Growth, Differentiation and Sexuality

a reduction of growth rate. These results show
that the FadA-dependent hyphal growth signalling
pathway has to be switched off, or at least the
activity has to be down-regulated in order to allow
asexual development to take place. In addition to
FadA, two other Gα subunits were identified in the
A. nidulans genome (Chang et al. 2004). Deletion
of ganB caused conidiation in submerged culture,
a condition under which sporulation is normally
repressed. Likewise, constitutive activation of the
protein caused a defect in conidiation. GanB thus
appears to negatively control asexual development.
It has not yet been discovered with which receptors
the G-proteins may interact, and also the nature
of the signals which are transduced through FadA,
GanA or GanB needs to be investigated. In the case
of FadA, it is possible that a low-molecular weight
compound is the upstream signal, because several
compounds are known to trigger developmental
decisions in A. nidulans (see Chap. 11, this
volume). In the case of GanB, Chang et al. (2004)
speculate that it could be involved in carbon source
sensing, because ganB deletion mutants germinate
even in the absence of a carbon source. It will be
the challenge of the near future to unravel the interconnection
and interdependencies between the
external signals, the GPCRs, and the G-proteins.
The analysis of fluffy mutants revealed another
very interesting aspect, this being the regulation of
development through low-molecular weight compounds.
A gene was isolated, fluG, which encodes
a protein with some similarity to prokaryotic glutamine
synthetases (Lee and Adams 1994b). The
fluffy phenotype of fluG mutants could be rescued
by neighbour colonies, and this effect occurred
even through dialysis membranes. These results
suggest that a low-molecular weight molecule is
absent in the mutant. The nature of the molecule
has not yet been determined (see Chap. 11, this
volume).
c) MAP-Kinase and Other Signalling Modules
MAP-kinase modules consist of three serine/threonine
protein kinases which are sequentially
activated by phosphorylation, and
eventually lead to the phosphorylation of target
proteins (Lengeler et al. 2000). In turn, these can
regulate transcription, the cell cycle, or other
cellular processes. Over the past decade, the role
and functioning of MAP-kinase pathways was
unravelled in a number of fungi, e.g. S. cerevisiae
and Ustilago maydis (see Chap. 18, this volume).
Fungal Asexual Sporulation 271
In A. nidulans, one MAP-kinase, SakA, was
characterized which is transiently activated in
early conidiogenesis, and involved in heat-shock,
osmotic and oxidative stress responses (Han
and Prade 2002, Kawasaki et al. 2002). Deletion
of the gene resulted also in changes in cellular
morphogenesis, in stress-sensitive conidia and
in premature sexual development, indicating an
involvement in different morphogenetic pathways.
Another MAP-kinase, MpkA, appears to be
involved mainly in the germination of asexual
spores and in hyphal morphogenesis (Bussink
and Osmani 1999). In addition to MAP-kinases,
a MAPKKK, named SteC, was identified serendipitously.
Deletion of the gene caused pleiotropic
phenotypes, suggesting the involvement of this
signalling module in several morphogenetic
pathways. ΔsteC A. nidulans strains failed to
form heterokaryons and displayed a block in
cleistothecium development. In addition, the
gene is required for correct conidiophore and
spore development (Wei et al. 2003). In about 2%
of the conidiophores, secondary conidiophores
developed on top of existing conidiophores,
and metulae often did not mature but rather
produced hyphal-like structures. Interestingly, an
up-regulation of transcription was observed in
metulae and phialides. This developmental stage
has been compared with pseudohyphal growth
of S. cerevisiae (see Chap. 1, this volume). The
expression pattern and the mutant phenotype at
the metula stage suggest a specific function during
thetimeperiodthatStuAandAbaAareactive
(see Sect. IV.A.2.f). Interestingly, S. cerevisiae
homologues of StuA (Phd1) and of AbaA (Tec1)
are required in pseudohyphal development, and
are likely to be a target of cAMP-dependent
protein kinase (Phd1) and a MAP-kinase cascade
(Tec1; Gimeno and Fink 1994; Gavrias et al.
1996; Gancedo 2001; Chou et al. 2004). It will be
interesting to see whether the activities of StuA
or AbaA are fine-tuned by posttranscriptional
regulation.
Another fundamental signalling module in
the regulation of eukaryotic development is the
COP9 signalosome. This well-conserved multiprotein
complex was recently also discovered in
A. nidulans, using an insertional mutagenesis
approach which inactivated the gene for the COP9
component CsnD (Busch et al. 2003). Deletion of
subunits of the COP9 multiprotein complex (CsnD,
CsnE) caused a pleiotropic phenotype with defects
in cell morphology, cleistothecium maturation