Basic HTML Version
Molecular Plant Breeding 2012, Vol.3, No.7, 63
-
79
http://mpb.sophiapublisher.com
72
exhibit certain unique structural characteristics such as
relatively high molecular mass and chemical stability,
which is mainly due to specific physicochemical
properties such as length and branching of the carbon
chains (usually between 20 to more than 40 carbons),
as well as the position and the kind of functional
groups and double bonds. The most abundant
components are long chain HC, wax esters, fatty
alcohols and free or sterified fatty acids.
The insect epicuticle contains lipoproteins, fats, and
waxy layers which would be barriers to EPF without
the action of lipases and lipoxygenases, as some of
these structures have anti-fungal activities, and EFP
can’t use them as substrates (Khachatourians, 1996).
In addition, some chains of saturated fatty acids even
inhibit the growth of some EPF. Lord et al (2002)
showed a role for the lipoxygenase pathway through
eicosanoid-mediated cellular immune response to the
B. bassiana
. James et al (2003) demonstrated that
conidial germination of
B. bassiana
and
P. fumosoroseus
are affected by cuticular lipids and silverleaf whitefly
(
B. argentifolii
). The whitefly nymphs produce thick
coating of long-chain wax esters affecting spore
germination.
4.4 The function of phospholipases in cuticle
penetration
Since lipids represent major chemical constituents of
the insect cuticle, enzymes capable of hydrolyzing
these compounds, such as phospholipases, could be
expected to be involved in the cuticle disruption
processes that occur during host invasion. Phospho-
lipases are a heterogeneous group of enzymes that are
able to hydrolyze one or more ester linkages in
glycerophospholipids. The action of phospholipases
can result in the destabilization of membranes, cell
lysis and the release of lipid second messengers
(Ghannoum, 2000). These enzymes are categorized
according to the location of the ester link that is
cleaved (Figure 2). Although phospholipase B (PLB)
refers to an enzyme that can remove both
sn
-
1
and
sn
-
2
fatty acids, this enzyme also has lysophospho-
lipase-transacylase activity.
Extracellular phospholipases have been implicated as
pathogenicity factors for bacteria, rickettsiae and
Figure 2 Sites of action of phospholipases
Note: A1, A2, B, C and D indicate cleavage sites of the corres-
ponding phospholipases (PLA1, PLA2, PLB, PLC and PLD)
protozoa. The type of phospholipase involved in
virulence varies with the organism. For example,
C.
perfringens
(Alape-Giron, 2000) secretes a phospholi-
pase C (PLC), whereas
T. gondii
secretes a phosphor-
lipase A (PLA). The importance of these enzymes,
especially PLB, for virulence has so far only been
verified in medically important fungi. PLB was
secreted by different clinically important fungal
species such as
Candida albicans
(Mukherjee, 2001),
Aspergillus fumigatus
and
Cryptococcus neoformans
.
The role of PLB in the pathogenicity of entomopatho-
genic fungi remains to be determined, even in the
best-studied species
M. anisopliae
.
5 Advantages of using fungi as insecticides
The advantages of using fungi as insecticides are: (1)
They have high degree of specificity for controlling
pest without affecting beneficial insect predators and
non-harmful parasites. (2) They have no hazard effects
on environment or the health of mammals which is
normally affected by chemical insecticide applications.
(3) They have different ways of infection; hence insect
resistance cannot be developed and they can be used
as prolonged pest control. (4) They have genes for
secretion of insect toxins; hence they have high
potentials for further development by biotechnological
research. (5) Some of them have endophytic capability;
hence they can play important roles in the activation