Growth, Differentiation and Sexuality

in the gene vivid, the effect of inhibitors of protein
kinase C, and the effect of regulatory mutations
of protein kinase C on Neurospora photoperiodism
could be interesting avenues of research to
unravel the molecular mechanisms of fungal photoperiodism.
f) Photoreceptor Genes in Neurospora
Selection for mutants that have lost the Neurospora
light responses has led to the isolation of wc
mutants, and the cloning and characterization
of the photoreceptor WC-1 (Ballario et al. 1996;
Froehlich et al. 2002; He et al. 2002). A photoreceptor
with a similar flavin-binding domain,
VIVID, was originally identified by a mutation
that promoted carotene biosynthesis but cloned
only after the isolation of a cDNA segment for
a protein with a PAS domain (Heintzen et al.
2001; Schwerdtfeger and Linden 2003). Genome
sequencing has increased the list of Neurospora
photoreceptors. A cDNA segment with similarity
to archeal rhodopsin photoreceptors has been
identified that has enabled the isolation of the
full-length gene nop-1, but its inactivation did
not result in a clear blind phenotype (Bieszke
et al. 1999a), despite the characterization of the
NOP-1 protein as a photoreceptor (Bieszke et al.
1999b; Brown et al. 2001; Bergo et al. 2002). In
addition, the complete genome sequence included
two phytochrome genes and one cryptochrome
gene (Galagan et al. 2003; Borkovich et al. 2004).
Cryptochromes are plant blue-light photoreceptors
(Cashmore 2003; Lin and Shalitin 2003), and
it is surprising that the Neurospora cryptochrome
gene was not identified by a mutation, despite the
extensive search for blind mutants (Linden et al.
1997a). It is possible that Neurospora rhodopsin
and cryptochrome have secondary or redundant
roles, and that a clear blind phenotype will be
observed only with a combination of mutations
in several genes. It is also possible that mutants
with subtle effects in light responses (Linden et al.
1997a) are affected in these genes, or that these
photoreceptors are important for the survival of
Neurospora in nature but are redundant in the
laboratory environment. The observation of light
responses in Neurospora wc-1 and wc-2 mutants,
although controversial, has suggested the presence
of additional photoreceptors (Dragovic et al. 2002).
More striking, however, is the identification of
two genes for phytochromes that are red lightabsorbing
photoreceptors (Schafer and Bowle
Photomorphogenesis and Gravitropism 239
2002), since the light responses identified and
investigated in Neurospora are caused by blue light
(Linden et al. 1997a). Interestingly, a far-red light
effect on DNA stability has been reported. DNA
damage and induction of mutations in conidia of
Neurospora after X-ray treatment were increased
by exposure to far-red light and were decreased by
exposure to red light. The far-red light effect was
reversed by a subsequent red-light exposure (Klein
and Klein 1962). The possibility that Neurospora
phytochromes are responsible for this far-red
light effect deserves further investigation. Red
light might also be involved in circadian clock
regulation.Theamountofblueandredlightin
nature changes throughout the day, with more
red than blue light at dawn and sunset. We can
speculate that red light- and blue light-absorbing
photoreceptors could cooperate to measure the
ratioofredandbluelightreceivedbyNeurospora.
Red light will be high and blue light low at dawn,
red light will decrease throughout the day with
a concomitant increase in blue light, and red light
will increase again as blue light decreases at sunset.
It is possible that this photoreceptor cooperation
will generate two separate inputs into a molecular
mechanism that will measure more precisely the
time of the day for circadian clock regulation.
A role for phytochromes as photoreceptors that
would allow the detection of red-light variations
during the day for circadian regulation has already
been suggested (Hellingwerf 2002). Clearly, the relationship
between red and blue light in circadian
clock regulation deserves further investigation.
2. Aspergillus nidulans
a) Photoconidiation
Asexual development in Aspergillus nidulans
(henceforth, Aspergillus) is a morphological
pathway that culminates with the production
of conidia, and involves a complex regulatory
network of gene regulation and cell differentiation
(reviewed by Adams et al. 1998; see Chap. 14, this
volume). Conidiation in Aspergillus is induced by
light, but other aspects of Aspergillus development
are also influenced by light: Aspergillus produces
hyphae and sexual structures in the dark, and
mainly conidiophores and conidia in the light.
Thus, the ratio of sexual to asexual development is
changed by light.
Light is only effective if it is applied up to 6 h after
conidiation has been induced. Surprisingly, only