Biomedical Engineering Reference
In-Depth Information
was concluded that
BbMPK1
is essential for penetration of the insect cuticle
both from the outside and from the inside-out (Zhang et al. 2010). An MAP
kinase
Bbhog1
(high osmolarity glycerol) has been reported in
B. bassiana
which is implicated in spore adhesion and appressorium formation (Zhang
et al. 2009). A mutation in the
Bbhog1
gene in
B. bassiana
was found to reduce
virulence (Zhang et al. 2009, Jin et al. 2010). A perilipin was found to regulate
the turgor pressure of infection structures (Wang and St. Leger 2007a).
Growth of the Infection Peg and Penetration of the
Fungus into the Insect
Breaching of the host cuticle
The insect cuticle is the fi rst barrier for the fungus. It is formed by several
layers, from outside to inside—the epicuticle, procuticle and epidermis.
The epicuticle is very thin (0.1-3 µm) multilayered hydrophobic lipid
layer and resistant to enzyme degradation (Hadley 1981). It has to
be physically disrupted for the passage of cuticle degrading fungal
enzymes. Recently, a suite of hydrocarbon assimilating enzymes of the
cytochrome P450 family implicated in insect waxy layer degradation has
been identifi ed in
B. bassiana
(Pedrini et al. 2010). Once the epicuticle is
breached, progress by the penetration peg through the cuticle may be
more or less direct through penetrant hyphae; penetrant structures may
also extend laterally (Hajek and St. Leger 1994). Successful infection is
possible only after penetration. A gene that displayed homology to vitamin
H (biotin)/tartrate transporter presumably required for the assimilation
of the lipid substrate was found to effect virulence in
B. bassiana
(Fan et
al. 2011). Biotin is an essential co-factor for some of the enzymes involved
in lipid and fatty acid degradation, with fatty acids produced during the
catabolism of alkanes (Fickers et al. 2005). Thus, biotin transporter may
be a mechanism of scavenging the needed co-factor from the host during
the infection process (Fang et al. 2011).
Extracellular pH is considered a major determinant in expression
of genes essential to the growth, differentiation and virulence in fungal
pathogens of humans, plants and insects (Jin et al. 2010). An alkaline pH
is required for growth of fungi. It has been found that alkalinization of
the insect cuticle is achieved by the fungus itself (Jin et al. 2010). There is
evidence that the transport of acids would change the extracellular acid-
base equilibrium. A gene
BJEN1
coding for a carboxylate transporter has
been discovered in
B. bassiana
which facilitates transport of carboxylates
into cells. Consequently a decrease of extracellular organic acids will lead
to alkalinization of the infected cuticle. Thus, when a mutant of
BbJEN1
