or calcium salt formation.
The primary wall consists chiefly of cellulose
made up of b-(1,4)-linked glucose units.
Hemicelluloses are also present; these comprise
several heterogeneous polymers with a backbone
of glucose, mannose or xylose in b-(1,4)-linkage,