Growth, Differentiation and Sexuality

404 H.A.B. Wösten and J.G.H. Wessels
Fig. 19.4. A–C Schematic presentation of the role of hydrophobins
in development of emergent structures of a S.
commune dikaryon (a similar scheme can be drawn for the
monokaryon which only produces the SC3 hydrophobin
and aerial hyphae). A In juvenile cultures, the hydrophobin
genes are silent and a substantial amount of submerged
mycelium can be produced. B After a feeding submerged
mycelium is formed, the hydrophobin genes are switched
on. The SC3 hydrophobin (and, to a lesser extent, the SC4
hydrophobin) is secreted into the medium in a water-soluble
form (stipples) at tips of growing hyphae, whereas the walls
of these hyphae are virtually devoid of hydrophobins (walls
ter et al. 2000a). By swapping promoters, it was
shown that SC4 can substitute for SC3 in formation
of hydrophobic aerial hyphae. However, hyphal attachment
to hydrophobic surfaces is only partially
restored because the hydrophilic side of the SC4
membrane has a lower affinity for the cell wall of
emergent hyphae than does SC3. Possibly, this is
related to the different lectin specificities of these
hydrophobins (van Wetter et al. 2000a). The exposed
carbohydrates of cell walls of aerial hyphae
and hyphae in fruiting-body tissue may be different,
requiring different lectin specificities to ensure
strong binding to the cell wall.
That hydrophobins are tailored to fulfil specific
functionsisalsoindicatedbysequenceanalysis.
SC3 of S. commune (de Vocht et al. 1998), ABH3
of A. bisporus (Lugones et al. 1998), COH1 of C.
cinereus (Ásgeirsdóttir et al. 1997) and POH1 of
of substrate hyphae drawn as thin lines). SC3 self-assembles
at the water–air interface, thus reducing the surface tension.
This allows hyphae to breach the interface to grow into the
air (C). While continuing apical secretion, surfaces of aerial
structures in contact with air are coated with an insoluble
hydrophobin membrane due to self-assembly at the cell
wall–air interface (hyphal walls drawn as thick lines). SC3
coats aerial hyphae, the hyphae surrounding the fruit bodies
(here represented by a fruit body in which the pileus has
not yet expanded) and the hymenium. SC4 coats air channels
within the fruiting body, thus ensuring gas exchange
(inset)
Pleurotus ostreatus (Ásgeirsdóttir et al. 1998) have
all been implicated to be involved in formation of
aerial hyphae. These hydrophobins are more related
to each other than are SC3 and ABH3 to the
other hydrophobins of S. commune and A. bisporus
respectively. Similarly, the fruiting body-specific
hydrophobins of S. commune cluster with HypB (de
Groot et al. 1999), ABH1/HypA (de Groot et al. 1996;
Lugones et al. 1996) andABH2/HypC (de Groot et al.
1996; Lugones et al. 1996) of A. bisporus.Thissuggests
that functional similarity is reflected in the
primary sequence of hydrophobins.
With the established roles of SC3, SC4 and
ABH1, we are only at the beginning of our
understanding of the functions of these proteins
in fruiting. Dikaryons seem to express several
hydrophobins which may have specific properties
or are expressed at a particular place. For instance,