A. Fungal Differentiation
Filamentous fungi of the Dikarya germinate
from single spores and initially grow in long,
tubular vegetative hyphae by tip extension
(Riquelme et al. 2018). The Spitzenko ¨rper,
which is located at the hyphal tip, acts as a
dynamic center for the organization and supply
of the vesicles required for material transport.
Hyphae fuse through anastomosis tubes at the
tips and form branched two dimensional networks, called mycelia. When a certain state
of developmental competence is reached, the
fungus can respond to external signals to proliferate through either asexual or sexual differentiation (Fig. 8.1a). In asexual reproduction,
the fungus produces spore-forming structures
called conidiophores, which carry the uninuclear, mitotic-derived asexual spores called
conidia (Adams et al. 1998). These airborne
spores are distributed by the wind. In sexual
reproduction, ascomycetes produce sexual
spores called ascospores in a sac-shaped
ascus.
1 In most species, each ascus carries
eight spores which are formed by meiosis and
subsequent mitotic division. The asci usually
form within the protecting fruiting bodies,
which can be closed and spherical (cleistothecia), closed and flask-like (perithecia), or open
and cup-shaped (apothecia) and which serve as
overwintering structures in the soil (Po ¨ggeler
et al. 2018). Whether asexual or sexual structures are formed depends strongly on the environmental conditions, such as nutrients, light,
temperature, or oxygen availability. Fungi
change their lifestyle also during the infection
process. Whereas some fungi can gain entry
into the host without forming specialized structures, there are many plant pathogenic fungi
that produce adhesion and penetration structures, such as appressoria and hyphopodia.
These penetration organs form tiny infection
hooks and penetrate the host using turgor pressure and/or by secreting large amounts of plant
cell wall-degrading enzymes (Lo Presti et al.
2015).
B. Fungal Secondary Metabolism
Whether a fungus initiates a differential program depends not only on environmental conditions but also on endogenous factors such as
the formation of primary or secondary metabolites including pheromones. Metabolic programs are tightly connected with morphogenic
differentiation through sophisticated signal
sensing and transduction mechanisms as well
as transcriptional networks. Whereas primary
metabolites (also called central metabolites) are
essential for the growth of an organism, secondary metabolites (also called specialized
metabolites or natural products) are dispensable but offer advantages in the natural habitat
of their producer. Usually, primary metabolites
are the precursors for the biosynthesis of secondary metabolites, and the biosynthesis
occurs during developmental or aging processes (Bayram et al. 2016). Several secondary
metabolites directly related to development are
known and show the close connection between
these two processes in fungi (Fig. 8.1a). These
secondary metabolites are involved, for example, in the initiation and regulation of development and in protection and survival. They are
also required for communication, competition,
and defense against other microorganisms as
well as for virulence in plant and animal infections (Macheleidt et al. 2016; Ku ¨nzler 2018).
Although the biological benefit of most secondary metabolites has not yet been understood, their biological activities have been
used for a very long time. Already our ancestors
used the healing extracts of fungi and plants as
medicine and nowadays many fungal compounds serve as lead structures for the synthesis of new drugs (Newman and Cragg 2012).
Examples of secondary metabolites with pharmaceutical relevance from ascomycetes are the
antibiotic penicillin, the anticancer drug taxol,
the cholesterol-lowering drug lovastatin, or the
immunosuppressant ciclosporin. However, the
biological activities of secondary metabolites
can also be harmful for us. Fungi synthesize
strong carcinogens and mycotoxins such as
aflatoxins produced by Aspergillus and trichothecenes produced by Fusarium (Gerke
and Braus 2014).
1 Ascus ¼ skin bag.
174
J. Gerke et al.
Filamentous fungi of the Dikarya germinate
from single spores and initially grow in long,
tubular vegetative hyphae by tip extension
(Riquelme et al. 2018). The Spitzenko ¨rper,
which is located at the hyphal tip, acts as a
dynamic center for the organization and supply
of the vesicles required for material transport.
Hyphae fuse through anastomosis tubes at the
tips and form branched two dimensional networks, called mycelia. When a certain state
of developmental competence is reached, the
fungus can respond to external signals to proliferate through either asexual or sexual differentiation (Fig. 8.1a). In asexual reproduction,
the fungus produces spore-forming structures
called conidiophores, which carry the uninuclear, mitotic-derived asexual spores called
conidia (Adams et al. 1998). These airborne
spores are distributed by the wind. In sexual
reproduction, ascomycetes produce sexual
spores called ascospores in a sac-shaped
ascus.
1 In most species, each ascus carries
eight spores which are formed by meiosis and
subsequent mitotic division. The asci usually
form within the protecting fruiting bodies,
which can be closed and spherical (cleistothecia), closed and flask-like (perithecia), or open
and cup-shaped (apothecia) and which serve as
overwintering structures in the soil (Po ¨ggeler
et al. 2018). Whether asexual or sexual structures are formed depends strongly on the environmental conditions, such as nutrients, light,
temperature, or oxygen availability. Fungi
change their lifestyle also during the infection
process. Whereas some fungi can gain entry
into the host without forming specialized structures, there are many plant pathogenic fungi
that produce adhesion and penetration structures, such as appressoria and hyphopodia.
These penetration organs form tiny infection
hooks and penetrate the host using turgor pressure and/or by secreting large amounts of plant
cell wall-degrading enzymes (Lo Presti et al.
2015).
B. Fungal Secondary Metabolism
Whether a fungus initiates a differential program depends not only on environmental conditions but also on endogenous factors such as
the formation of primary or secondary metabolites including pheromones. Metabolic programs are tightly connected with morphogenic
differentiation through sophisticated signal
sensing and transduction mechanisms as well
as transcriptional networks. Whereas primary
metabolites (also called central metabolites) are
essential for the growth of an organism, secondary metabolites (also called specialized
metabolites or natural products) are dispensable but offer advantages in the natural habitat
of their producer. Usually, primary metabolites
are the precursors for the biosynthesis of secondary metabolites, and the biosynthesis
occurs during developmental or aging processes (Bayram et al. 2016). Several secondary
metabolites directly related to development are
known and show the close connection between
these two processes in fungi (Fig. 8.1a). These
secondary metabolites are involved, for example, in the initiation and regulation of development and in protection and survival. They are
also required for communication, competition,
and defense against other microorganisms as
well as for virulence in plant and animal infections (Macheleidt et al. 2016; Ku ¨nzler 2018).
Although the biological benefit of most secondary metabolites has not yet been understood, their biological activities have been
used for a very long time. Already our ancestors
used the healing extracts of fungi and plants as
medicine and nowadays many fungal compounds serve as lead structures for the synthesis of new drugs (Newman and Cragg 2012).
Examples of secondary metabolites with pharmaceutical relevance from ascomycetes are the
antibiotic penicillin, the anticancer drug taxol,
the cholesterol-lowering drug lovastatin, or the
immunosuppressant ciclosporin. However, the
biological activities of secondary metabolites
can also be harmful for us. Fungi synthesize
strong carcinogens and mycotoxins such as
aflatoxins produced by Aspergillus and trichothecenes produced by Fusarium (Gerke
and Braus 2014).
1 Ascus ¼ skin bag.
174
J. Gerke et al.