Growth, Differentiation and Sexuality

senescence. However, no specific experiments on
backgrowth inhibition were performed.
The combined evidence indicates that autoregulators
are very probably involved at various stages
of colony morphogenesis, despite our current ignorance
of their chemical nature.
IV. Asexual Development
Behind the peripheral growth region, the colony
not only adapts its structure for intercommunication
by hyphal anastomosis but additionally, it often
prepares aerial hyphae in distal regions for the
production of asexual spores as dispersal propagules.
The autoregulatory signals reporting on the
emergence of hyphae from the substratum to the
air have been assigned to secondary metabolites.
Early studies of asexual spore production point
to the existence of self-produced metabolites as active
agents in sporulation. Perhaps the most precise
example of this comes from the work of Hadley
and Harrold (1958) who described an unidentified
endogenous factor believed to enhance 1–10 mM
calcium-induced conidiation (asexual sporulation)
in liquid cultures of Penicillium notatum.Anequivalent
factor was only recently isolated and identified
in the closely related Penicillium cyclopium.
Moreover, it was shown to be both necessary and
sufficient for conidiation induction, whilst calcium
was demonstrated to act as the enhancer. The factor
was hence given the common name of conidiogenone
(Roncal et al. 2002).
Conidiogenone is aditerpene (1R ∗ ,3R ∗ ,7R ∗ ,8S ∗ ,
11R ∗ ,14S ∗ ,15R ∗ )-3,6,6,11,15 pentamethyl-tetracyclo[9.4.0.0
1,8 .0 3,7 ]-pentadecan-12-one, Fig. 11.3a)
which is produced throughout the growth phase
of the mycelium under submerged conditions. On
emergence to the air, the autoregulator is believed
to accumulate at the surface of hyphae, thus attaining
concentrations which surpass the threshold
levels (350 pM). This rapid accumulation has been
proposed to result in binding of conidiogenone to
an as yet unidentified receptor at the cell surface,
thus triggering a signal transduction pathway
leading to spore production. For more details
on conidiation induction, see Fischer and Kües
(Asexual sporulation in mycelial fungi, Chap. 14,
this volume). When the hypha is in a liquid
environment, conidiogenone is diluted in the
bulk medium, thereby remaining at sub-threshold
levels. In the exceptional case of some Penicillia,
Fungal Mycelial Signals 207
Fig. 11.3. A–D Autoregulatory signals involved in conidiation
induction: A conidiogenone, B conidiogenol, C sporogen
AO-1, and D butyrolactone I
the presence of calcium exerts changes at the cell
surface and this reduces the threshold level fivefold,
thus facilitating conidiation under submerged
conditions.
In order to avoid long-term signal accumulation
which would result in inappropriate conidiation
induction, conidiogenone is continuously
converted to an inactive derivative, conidiogenol
(1R ∗ ,3R ∗ ,7R ∗ ,8S ∗ ,11R ∗ ,14S ∗ ,15R ∗ )-14-hydroxy-3,6,
6,11,15 pentamethyl-tetracyclo[9.4.0.0 1,8 .0 3,7 ]-pentadecan-12,14-diol,
Fig. 11.3b), by the reduction of
a single keto group (Roncal et al. 2002).
There are other instances in which selfproduced
secondary metabolites have been
reported to exert a role as autoregulators in
asexual spore production. The partially elucidated
diterpene sporogen-PF-1 is produced by
Penicillium funiculosum under blue light, and
promotes the production of conidia (Katayama
et al. 1989). The sesquiterpene sporogen-AO1
((1aR,6R,7bR)-5,6,7,7a-tetrahydro-6-hydroxy-7,7adimethyl-1a-(prop-1-en-2-yl)naphtho[2,1-b]oxiren-
2(1aH,4H,7bH)-one; Fig. 11.3c) has been reported
to exert sporogenic effects in Aspergillus oryzae
(Tanaka et al. 1984). Butyrolactone I (methyl
2-(4-hydroxy-3-(3-methylbut-2-enyl)benzyl)-tetrahydro-3-(4-hydroxyphenyl)-4,5-dioxofuran-2-carboxylate;
Fig. 11.3d), a small γ-butyrolactonecontaining
metabolite, has been found in
Aspergillus terreus cultures, with significant
effects on branching and asexual spore pro-