Growth, Differentiation and Sexuality

it has been suggested that hydrophobins could
be involved in aggregating aerial hyphae during
fruiting-body morphogenesis (see The Mycota,
Vol. I, 1st edn., Chap. 21).
B. SC7 and SC14
SC7 and SC14 were isolated as genes highly
expressed during fruiting of S. commune (Mulder
and Wessels 1986). Like the SC1, SC4 and SC6
hydrophobin genes, these genes are regulated by
the mating-type genes (Ruiters et al. 1988), the FBF
gene (Springer and Wessels 1989) and the THN
gene (Wessels et al. 1991b). The coding sequences
of SC7 and SC14 are 70% identical at the nucleotide
level, while the encoded proteins have 87% similarity
in amino acids (Schuren et al. 1993c). SC7
and SC14 are rather hydrophilic proteins with
homology to pathogenesis-related proteins of
plants, testis-specific proteins from mammals, and
venom allergen proteins from insects. By using
an antibody against a bacterial fusion protein, it
was shown that SC7 is secreted in the medium,
and is loosely bound to the extracellular matrix
which binds hyphae in the fruiting body together
(Schuren et al. 1993c). The precise role of SC7 and
SC14 remains to be established.
C. Lectins
Lectins have been implicated in fruiting-body
development of various basidiomycetes (Wang
et al. 1998). Although a large number of these
carbohydrate-binding proteins have been isolated,
their role in fruiting is still unclear. The
best-studied lectins with respect to fruitingbody
development are CGL1 and CGL2 of C.
cinereus. These β-galactoside binding lectins,
called galectins, show 87% sequence identity
(Charlton et al. 1992; Cooper et al. 1997) and
have been shown to be specifically produced
during fruiting-body formation (Boulianne et al.
2000). CGL2 is produced in the dark in aerial
mycelium which forms the primary hyphal
knots. Production of the lectin proceeds until
completion of tissue differentiation in the primordia.
Expression of cgl2 is regulated by the
A mating-type genes, is inhibited by constant
light and is subject to carbon and/or nitrogen
repression. The promoter of cgl2 contains
a cAMP response element with the consensus
sequence TGCGTC (Bertossa et al. 2004),
Fruiting in Basidiomycetes 405
which is in agreement with a role of this second
messenger in fruiting-body development (see
above). Within the promoter sequence of cgl2,
no motif could be defined acting in light or A
mating-type regulation (Bertossa et al. 2004).
Possibly, regulation is not direct but acts via
transcription factors which have not yet been
identified.
CGL1 production starts later than that of CGL2.
It is produced at the moment the light-induced
compact secondary hyphal knots are produced.
Expression of cgl1 continues throughout primordial
development, reaching its maximum just prior
to meiosis (Charlton et al. 1992; Boulianne et al.
2000). CGL1 and CGL2 are located in cell walls
and in the extracellular matrix in the outer layers
of the cap and stipe of primordia (Boulianne
et al. 2000). By incubating fluorescently labelled
galectinsonsectionsofmushroom,itwasshown
that ligands of the galectins are located mainly
in the gill tissue (Walser et al. 2005). This was
rather surprising, although ligands were also localized
at the outer surface layers. These outer
layers are subjected to strong forces during stipe
elongation and cap expansion (see The Mycota,
Vol. I, 1st edn., Chap. 22). It was proposed that
the galectins and their receptors are involved in
resisting these stretching forces by attaching hyphal
cells to each other (Boulianne et al. 2000).
Recently, the structure of CGL2 was determined
(Walser et al. 2004). The protein forms a clover
leaf-shaped tetramer. Each monomer has a galectin
fold composed of two antiparallel, six-stranded βsheets
forming a β-sandwich. The orientation of
the binding sites in the tetramer makes CGL2 an
excellent candidate to function as a cross-linker.
Experimental evidence showed that β-galactosides
derivatised at the 2 ′ and 3 ′ positions most strongly
interact with CGL1 and CGL2 (Walser et al. 2004,
2005). Such derivatised galactosides are found in
lipids of basidiomycetes (Jennemann et al. 1999,
2001).
Galectins have also been isolated from other
basidiomycetes. A galectin called ACG, with 37%
identity to CGL1, was isolated from Agrocybe cylindracea
(synonym A. aegerita) (Yagi et al. 2001).
Walser et al. (2003) suggested that this lectin is the
same as the lectin AAL from A. aegerita (Sun et al.
2003). AAL has the property to promote differentiation
of fruiting-body primordia of Aaegerita
as well as Auricularia polytricha. Thisindicates
that galectins could also function as a signal for
fruiting-body initiation.