Growth, Differentiation and Sexuality

expression in the stipe suggests that it could have
a function similar to that of the eln2 gene of C.
cinereus.
F. Enzymes Involved
in Carbohydrate Metabolism
Due to substrate limitation in most laboratory
cultures of S. commune, only a few primordia
can grow into typical, fan-shaped fruiting bodies
(Fig. 19.2), which can measure a few centimetres
across. During enlargement, here based on
proliferation of hyphae (Wessels 1993a) and not
on hyphal inflation, as in agarics (Hammad et al.
1993, see The Mycota, Vol. I, 1st edn., Chap. 22),
nitrogenous compounds and carbohydrates are
retrieved from preformed substrate mycelium
and from abortive primordia (Wessels 1965;
Niederpruem and Wessels 1969; Wessels and
Sietsma 1979). Glycogen degradation occurs
mainly by a glucoamylase (Yli-Mattila and Raudaskoski
1992) while degradation of water-soluble
β-(1,3)/β(1,6)-glucan occurs by a β-(1,3)-glucan
glucohydrolase (Wessels 1969). The latter glucan
occurs as a jelly material around hyphae and in
the medium (Wessels et al. 1972). In addition,
a glucan of similar structure, but occurring as
a major alkali-insoluble component of hyphal
walls (R-glucan) due to linkage to chitin (Sietsma
and Wessels 1977, 1979), is degraded in substrate
hyphae and in abortive primordia to provide for
the needs of growing fruiting bodies. Breakdown
of this glucan is initiated by a catabolite-repressed
β-(1,6)-glucan glucanohydrolase (Wessels 1966,
1969). Because other cell components are also
degraded and reused for growth of the fruiting
bodies, most of what remains of the supporting
structures are empty hyphae with walls containing
mainly (1,3)-α-glucan, notwithstanding the
apparent possibility of this fungus to make 1,3-αglucanase
under other circumstances (Reyes et al.
1980). A mutant in which R-glucan degradation
is blocked is deficient in outgrowth of primordia
(Wessels 1965, 1966). In the wild-type strain, high
concentrations of carbon dioxide lead to synthesis
of an altered R-glucan which is less susceptible to
enzymatic degradation and, thus, cannot sustain
outgrowth of primordia (Sietsma et al. 1977).
Also spore production in S. commune depends on
degradation of previously synthesised polymers
(Bromberg and Schwalb 1976). Even in the presence
of a carbon source in the medium, 30% of the
Fruiting in Basidiomycetes 407
total material in spores derives from previously
synthesised material. In agarics, such as C. cinereus
(Madelin 1960; Ji and Moore 1993), Flammulina
velutipes (Kitamoto and Gruen 1976; Gruen and
Wong 1982) and C. congregatus (Robert 1977),
fruiting-body formation has also been shown to
be associated with breakdown of polysaccharides
in the substrate mycelium.
During transition from vegetative growth
to fruiting-body development in S. commune,
the respiratory quotient of cultures changes from
above 2 to around unity, indicating the operation of
purely oxidative metabolism in the fruiting bodies
but some fermentative activity in the substrate
mycelium (Wessels 1965). This may be a consequence
of the greater oxygen availability to hyphae
in the emerging fruiting bodies. Of enzymes involved
in respiratory activity, Schwalb (1974) noted
a marked decrease in the activity of phosphoglucomutase
in the fruiting bodies. This was linked to the
appearance of specific proteases in fruiting bodies
which inactivated the enzyme (Schwalb 1977).
V. Conclusions
Establishment of the dikaryotic mycelium and
formation of fruiting bodies are highly complex
developmental programmes. A wide variety of proteins
are expected to regulate and coordinate these
programmes or to fulfil enzymatic conversions or
structural roles. With the identification of the first
genes during the last two decades, we are only at the
beginning of understanding fruiting-body formation.
A number of genes have been isolated which
have no homologues in the protein databases, and
for which functions are unknown. Other genes have
been studied in more detail and, based on these
studies, functions have been proposed or assigned.
Gene expression during establishment of the
dikaryon and emergence of fruiting bodies in basidiomycetes
has been linked to the combinatorial
activities of the mating-type genes. These genes encode
DNA-binding proteins and pheromones and
their receptors. The regulation of fruiting genes by
the mating-type genes seems to be quite indirect,
and other regulatory genes such as FRT1 and THN
of S. commune and pcc1 and clp1 of C. cinereus
have been shown to influence emergent growth,
and fruiting-body formation in particular. The latter
gene appears to be a direct target of the homeodomain
protein encoded in the A mating-type