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Molecular Plant Breeding 2012, Vol.3, No.7, 63
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toxic activity against pest, like hirsutellin A (Enrique
and Alain, 2004). However, the major obstacle
limiting the market share of these fungi as mycoin-
secticides is their slow killing speed and increase in
market share is directly proportional to killing speed
(St Leger and Wang, 2009).
A large number of studies were conducted to
potentiate and improve the virulence of entomopatho-
genic fungi to a greater extent than their individual
activities and thus increasing the market share. The
extensive transcriptomic and genetic study of entomo-
pathogenic fungal infection process revealed that a
number of different genes were involved in the
pathogenicity (Freimoser et al., 2003; Wang et al.,
2005; Cho et al., 2006a, 2006b, 2007) such as
chitinases (Bagga et al., 2004; Fang et al., 2005),
guanine nucleotide-binding proteins and its regulator
(Fang et al., 2007, 2008), adhesin which helps in
attachment of spore (Wang and St. Leger, 2007a), a
perilipin-like protein that regulates appressorium
turgor pressure and differentiation (Wang and St.
Leger, 2007b) and a cell protective coat protein
helping in escaping the pathogen from the host
immunity recognition (Wang and St. Leger, 2006).
Similarly, an increased virulence of the entomopatho-
genic fungi was observed with over-expression of
virulence genes such as subtilisin protease PR1A (St.
Leger et al., 1996), subtilisin protease PII gene
(Ahman et al., 2002), and hybrid chitinase containing
a chitin binding domain (Fan et al., 2007). Wang
and St. Leger (2007c) modified scorpion neurotoxin
peptide, AAIT using
Metarhizium’s
codon preferences
under the control of mc11 gene promoter for fungal
transformation. The LC50 of the transgenic strain
AaIT-Ma549 was reduced 22
-
fold against the
Manduca
sexta
and nine fold against the
Aedes aegypti
(Wang
and St. Leger, 2007c). AaIT-Ma549 was also tested
against the coleopteran coffee pest,
Hypothenemus
hampei
, reducing LC50 by 15.7-fold and the average
survival time by 20% (Pava-Ripoll et al., 2008).
3
Beauveria bassiana
(Clavicipitaceae)
The entomopathogenic mitosporic ascomycete,
Beau
-
veria bassiana
(Bals.) Vuill. is an important natural
pathogen of insects and it has been developed as a
microbial insecticide for use against many major
arthropod pests in agricultural, urban, forest, livestock
and aquatic environments (Charnley and Collins, 2007;
Faria and Wraight, 2007). It has been developed as a
microbial insecticide for use against many major
pests, including
lepidopterans
and
orthopterans
. About
33.9% of the mycoinsecticides is based on
B. bassiana
,
followed by
Metarhizium anisopliae
(33.9%),
Isaria
fumosorosea
(5.8%) and
Beauveria brongniartii
(4.1%)
(Faria and Wraight, 2007); however, to increase the
market share of
B. bassiana
, the killing speed which is
the major hindrance limiting their use as mycoinsec-
ticides should be accelerated, (St Leger and Wang,
2009). As natural strains of these fungi often lack
sufficient virulence or tolerance to adversity (Braga et
al., 2001a, 2001b; Ying and Feng, 2004; Rangel et al.,
2005), genetic manipulation is necessary to improve
their efficacy and ecological fitness (Roberts and St.
Leger, 2004; Fang et al., 2005). The importance of
the
B. bassiana
and
B. brongniartii
can be briefly
understood from table 3 below.
Research studies are carried out to optimize the
preparation and application of fungal inoculums
(Wraight et al., 2001) and (St Leger and Wang, 2009)
to improve the virulence of mycoinsecticides by
genetic modification. Based on this approach, most
studies on the virulence factors of entomopathogenic
fungi have been directed to elucidate the most relevant
cuticle degrading enzymes (Griesch and Vilcinskas,
1998; Khachatourians, 1996; St Leger et al., 1996),
because their over-expression in engineered strains
results in more fungi that are more deadly toward
insects (Fang et al., 2005; St Leger et al., 1996).
Similarly, over-expression of a chitinase gene (Bbchit1)
was found to enhance the virulence of
Beauveria
bassiana
against aphids (
Myzus persicae
) (Fang et
al., 2005). Importantly, different genes involved in
pathogenicity have been characterized from
M.
anisopliae
or
B. bassiana
and some of these genes
include cuticle degrading enzymes (Bagga et al., 2004;
Fang et al., 2005), G protein and its regulator (Fang et
al., 2007), adhesin that mediates spore attachment and
fungal differentiation (Wang and St. Leger, 2007a), a
perilipin-like protein that regulates appressorium
turgor pressure and differentiation (Wang and St.
Leger, 2007b), and a cell protective coat protein