Biomedical Engineering Reference
In-Depth Information
A knowledge of the genes involved in pathogenicity and virulence
of these fungi can facilitate their biotechnological exploitation—either for
strain improvement for use as biopesticide or in transgenic plant approach
as the Bt toxins.
The entomopathogenic fungal infection comprises two main events—
pre and post penetration of the insect. The pre penetration event includes
the attachment and adhesion of the fungal propagule to the host (insect)
surface, sensing of the host, formation of the infection peg and appressorium
and breaching of the insect cuticle. Post penetration incident involves
evasion of host or a battle with the host immune system, utilization of
the nutrients in the host and growth and spread in the insect, secretion
of toxins all of which fi nally cause death of the insect. Subsequently, the
fungus grows out of the insect and sporulates on the cadaver. A comparative
transcriptome analysis through EST and cDNA microarray analysis in
Metarhizium anisopliae
showed that different sub set of genes are expressed
on the cuticle and hemolymph (Wang and St. Leger 2005). The genes
involved in these two events are described below. A few fungal molecules
were proven critical to the outcome of infection. These molecules have been
classifi ed as penetration effectors, toxins, enzymes that reduce host defense,
transporters that protect the fungus from host defenses and components
of signal transduction pathways that enable the fungus to sense the host
environment (Pedrini et al. 2007).
GENES INVOLVED IN PRE-PENETRATION EVENTS
(GETTING SET FOR INFECTION)
Adhesion to the Host
For most entomopathogenic fungi, location of the host is a random event
aided by wind or water and the spores attach passively to the insect.
Fargues (1984) proposed adhesion to occur in three successive steps: (a)
adsorption of the fungal propagule to the cuticular surface, (b) adhesion
or consolidation of the interface between the fungal propagule and the
epicuticle (c) fungal germination and development on the insect cuticular
surface, until appressorium is developed to start the penetration stage.
The conidia of
B. bassiana
have an outer layer fabricated from interwoven
fascicles of hydrophobic rodlets; the rodlet layer is unique to the aerial
conidia and is not found on blastospore or the vegetative cells (Boucias
et al. 1988, Bidochka et al. 1995). The rodlet layer is constituted by an
amphipathic hydrophobin protein containing eight cysteine residues in
a conserved pattern (Bidochka et al. 1995). It is believed (Boucias et al.
1988, Wang and St. Leger 2007a) that adhesion of the conidia to the insect
cuticle happens due to non-specifi c hydrophobic forces exerted by the
