Introduction to Fungi, Third Edition

526 HOMOBASIDIOMYCETES
An equally important ecological role played by
the Homobasidiomycetes in the global environment
is that of saprotrophs involved in the
decomposition of the two most abundant organic
carbon sources, cellulose and lignin, thereby
releasing nutrients locked up in wood and leaf
litter. The degradation of wood is achieved in
two different ways, white-rot and brown-rot,
and these are described briefly on pp. 527 532.
A few wood-rotting species are plant pathogens,
e.g. Armillaria mellea, Phellinus noxius and Crinipellis
perniciosa, whereas others, notably the dry rot
fungus Serpula lacrymans, cause economic damage
of timber built into houses.
19.3.1 Ectomycorrhiza
Many members of the Homobasidiomycetes
(including gasteromycetes), as well as a few
Ascomycota such as the truffles (Tuber spp.; see
p. 423), form ectomycorrhizal associations with
coniferous and broad-leaved trees. These are
distinguished from the vesicular arbuscular
mycorrhiza between herbaceous plants and
Zygomycota (see p. 217) in several key features
which have been reviewed by Smith and Read
(1997) and Peterson et al. (2004). The most
immediately obvious is that the bulk of the
fungal biomass is located outside the plant root,
hence the term ectomycorrhiza. The colonization
of a tree root by an ectomycorrhizal fungus has
reciprocal morphogenetic effects. Lateral roots
show stunted growth accompanied by increased
branching which is often dichotomous
(Fig. 19.10a) in response to fungal colonization,
and they are covered by a thick sheath of hyphae,
the mantle (Fig. 19.10b). There is also a limited
colonization of the root cortex in the shape of
the Hartig net, a system of unusually richly
branched intercellular hyphae (Fig. 19.10b). The
sequence of colonization events probably starts
with hyphae being initially attracted to root tips
by their exudates or possibly those of microorganisms
associated with the rhizosphere, such
as fluorescent pseudomonads (Garbaye, 1994;
Smith & Read, 1997). Following contact with a
root hair, hyphae grow alongside it until they
meet the surface of the main root (Thomson
et al., 1989). There, morphogenetic changes are
initiated, such as hyphal branching and anastomosis
which lead to the establishment of the
mantle (Massicotte et al., 1987). These changes
may result from the specific recognition of wall
surface molecules between the root and fungus
hypha (Giollant et al., 1993; Lapeyrie & Mendgen,
1993). Outside the mantle, the mycelium
may extend into the soil by a few centimetres,
or much further if the fungus is capable of
forming mycelial cords (see p. 581). The roots of
different plants in complex forest ecosystems
may be linked by common ectomycorrhizal
fungi, and there may be a net transfer of
carbon from sunlit plants to those growing in
the shade (Leake et al., 2004). Mineral nutrients,
notably phosphate, as well as water are transported
from the fungus to the plant.
Carbohydrates travel the opposite way. Sucrose,
the main transport carbohydrate in plants,
is secreted into the apoplast and hydrolysed to
give fructose and glucose. The latter is taken up
by the fungus and converted to glycogen, trehalose
or polyols (Smith & Read, 1997). Some 10% of
the net photosynthetic assimilate may be allocated
to mycorrhizal fungi which make up
20 30% of the microbial biomass in forest soils
(Leake et al., 2004). Fruit bodies are a major sink
for translocated carbon. Colourless (achlorophyllous)
plants may obtain their carbon by plugging
into the mycorrhizal network, a strategy known
as mycoheterotrophy (Smith & Read, 1997).
Fossil records of ectomycorrhizal associations
date back some 50 million years (LePage et al.,
1997), although they are more likely to be
around 200 million years old (Cairney, 2000).
Mycorrhizae of this kind are particularly prominent
in nutrient-poor or dry soils. Many ectomycorrhizal
fungi have retained the capacity
to produce hydrolytic enzymes and are capable
of solubilizing, for example, phosphorus and
nitrogen from complex sources (Perez-Moreno &
Read, 2000). They are therefore often associated
with humus layers in which saprotrophic
species also grow. Ectomycorrhizal fungi may
show a greater or lesser degree of host specificity;
for instance, Suillus grevillei is associated almost
exclusively with larch (Larix spp.), whereas
Amanita muscaria (fly agaric), Boletus edulis
(cep or penny bun) or Cantharellus cibarius