Growth, Differentiation and Sexuality

242 L.M. Corrochano and P. Galland
not function as a transcriptional regulator. The inhibition
of mycelial growth by light may promote
the growth of Tuber in the soil (Ambra et al. 2004).
The increase of branching by light in Neurospora
(Lauter et al. 1998) and in Tuber (Ambra et al. 2004)
mayberesponsibleforthecompactaspectoftheir
colonies when they grow in the light.
Colletotrichum trifolii is a plant pathogen causing
anthracnose in alfalfa. Light promotes hyphal
branching in C. trifolii (Chen and Dickman 2002).
The C. trifolii gene tb-3 is the homolog of the Neurospora
cot-1 gene for a Ser/Thr protein kinase. Like
cot-1, tb-3 is also induced by light but TB3, unlike
COT-1, may act as a transcriptional regulator for
hyphal branching, as suggested by the nuclear localization
of TB3 and the presence of polyglutamine
repeats that serve as transcriptional activation domains
in yeast (Chen and Dickman 2002).
B. Zygomycota
1. Phycomyces blakesleeanus
a) Photophorogenesis
The zygomycete Phycomyces blakesleeanus (Cerdá-
Olmedo and Lipson 1987; Cerdá-Olmedo 2001)
develops two types of fruiting bodies (sporangiophores)
of very different size, macrophores
and microphores (Thornton 1972). Blue light
stimulates macrophorogenesis and inhibits microphorogenesis
(photophorogenesis; Fig. 13.1;
reviewed in Corrochano and Cerdá-Olmedo
1991, 1992), and has a prominent role on other
aspects of Phycomyces blakesleeanus (henceforth,
Phycomyces) biology, such as the phototropism of
the macrophores and the stimulation of β-carotene
biosynthesis in the mycelium (Galland 1990, 2001;
Bejarano et al. 1991).
Sporangiophore development in Phycomyces is
highly synchronized. Only vegetative mycelium is
detected at the age of 48 h, when the mycelium is
ready to develop sporangiophores and is sensitive
to blue light. Sporangiophores appear soon thereafter,
and can be easily collected at the age of 72 h.
The maximum number of sporangiophores is obtained
at the age of 96 h, and remains constant for
several days. The final numbers of macrophores
and microphores in the cultures depend on the
blue-light fluence applied at the age of 48 h (Corrochano
and Cerdá-Olmedo 1988, 1990). The effect
of blue light on sporangiophore development
follows a two-step stimulus-response curve with
thresholds at 10 −4 and 1 J/m 2 , which suggests the
presence of different photosystems optimized to
operate at different light fluences (Fig. 13.2; Corrochano
and Cerdá-Olmedo 1990).
b) Photophorogenesis Mutants
Mutants defective in macrophore phototropism,
genotype mad, have been isolated and some of
these, madA and madB, are also defective in
photophorogenesis and other light responses, an
indication that their protein products are required
for all light responses in Phycomyces (reviewed in
Cerdá-Olmedo and Corrochano 2001). A search for
mutants in photomicrophorogenesis, pim mutants,
identified three with a higher threshold for this
photoresponse. The pim mutants had a normal
phototropism and photocarotenogenesis, except
for one that showed a higher threshold for photocarotenogenesis
(Flores et al. 1998). A madJ
mutant, blind for phototropism, had a higher
threshold for photomicrophorogenesis whereas
its photomacrophorogenesis and photocarotenogenesis
remained normal (Flores et al. 1998).
These results suggest the presence of a complex
photosensory system with separate transduction
pathways for photomicrophorogenesis and photomacrophorogenesis,
as already indicated by
differences in their action spectra (Corrochano
et al. 1988).
c) Light Transduction Chain
The molecular basis of photophorogenesis in
Phycomyces remains largely unknown. Experiments
with chemical inhibitors have shown that
heterotrimeric G proteins and protein phosphorylation
may play a role in the transduction pathway
for photophorogenesis (Tsolakis et al. 1999, 2004).
Chemical analysis and inhibitor treatments have
suggested that pteridines and NO synthase participate
in blue-light signaling for photophorogenesis
(Maier and Ninnemann 1995; Maier et al. 2001).
Light should activate gene transcription at
the moment of sporangiophore development.
The role of differential gene expression during
photophorogenesis in Phycomyces was investigated
with a method based on the polymerase chain
reaction with arbitrary primers (Corrochano
2002). The method facilitated the visualization
of Phycomyces cDNAs differentially expressed
during sporangiophore development, or after light
induction in 48 h-old mycelia when the fungus
is sensitive to light. A segment of a cDNA from