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Molecular Pathogens 2012, Vol.3, No.4, 19
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2003). Chitosanase is an enzyme, capable of breaking
the β-1,4 linkages between N-acetyl-D-glucosamine
and D-glucosamine residues in a partially acetylated
fungal cell wall polymer.
Many plants take advantage of this hydrolytic
action as a component of a larger post-attack defense
response when attacked by pathogenic fungi (Agrios,
1997), but these enzymes may also function in
pathogenesis-related (PR) signal transduction. After
hydrolysis with a chitosanase or chitinase, fungal cell
wall fragments are released, which act as elicitors of
plant defense responses such as stomatal closure (Lee
et al., 1999), lignification (Vander et al., 1998;
Moerschbacher et al., 1988), and PR gene induction
(Jabs et al., 1997). The responses elicited by these
molecules depend on the length and degree of acetylation
of the released fragments (Vander et al., 1998).
Many plant species have been transformed with
chitinases (Punja, 2001) and these studies showed
great variability in the antifungal effectiveness among
chitinases from different sources. The
in vitro
antifungal potential of chitinases has been documented
(Collinge et al., 1993; Ji and Kuc, 1996) and the
introduction of chitinase genes into plants under the
control of a constitutive promoter has been found to
augment plant resistance to fungal pathogens in
greenhouse studies (Broglie et al., 1991; Lin et al.,
1995; Tabei et al., 1998) and field trials (Grison et al.,
1996). Transgenic rice plants with constitutive
expression of a rice chitinase gene,
chi11
, under
control of the cauliflower mosaic virus (CaMV) 35S
promoter have been developed (Lin et al., 1995). The
transgenic rice plants and their progeny revealed a
high expression of chitinase and had improved
resistance to sheath blight (Lin et al., 1995).
Chitinase genes have been cloned and the gene
products of chitinase genes have been used in
bioassays against many fungi of economic interest. A
Trichoderma
chitinase was cloned in
E. coli
and the
transformed bacteria were used in irrigation to see the
impact on
S. rolfsii
and a noteworthy reduction was
observed in pathogen population in the irrigated field
(Chet et al., 1993). In comparison with the enzymes of
plant origin, chitinases and (1,3) β-glucanases of
Trichoderma
origin are 100 times stronger and
powerful than their counterparts of plant origin and
are also non toxic to plant tissues even at very high
concentrations (Lorito et al., 1994; Lorito et al., 1996)
and their antifungal activity is boost up synergistically
when used in combination with PR proteins,
biocontrol bacteria and fungicide or toxin (Lorito et al.,
1998; Steyaert et al., 2004). Chitinases from
Trichoderma harzianum
in addition to antifungal
activity have also been revealed to augment tolerance
against salinity and heavy metals in transgenic
tobacco (Dana et al., 2006).
Chitosan is a component of fungal cell wall and
degraded by an enzyme chitosanase, and this enzyme
is a proficient candidate for slowing down the infection
process (Hendrix and Stewart, 2002). Oligomers
produced after the hydrolysis are shorter than
oligomers formed after the hydrolysis of chitin by an
enzyme, chitinase and used in the elicitation of
defense response in the plant system such as stomatal
closure and cell wall lignification. Small oligomers
are stronger elicitors of defense than larger ones
(Vander et al., 1998 and Lee et al., 1999). In most
fungi cell wall, chitin, chitosan, and β-(1,3) glucan
are the major structural polymers. Some part of chitin
is always de-acetylated to convert it into chitosan.
Chitinases are imperative and significant component of
plant defense system (Jones et al., 1986; Collinge et al.,
1993). This group of enzyme hydrolyses β-(1,4)
linkages endolytically in chitin molecules (Cabib,
1987). These enzymes are present naturally in fungi
as well as in plants but in fungi they play a vital role
in cell division and differentiation and help
saprophytic and mycoparasitic fungi to get their food
(Cabib, 1987; Kuranda and Robin, 1991). Literature
supports the Co-expression of chitinase and
chitosanase to enhance plant defense against fungal
pathogens and proved their synergistic work (Ayoo et
al., 2011; Ahmad, 2009).
4 Synergistic Effect of PR Genes Proteins
and Antifungal Potential
Production of various cell wall degrading enzymes of
various classes by biocontrol fungi
Gliocladium virens
and
Trichoderma harzianum
suppresses germination
of spores of
Botrytis cinerea
in vitro
assay (Lorito et