Chel et al. 2016; Fischer et al. 2016). The developmental light responses vary considerably
between different fungi. Blue or red light can
promote asexual spore formation in A. nidulans (Fig. 8.1a) but inhibit sporulation in Botrytis cinerea (Tan 1974). Similarly, A. nidulans
sexual fruiting bodies are preferentially formed
in darkness, but Trichoderma reesei forms its
corresponding sexual structures in light (Po ¨ggeler et al. 2018).
Typical fungal photosensory systems
include light sensors specific for different wavelengths from 350 to 650 nm distributed into
four types of light receptor proteins. The
white collar proteins perceive blue light, phytochromes sense red light, opsins are specific for
green light, and cryptochromes react to blue
and ultraviolet (UV) light. The light perception
occurs through binding of the photoreceptor to
a chromophore, which are flavin for blue and
UV light, linear tetrapyrrole for red light and
retinal for green light.
White collar (WC) proteins are blue-light
regulated transcription factors with a GATAtype zinc finger DNA binding domain and
three PAS (Per, Arnt, Sim) domains directly
controlling gene expression. One of the PAS
domains is a LOV (light oxygen voltage)
domain, which is able to bind the chromophore
flavin. In Neurospora crassa, two WC proteins
are present, WC-1 and WC-2 (Wu et al. 2014).
They form a heterodimeric complex through
the PAS domains, which binds to light response
elements in the dark and activates lightinduced transcription after phosphorylation
and dissociation of WC-1. The complex regulates approximately 20% of the N. crassa genes
encoding transcription factors as large hierarchical network responding to light and acting
downstream of the WC complex in light and
circadian clock control (Wu et al. 2014).
Besides the typical WC proteins, blue light
photoreceptors with only one LOV domain
(vivid, VVD) exist that act in fine-tuning the
light response (Malzahn et al. 2010).
N. crassa biosynthesis of carotenoid secondary metabolites and asexual spore formation are dependent on light and controlled by
the circadian clock (Rodriguez-Romero et al.
2010). WC-1 comprises a flavin mononucleotide binding LOV domain for environmental
sensing of light, oxygen, and voltage. The WC1 and WC-2 zinc fingers dimerize to form the
white collar complex transcription factor for
DNA binding. WC-1 and WC-2 complexes are
conserved in numerous fungi with sometimes
several orthologs. WC-1 homologs of pathogenic species are connected to virulence, probably through secondary metabolite production
(Idnurm et al. 2010; Fischer et al. 2016). The
white collar complex of Aspergilli, LreA/LreB
(light response), activates the expression of
the brlA (bristle A) gene encoding the central
transcription factor of the conidiation pathway
in response to light (Ruger-Herreros et al. 2011,
Fig. 8.2). The brlA promoter is repressed by
binding factors such as NsdD (never in sexual
development D) or the velvet domain protein
VosA (viability of spores A) (Ni and Yu 2007).
brlA must be activated through a complex cascade of early genes for proteins which produce
small signal molecules (fluG: fluffy G) or act as
transcription factors (flbB-E: fluffy low brlA BE; sclB: sclerotia-like B). Activation of brlA by
light then activates the downstream pathway
with the genes abaA (abacus) and wetA (wetwhite; Fig. 8.2; Thieme et al. 2018).
Phytochromes are multi-domain photoreceptors that bind linear tetrapyrrole as chromophore at their N-terminal photosensory domain
and have red/far-red light absorbance properties. The tetrapyrrole undergoes a conformational change upon red light induction and
thereby changes its spectroscopic properties
by shifting the absorption maximum toward
far-red light. The photosensory domain consists of a PAS, a GAF (vertebrate cGMP-specific
phosphodiesterases, cyanobacterial adenylate
cyclases, transcription activator FhlA) and a
PHY
(phytochrome-specific
PAS-related)
domain. The further domains are a histidine
kinase domain and a response regulator
domain. Phytochromes seem to be absent
from Mucoromycotina fungi but might be present in some Chytridiomycetes and have been
extensively studied in Aspergilli (Fischer et al.
