Growth, Differentiation and Sexuality

and Robinson 1969). Related compounds such
as 1-octen-3-ol (Fig. 11.1j) have been recently
reported to act in the cereal pathogen Penicillium
paneum (Chitarra et al. 2004), in which the action
of the autoinhibitor was species specific.
The marked differences in specificity between
biotrophic pathogens and saprophytes
may originate from the fact that the former
occupy a specific niche in which the host may
also generate molecules which play an important
role in the infection process. These often
comprise volatile alcohols of various chain
lengths, such as nonyl alcohol (Allen 1972),
trans-2-hexen-1-ol (Collins et al. 2001) and surface
waxes (Podila et al. 1993). These stimulatory
signals share molecular and physicochemical
characteristics with germination autoinhibitors
and must, therefore, be clearly distinguished
by the spore. The general consensus is that
mono-unsaturated aliphatic chain-containing
self-inhibitors are cis isomers, whereas the stimulators
are mostly in the trans conformation.
In addition, the positioning of the unsaturated
bonds is important for differentiation
between stimulators and inhibitors (Wang et al.
2002).
Saprophytes need not differentiate necessarily
between self and non-self, but may preferentially
need to determine the extent of occupancy of the
substratum in a first instance, independently of the
nature of the occupants. This may be the principal
reason for a less varied array of reported germination
autoinhibitors in saprophytic fungi. Nevertheless,
as work progresses on the chemical ecology
of these signals, this current understanding may
be confirmed or reformed by the weight of new
evidence.
The mechanism of action of germination
autoinhibitors has been examined in some detail.
Nonanoic acid has been postulated to provoke
alterations in membrane permeability and, consequently,
in intracellular pH (Breewer et al.
1997). Other studies on the effect of germination
inhibitors of mildew point to a direct involvement
of K + channel activation and consequent loss of
intracellular potassium (Wang et al. 2002). The
process of germination itself appears to involve
the independent participation of RAS and cAMP
signalling pathways (Fillinger et al. 2002). Autoinhibitors
have been shown to block calmodulin
gene expression (Liu and Kalattukudy 1999) but
whether this is directly or indirectly exerted by the
germination effectors is not yet clear.
Fungal Mycelial Signals 205
Although mycelial fungi are known mostly to
produce germination inhibitors, the higher fungi
possess self-produced germination autoinducers.
These are also volatile substances which are
produced by spores and mycelia, and which have
been shown to be necessary to trigger germination.
Among them, isovaleric acid is the best known
autoinducer, and it is produced by spores and
mycelia of Agaricus bisporus (Rast and Stäuble
1970).
The currently accepted interpretation of the
role of germination autoinducers is that they could
maximize the opportunity of compatible spores to
form a dikaryon (an important feature of the life
cycle of the producer). This distinguishing feature
would make it advantageous for maximum spore
germinationinanenvironmentcrowdedwithcompatible
spores.
In all, the information available on germination
autoinhibitors and autostimulators is still limited
and dated. An interdisciplinary approach to
the study of these molecules and their mode of action
should yield important new knowledge in the
future.
III. Colony Morphogenesis
Having completed the germination process, emerging
hyphae extend away from the germling, eventuallygivingrisetoaradialcolony.Thedevelopmentofacolonyfromagermlingtoamultifunctionalliving
body has been described as a complex process
(Fig. 11.2A; Buller 1933). At the advancing edge,
apical extension growth is clearly dominant, with
few ramifications. Older subapical zones display
lateral branches which fuse with already existing
leading hyphae, or amongst themselves. This transforms
the initial radial pattern into a matrix, with
important functional consequences for the colony.
At distal regions approaching the spatial and temporal
origin of the colony, living vacuolated cells
survive, support intercellular transport and the formation
of long-term survival structures. There is
no record of hyphal growth from the periphery
into these regions. The colony hence appears to
undertake a self-organising plan. The nature of the
autoregulatory signals responsible for the undertaking
of this plan remains largely unknown, but
some information is available on the subject.
Newly formed hyphae elongate by apical
extension growth, and the process by which they