Biomedical Engineering Reference
In-Depth Information
weakens the host immune response (Xia et al. 2001). The insect hemolymph
being solute rich with a high osmotic pressure, aids in immune defense
(Charnley 2003). In
M. anisopliae
strain 2575, an osmosensor was
found which mediates cellular responses to high osmotic pressure in
hemolymph (Wang et al. 2008). In the cDNA library of
M. anisopliae
cultured in hemolymph, an EST was found which was similar to choline
dehydrogenase of
Aspergillus terreu
(Zhang and Xia 2009).This enzyme
catalyzes formation of glycine-betaine from choline. Glycine-betaine is an
osmolyte and is believed to provide osmotic balance with the environment
and protect enzymes and other cellular components from high osmotic
pressure (Pollard and Wyn Jones 1979). A collagenous protein (MCL1) has
been detected in
M. anisopliae
cultured in hemolyph (Wang and St. Leger
2006). It is required for evading insect immune responses. MCL1 contains
a collagenous domain to mask antigenic structural components of the
fungal cell wall and functions as an anti-adhesive protective coat against
phagocytosis and encapsulation from hemocytes (Wang and St. Leger
2006). In
M. anisopliae
an EST was found in the cDNA library made from
the fungus in the hemolymph which was similar to pisatin demethylase, a
putative detoxifying enzyme in
Nectria haematococca
(Hirschi and VanEtten
1996, Zhang and Xia 2009). RT-PCR analysis confi rmed that this gene was
highly up-regulated in hemolymph, especially at the early stages during
colonization of host hemolymph (Zhang and Xia 2009). It is presumed that
this enzyme allows fungal growth within the host despite the presence of
the host's defensive compounds (Zhang and Xia 2009).
Toxins
Many toxins have been reported in
B. bassiana
and
M. anisopliae
. Among
them, the low molecular weight compounds—beauvericin, enniatins,
isarolides and bassianolide have been demonstrated to be insecticidal
(Gupta et al. 1994; Castlebury et al. 1999). Beauvericin, an ionophoric cyclic
peptide of six amino acids synthesized by the multifunctional enzyme
beauvericin synthetase (without the aid of a ribosome—non ribosomal
peptides) was found toxic to brine shrimp and mosquito larvae (Hamill
et al. 1969). At least twenty six different destruxins have been identifi ed
in
Metarhizium anisopliae.
Destruxins are secondary metabolites; fungi
are small peptides—cyclic depsipeptides. Destruxins A, B and E were
predominant in
M. anisopliae
(Gupta et al. 1989, Amiri- Besheli et al. 2000).
Destruxins A and E are more toxic than the others (Dumas et al. 1994).
These toxins have often been implicated as the cause of death of insects
infected with
M
.
anisopliae
(Butt et al. 1994, Vestergaard et al. 1995). A
direct correlation between destruxin production and pathogenicity was
not always possible as strains which produced low levels of these toxins
