Biomedical Engineering Reference
In-Depth Information
(created by insertional mutagenesis) in B. bassiana was bioassayed on aphids,
the pH value on the insect cuticles remained acidic (due to accumulated
organic acids) and there was a decrease in virulence (Jin et al. 2010).
A multitude of enzymes have been reported to be secreted by the
fungus for penetration of the insect epidermis. There is some evidence
that cuticle penetration and degrading enzymes are regulated by nutrient
source of carbon (C) and nitrogen (N) repression/derepression. Protease
PR1A production was found to be infl uenced by both C and N derepression
and specifi c induction by an as-yet unidentifi ed proteinaceous component
of insect cuticle (St. Leger et al. 1992a, Paterson et al. 1993). Other enzymes
involved in penetration of the insect cuticle are also reported to be regulated
by C and N repression, including the trypsin-like protease PR2 (Smithson
et al. 1995). An EST belonging to the Nmr family which is implicated in
nitrogen metabolism repression has been found in B. bassiana cultured on
cuticle medium of four different insects but not when cultured on synthetic
medium (Khan et al. 2007) iterating the importance of repression of nitrogen
metabolism in infection.
M. anisopliae has been successfully genetically engineered for over
expression of cuticle degrading enzymes, resulting in a considerable
shortening in the time to kill agronomic pests (St. Leger et al. 1996 b, Hu
and St. Leger 2002, Fang et al. 2005). Following is an account of the various
enzymes used by the entomopathogenic fungi for negotiating the cuticle.
The insect cuticle is made of lipids, proteins and chitin. Enzymes that act
on these substrates have been implicated in pathogenicity of entomogenous
fungi.
Proteases
Distinct extracellular serine proteases such as subtilisin-like proteases,
trypsin-like proteases and metalloproteases and several families of exo-
acting peptidases are believed to be important for host cuticle degradation
(Bagga et al. 2004, St. Leger et al. 1997). The most important cuticle
degrading enzymes are the subtilisin-like Pr1 and Pr1J (Freimoser et al.
2005). M. anisopliae Pr1A is the most studied fungal subtilisin protease and
the only one generally accepted to have a role in insect cuticle breach (Shah
et al. 2005). It is the predominant protein produced during insect cuticle
degradation and its transcripts are 10 times more abundant than Pr1J, the
second most expressed protease (Bagga et al. 2004). The Pr1A gene from
M. anisopliae was cloned and sequenced (Zhang et al. 2008). The Pr1 and
Pr2 genes were found to be highly expressed on cuticle medium in the
entomopathogenic fungus Nomuraea rileyi as well (Nunes et al. 2010). N.
rileyi does not form appressoria and produces only a mucilaginous mass
around the germ tube and completely relies on enzymatic breaching
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