2016). The A. nidulans phytochrome FphA
interacts with the white collar orthologs LreA
and LreB, corresponding to WC-1 and WC-2,
and the velvet transcription factor VeA (see
8 Coordination of Fungal Secondary Metabolism and Development
181
between different fungi. Blue or red light can
promote asexual spore formation in A. nidulans (Fig. 8.1a) but inhibit sporulation in Botrytis cinerea (Tan 1974). Similarly, A. nidulans
sexual fruiting bodies are preferentially formed
in darkness, but Trichoderma reesei forms its
corresponding sexual structures in light (Po ¨ggeler et al. 2018).
Typical fungal photosensory systems
include light sensors specific for different wavelengths from 350 to 650 nm distributed into
four types of light receptor proteins. The
white collar proteins perceive blue light, phytochromes sense red light, opsins are specific for
green light, and cryptochromes react to blue
and ultraviolet (UV) light. The light perception
occurs through binding of the photoreceptor to
a chromophore, which are flavin for blue and
UV light, linear tetrapyrrole for red light and
retinal for green light.
White collar (WC) proteins are blue-light
regulated transcription factors with a GATAtype zinc finger DNA binding domain and
three PAS (Per, Arnt, Sim) domains directly
controlling gene expression. One of the PAS
domains is a LOV (light oxygen voltage)
domain, which is able to bind the chromophore
flavin. In Neurospora crassa, two WC proteins
are present, WC-1 and WC-2 (Wu et al. 2014).
They form a heterodimeric complex through
the PAS domains, which binds to light response
elements in the dark and activates lightinduced transcription after phosphorylation
and dissociation of WC-1. The complex regulates approximately 20% of the N. crassa genes
encoding transcription factors as large hierarchical network responding to light and acting
downstream of the WC complex in light and
circadian clock control (Wu et al. 2014).
Besides the typical WC proteins, blue light
photoreceptors with only one LOV domain
(vivid, VVD) exist that act in fine-tuning the
light response (Malzahn et al. 2010).
N. crassa biosynthesis of carotenoid secondary metabolites and asexual spore formation are dependent on light and controlled by
the circadian clock (Rodriguez-Romero et al.
2010). WC-1 comprises a flavin mononucleotide binding LOV domain for environmental
sensing of light, oxygen, and voltage. The WC1 and WC-2 zinc fingers dimerize to form the
white collar complex transcription factor for
DNA binding. WC-1 and WC-2 complexes are
conserved in numerous fungi with sometimes
several orthologs. WC-1 homologs of pathogenic species are connected to virulence, probably through secondary metabolite production
(Idnurm et al. 2010; Fischer et al. 2016). The
white collar complex of Aspergilli, LreA/LreB
(light response), activates the expression of
the brlA (bristle A) gene encoding the central
transcription factor of the conidiation pathway
in response to light (Ruger-Herreros et al. 2011,
Fig. 8.2). The brlA promoter is repressed by
binding factors such as NsdD (never in sexual
development D) or the velvet domain protein
VosA (viability of spores A) (Ni and Yu 2007).
brlA must be activated through a complex cascade of early genes for proteins which produce
small signal molecules (fluG: fluffy G) or act as
transcription factors (flbB-E: fluffy low brlA BE; sclB: sclerotia-like B). Activation of brlA by
light then activates the downstream pathway
with the genes abaA (abacus) and wetA (wetwhite; Fig. 8.2; Thieme et al. 2018).
Phytochromes are multi-domain photoreceptors that bind linear tetrapyrrole as chromophore at their N-terminal photosensory domain
and have red/far-red light absorbance properties. The tetrapyrrole undergoes a conformational change upon red light induction and
thereby changes its spectroscopic properties
by shifting the absorption maximum toward
far-red light. The photosensory domain consists of a PAS, a GAF (vertebrate cGMP-specific
phosphodiesterases, cyanobacterial adenylate
cyclases, transcription activator FhlA) and a
PHY
(phytochrome-specific
PAS-related)
domain. The further domains are a histidine
kinase domain and a response regulator
domain. Phytochromes seem to be absent
from Mucoromycotina fungi but might be present in some Chytridiomycetes and have been
extensively studied in Aspergilli (Fischer et al.
2016). The A. nidulans phytochrome FphA
interacts with the white collar orthologs LreA
and LreB, corresponding to WC-1 and WC-2,
and the velvet transcription factor VeA (see
8 Coordination of Fungal Secondary Metabolism and Development
181