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Molecular Pathogens 2012, Vol.3, No.4, 19- 26
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proteins. For example polygalacturonase-inhibiting
protein (PGIP) in plants specifically inhibits
endopolygalacturonases and throws it to the extra-cellular
matrix. This results in the production of large
oligomeric degradation products. These products then
act as elicitors of defense responses of plants (Cervone
et al., 1989).
1 Plant Disease Resistance Mechanisms
Gene for gene interaction (Plant defense system)
between resistance (R) and avirulence (avr) genes
from plant and pathogen respectively is activated to
control resistance (Dangl and Jones, 2001; Jones and
Dangl, 2006). Plant R gene coded protein recognizes
pathogen avr gene product and stimulate hypersensitive
response (HR). Hypersensitive response is basically a
type of programmed cell death that happens at or near
the point of pathogen attack (Morel and Dangl, 1999;
Heath, 2000) which leads to the death of affected plant
cell and restrict the spread of pathogen to the other
parts of the plant. Receptor like proteins are encoded
by several of R genes which have been recognized
(Bent, 1996). Most of the higher plants have innate
defense mechanisms.
Activation of inducible defense response leads to the
restriction of pathogen spread in incompatible
interaction and resulting in system acquired resistance
(SAR). These proteins Possess antifungal activity and
hit the cellular components of the pathogen cell wall
e.g. chitinases, β-1,3-glucanases and fungal membrane
permeability is also affected by thaumatin like
proteins (TLP) (Linthorst, 1991). Three chitinases
isoforms were found to be induced simultaneously in
maize after inoculation of maize seed by the fungus
Fusarium moniliforme (Cordero et al., 1994). In wheat
7 constitutively expressed and 3 pathogen induced
chitinases isoforms have been reported by Botha et al.,
(1998). A chitinase (33 kD) was isolated in wheat and
expressed in E.coli to demonstrate its antifungal role
in vitro (Singh et al., 2007).
2 Secondary Metabolites and Stress Resistance
In response to infection or abiotic stress some
secondary metabolites collectively termed as
phytoalexins are also produced. They are more than
350 in number reported from plant families. They are
not the ultimate solution of fungal pathogens although
generated in all parts of plants including vegetative
and reproductive. They control the pathogens to some
extents. Most of the fungal pathogens have
detoxifying mechanism for phytoalexins (Kuć, 1995;
Etten et al., 1995). Phenylpropanoid pathway depends
basically on phenylalanine ammonia-lyase (PAL) for
the production of phytoalexins and salicylic acid
which are the key pathways in plant defense response.
In order to check the potential of it, PAL gene was
transferred into tobacco. The resulting transgenics
were resistant to different Phytophthora but the
transgenic themselves were showing stunted growth
(Way et al., 2002).
Thionins play their role by creating hypersensitive
death of the cell and do it by forming cations ion
channels which bind to the Phosphatidylserine group
of lipid bilayer resulting in permeability and oxidative
burst of cell membrane. in vitro studies showed an
inhibited growth of 20 different fungi including
Botrytis cinerea, Fusarium , Phytophthora infestans
and Rhizoctonia solani when they were exposed to
thionins (Cammue et al., 1992; Molina et al., 1993;
Bussing et al., 1998; Hughes et al., 2000; Hilpert et al.,
2001; Coulon et al., 2002). Plant defensins are another
class of small cystein-rich proteins and they are
structural and functional homologues of insect and
mammalian proteins that have well established roles
in host defense. Definsins act upon fungal cell wall
and permealize their cell membrane by interacting
with sphingolipids but not with phosphoglycerolipids.
Definsins are constitutively produced in flower and
seeds and are induced upon fungal infection in other
plant parts. In the known plant defensins many are
related to insect’s defensins containing eight
disulphide linked cysteins (Thevissen et al., 2000).
3 Chitinases and Chitosanases in plant
defense response
Plants adopt a variety of ways and mechanisms to
defend against pathogens and many pathways are
involved in the development of disease resistance. The
defense response/PR genes function in a variety of
ways to hinder fungal infection and the expression of
these genes in transgenic plants has been shown to
augment fungal resistance (Muehlbauer and Bushnell,