Growth, Differentiation and Sexuality

402 H.A.B. Wösten and J.G.H. Wessels
nuclei) inactivates the MATB-on pathway (Ásgeirsdóttir
et al. 1995), and possibly also the MATA-on
pathway. This would thus result in a monokaryonlike
gene expression. The binucleate state can be
experimentally disrupted by growing the dikaryon
in liquid shaken cultures at high rpm (round per
minute)orbygrowingthehyphaeonahydrophobic
solid. This is accompanied by cessation of SC4
and SC7 secretion. Instead, SC3 is now produced
(Schuurs et al. 1998). It appears that when the nuclear
distance exceeds 3–4 μm,geneexpressionin
the dikaryon shifts to the monokaryotic type (Wessels
et al. 1998).
Although these findings are relevant for the interaction
of MATB genes (Schuurs et al. 1998; Wessels
et al. 1998), they also bear on the role of the
mating-type genes in regulating fruiting. A dikaryotic
type of gene expression (e.g. of SC4 and SC7
but not SC3) conducive to fruiting-body formation
is possible only in the binucleate state. Aerial
hyphae formed by the dikaryon are typically nonclamped,
have widely separated nuclei and produce
SC3. Non-clamped hyphae also form the outer layer
of the fruiting bodies (Fig. 19.2), and these produce
SC3 (Ásgeirsdóttir et al. 1995). By contrast, the central
plectenchyma of the fruiting bodies produces
SC4 and SC7 but not SC3.
IV. Proteins Involved in Fruiting
When 4-day-old surface cultures of co-isogenic
S. commune monokaryons (only aerial hyphae,
Fig. 19.3A) and dikaryons (mainly fruiting bodies,
Fig. 19.3C) were compared, clear differences
were seen in the proteins synthesised at this
stage (de Vries and Wessels 1984). Among 400
proteins, pulse-labelled with [ 35 S] sulphate and
analysed on two-dimensional gels, only eight
proteins appeared to be synthesised exclusively in
the monokaryon whereas the fruiting dikaryon
synthesised 37 abundant proteins not detected in
the monokaryon. Total RNA::cDNA hybridisations
and in vitro translations of total RNA showed the
presence of about 30 unique, abundant mRNAs
in the dikaryon, accounting for about 5% of the
mRNA mass (Hoge et al. 1982). No mRNAs unique
to the monokaryon were detected; apparently, an
additional set of genes is activated in the dikaryon
during fruiting.
Complementary DNA (cDNA) synthesised on
poly(A)RNA of the fruiting dikaryon of S. commune
was cloned, and clones containing sequences expressed
in this dikaryon, and not in the co-isogenic
monokaryons, were selected (Dons et al. 1984; Mulder
and Wessels 1986). Among the mRNAs detected
with these clones, those for hydrophobins SC1, SC4
and SC6 (see below) were most abundant, as was
the mRNA for the SC3 hydrophobin, which was expressedinbothmonokaryonanddikaryon(Mulder
and Wessels 1986). It was found that formation of
these mRNAs is controlled at the transcriptional
level (Schuren et al. 1993a,b).
In other basidiomycetes, a number of other
genes have been cloned which are specifically expressed
during fruiting-body development. However,
only few of these genes have been studied in
detail, and a function has been assigned to even
fewer of them. For instance, future research should
establish the role of PRI3 and PRI4 in fruitingbody
development of A. aegeritae (Sirand-Pugnet
and Labarère 2002; Sirand-Pugnet et al. 2003) and
of the cysteine-rich PRIA of L. edodes (Kajiwara
et al. 1992). These genes encode proteins without
homologues in the databases. Recently, it was established
that over-expression of the priA gene in
L. edodes led to decreased intracellular zinc ion accumulation
(Ishizaki and Shishido 2000). It is not
clearwhyaroleinregulationofintracellularzinc
concentration is important in the early stages of
fruiting-body development.
A. Hydrophobins
Hydrophobins are secreted proteins which fulfil
a wide spectrum of functions in fungal growth
and development in general, and in fruiting-body
formation in particular (Wessels 1997; Wösten
and Wessels 1997; Wösten 2001; see The Mycota,
Vol. VIII, Chap. 7). Based on their hydropathy
patterns and their solubility characteristics, class I
and class II hydrophobins were distinguished
(Wessels 1994). In the basidiomycetes, only class I
hydrophobins have been identified and, thus,
classIIhydrophobinsseemnottobeinvolved
in fruiting-body formation in this phylum of the
fungal kingdom.
Hydrophobins can function in a soluble state by
affecting hyphal wall composition (van Wetter et al.
2000b). However, the mechanism underlying most
functions is based on the property of hydrophobins
to self-assemble at a hydrophilic/hydrophobic interface
in an amphipathic membrane (Wösten et al.
1993, 1994a,b, 1995, 1999). This membrane of about