Growth, Differentiation and Sexuality

3. Trichoderma
a) Photoconidiation
Conidiation in the ascomycete Trichoderma is induced
by a pulse of blue light (Horwitz et al. 1990).
In addition, light inhibits hyphal growth, blue and
red light being equally effective (Casas-Flores et al.
2004). In Trichoderma atroviride,thegenephr1 for
the DNA repair enzyme photolyase is transiently
expressed in mycelia and conidiophores after
illumination. The gene is also developmentally
regulated, as the mRNA accumulates in the dark
in conidiophores during conidial development.
The threshold for phr1 photoactivation is about
10 μmol/m2 , which is similar to the threshold for
photoinduction of conidiation, suggesting that
both photoresponses could use similar photoreceptors
(Berrocal-Tito et al. 2000). However, phr1
photoinduction is not blocked in non-conidiating
mutants or in a photoreception mutant, a result
that suggests that gene photoactivation does not
require a fully active conidiation pathway, and that
gene photoactivation and conidial photoinduction
may not share all the elements of their respective
phototransduction pathways (Berrocal-Tito et al.
1999). In Trichoderma, blue light is used as a signal
to prevent the harmful effect of ultraviolet light
by inducing the development of resistant conidia
and the expression of the photolyase gene phr1
(Berrocal-Tito et al. 1999, 2000). In addition,
sporulation induced by light represses the gene
for glyceraldehyde-3-phosphate dehydrogenase
(Puyesky et al. 1997), and stimulates the expression
of specific proteins (Puyesky et al. 1999).
The primary effect of light in the related fungus
Trichoderma viridae was investigated with cell-free
extracts that promoted the phosphorylation of
two proteins after incubation with light (Gresik
et al. 1989). Light also promoted dephosphorylation
of a 33-kDa protein in Neurospora extracts
(Lauter and Russo 1990), and phosphorylation
of a 15-kDa protein (Oda and Hasunuma 1994).
The relationship between light and phosphorylation
was further supported by the discovery of
light-dependent phosphorylation and dephosphorylation
of the Neurospora photoreceptor
WC-1 and its partner WC-2 (Talora et al. 1999;
Schwerdtfeger and Linden 2000, 2001). We can
speculate that phosphorylation will play a similar
role in Trichoderma photoreception.
Two genes with similarities to the Neurospora
wc genes have been isolated in Trichoderma
atroviride. These genes, named blr-1 and blr-2,are
Photomorphogenesis and Gravitropism 241
required for the photoinduction of conidiation,
and for the photoinduction of phr1, but not for
the light-dependent inhibition of hyphal growth
(Casas-Flores et al. 2004). Curiously, mutants in
any of the blr genes have a strong light-dependent
inhibition of hyphal growth. The phenotype of
the blr mutants and the domains present in the
BLR proteins indicated that they should play
a major role as the photoreceptor system for the
photoinduction of conidia in Trichoderma,assuggested
for WC in Neurospora. Thenovelred-light
inhibition of hyphal growth that is exacerbated
in the blr mutants suggested the presence of
additional Trichoderma photoreceptors showing
complex interactions with the BLR photoreceptor
(Casas-Flores et al. 2004).
A gene with similarities to the Neurospora photoreceptor
gene vivid has been isolated in Hypocrea
jecorina (Trichoderma reesei).Thegeneisinduced
by the presence of cellulose, suggesting a connection
between light reception and carbon utilization
in this fungus (Schmoll et al. 2004).
4. Other Ascomycetes
The ascomycete Paecilomyces fumosoroseus is an
entomopathogenic fungus that invades insects
after conidial contact and germination with the
insect cuticle. Blue light has a dual role in conidial
production, with a stimulatory and inhibitory
effect depending on the fluence rate used. When
the fungus is grown in the dark, only vegetative
hyphae are present. However, 5 min of blue light is
enough to induce conidial development (Sánchez-
Murillo et al. 2004). A period of competence to
light has been observed, since light is effective
only when applied between 72 and 96 h of growth.
The blue-light threshold is 180 μmol/m 2 , and
the maximum conidial yield was obtained with
540 μmol/m 2 . Higher fluence rates decreased
conidiation, suggesting the presence of a complex
photosensory system (Sánchez-Murillo et al.
2004).
The ascomycete Tuber borchii is appreciated
for the production of trufles, ascocarps developed
as symbiotic mycorrhizae with host plants. Blue
light inhibits colony growth in Tuber and in Neurospora.
The effect of light in colony growth in Neurospora
requires the photoreceptor WC-1 (Lauter
et al. 1998; Ambra et al. 2004). The Tuber homolog
of wc-1 is induced by light, and its protein product
may function as a photoreceptor but lacks a polyglutamine
activator domain, suggesting that it may