Growth, Differentiation and Sexuality

238 L.M. Corrochano and P. Galland
d) Effect of Light on Sexual Development
The female sexual structures (protoperithecia) of
Neurospora are induced by blue-light illumination
(Degli-Innocenti et al. 1983; see Chap. 16, this volume).
The effect of light on protoperithecia formation
is enhanced if the culture medium lacks
nitrogen. Under these conditions, the formation
of conidia is strongly reduced. If an inhibitor of
DNA methylation (5-azacytidine) is added to the
growth medium, conidiation is induced and the
protoperithecia are inhibited, suggesting a role for
DNA methylation of regulatory genes on the choice
of developmental pathway (Kritsky et al. 2002).
The shape of fertilized perithecia culminates
with a beak that is located at the top when the culture
has been illuminated. However, cultures kept
in the dark form beaks at a random location in each
perithecium. Light-induced polarity of perithecial
beaks requires the product of the wc-1 gene (Oda
and Hasunuma 1997). In addition, the perithecial
beaks show a positive phototropism when illuminated
with blue light (Harding and Melles 1983).
Blue light induces the phosphorylation of a 15kDa
protein (Oda and Hasunuma 1994) that has
been identified as a nucleoside diphosphate kinase
encoded by the ndk-1 gene (Ogura et al. 1999).
Apointmutationinndk-1 reduced the NDK-1 activity
and prevented the effect of light on perithecial
beak polarity, without affecting phototropism (Oda
and Hasunuma 1997; Ogura et al. 2001). These results
suggest that the kinase activity of NDK-1 may
be an element of the signal transduction pathway
for the light regulation of perithecial beak polarity
(Ogura et al. 2001). Other signals may also play
a role in light-induced perithecial beak polarity.
A sod-1 mutant altered in the enzyme Cu, Zn superoxide
dismutase had lost the light-induced polarity
of perithecia, and also showed an enhanced
synthesis of carotenes and a higher expression of
the al genes for the enzymes involved in carotene
biosynthesis. These data indicate that intracellular
reactive oxygen regulated by SOD-1 should have
a role in the light transduction pathway (Yoshida
and Hasunuma 2004). SOD reduces the level of superoxide
anion. The role of oxidative stress in Neurospora
development received support from the
observation that the lack of a catalase in a cat-3
mutant induced carotene synthesis, hyphal adhesion,
and the development of more aerial hyphae
and conidia than in the wild type (Michán et al.
2003). Evidently, superoxide dismutase and catalase
cooperate to reduce oxidative stress. These re-
sults seem to support a proposal that microbial
differentiation is a mechanism to cope with excess
amounts of reactive oxygen in the cell (Aguirre et al.
2005).
e) Photoperiodism in Neurospora
Photoperiodism, the role of day or night length
in biological activity, has attracted little attention
in fungi (Roenneberg and Merrow 2001). In Neurospora,
a photoperiodic response has been described
for sexual and asexual reproduction, maximum
protoperithecial and conidial yields having
been recorded for periodic cycles of 14 and 12 h
of light, respectively, followed by a dark period to
complete a 24-h cycle. Light cycles with shorter
or longer illuminations resulted in lower yields of
reproductive structures, suggesting that the photoperiodic
response did not rely on the amount of
total light exposure. The involvement of the circadian
clock in Neurospora photoperiodism was
shown by the lack of a photoperiodic response in
an frq mutant with a major defect in the circadian
clock (Tan et al. 2004). Carotene biosynthesis
also showed a photoperiodic maximum but, unlike
photoperiodism for reproductive structures,
the maximum was found with cycles of 14–20 h of
light. Light/dark cycles with more hours of light
yielded lower carotene accumulation (Tan et al.
2004). The photoactivation of the genes for the enzymes
required for carotene biosynthesis ceases
after the illumination time has been extended for
a certain period of time, and further incubation in
the dark is required before these genes can be photoactivated
again (Baima et al. 1991; Arpaia et al.
1999; Schwerdtfeger and Linden 2001, 2003). This
phenomenon, dubbed light adaptation, has been
described for other light-activated genes in Neurospora
(Arpaia et al. 1993, 1995; Lauter and Yanofsky
1993), and is modified by mutations in the gene
vivid (Heintzen et al. 2001; Schwerdtfeger and Linden
2001, 2003; Shrode et al. 2001), by inhibitors
or mutations of the protein kinase C (Arpaia et al.
1999; Franchi et al. 2005), and it requires protein
synthesis (Schwerdtfeger and Linden 2001). One
can speculate that light adaptation of gene expression
could play a role in Neurospora photoperiodism.
The requirement of a particular day/night
duration to obtain a sustained gene photoactivation
with different levels of gene expression depending
on day/night duration could be one of the
mechanisms used to measure day or night length
for a biological response. The effect of mutations