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Molecular Pathogens 2012, Vol.3, No.4, 19- 26
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depolymerization of cell-wall by the combined action
of chitinases and glucanases could kill fungi in vitro
(Mauch et al., 1988).
Chitin is abundantly occurring natural polymer of
β-1,4- linked N-acetylglucosamine (GlcNac). Chitin
hydrolysate may be utilized as a carbon and nitrogen
source in the synthesis of single-cell proteins
(Revah-Moiseev and Carroad, 1981). These two
enzymes act synergistically in the partial degradation
of fungal cell-walls, and reported that these two
enzymes in combination strongly suppress growth of
various fungi like those suppressed by chitinase or
β-1,3- glucanase alone (Vögeli et al., 1988). A similar
enhance in the behavior of these enzymes is significant
for their finest role in plant defense (Pan et al., 1991).
It is also reported that chitinase is synthesized by a
number of microbes, those living in chitin-containing
habitats like soil, sediments and marine environment
(Gooday, 1990). In plants, chitinases are present
constitutively and are induced systematically also
upon treatment with biotic as well as abiotic inducers
(Viswanathan and Samiyappan, 2001). PR-proteins
accumulation is related with SAR in plants (Ryals et
al., 1996). Chitinase and β-1,3 -glucanase have not
only the potential to hydrolyze cell components like
chitin and β-1,3-glucan.They release elicitors from
the walls of fungi, which in turn arouse various
defense responses in plants (Ren and West, 1992).
The first report on developing fungus-resistant
transgenics came in 1991. Broglie et al., (1991)
constitutively expressed bean chitinase gene in
tobacco and Brassica napus and the plants showed
enhanced resistance to Rhizoctonia solani . Since
then there have been a number of reports on
transgenics developed by constitutively expressing
PR-protein genes. Various PR proteins may be acting
synergistically in vivo and reveal improve restriction
of fungal growth and development when tested in
combinations in vitro. Transgenic plants expressing
more than one PR protein genes in a constitutive
manner were developed. Desire gene introduction
with host plant under constitutively high expressing
promoter may cause silencing of the transgene as
well as its endogenous homologue leading to a high
proportion of progeny losing its enhanced resistance.
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.
Over-expression of genes encoded for chitinase, did
not show resistance for chitin deprived fungi. Fungus
can alter its cell wall by biosynthesis of chitosan and
glucan in place of chitin and may be pathogenic. It
can evolve mechanisms for detoxification of certain
phytoalexins. Sexually reproducing fungi may build
up resistance rapidly. Plants have passive defense
lines such as cell walls, wax layers and chemical
barriers against pathogens. When the pathogens
invade this first defense line, there is also a second
defense line, which is mounted by proteins encoded
by specific resistance (R) genes. This line of defense
is best attributed genetically by the gene for gene
model. It entails a pathogen protein encoded by avr
gene to be identified by a plant protein encoded by a
resistance (R) gene. This stimulates an array of
defense mechanisms, like the hypersensitive response.
During the last decade above 30 resistance genes
which confer resistance against a variety of pathogens,
like bacteria, nematodes, viruses fungi and even
aphids have been cloned from both monocots and
dicots. These genes for resistance have homology
with each other and their products are also highly
similar. R proteins have leucine-rich repeat (LRR)
domain. H2O2 revealed its detrimental effect on the
growth and development of microbes. An enzyme,
glucose oxidase (GO), is present in the microbes
causes the oxidation of β-D-glucose, producing
gluconic acid and H2O2. GO has not been found in
animals and plants.
Synthesis of lytic enzymes, like β-1,3 -glucanase and
chitinase by many PGPR strains possess principal
antagonistic characteristics. Chitinase and β- 1,
3 -glucanase can work in defense against various
fungal pathogens. These lytic enzymes have
hydrolytic activity and breakage the cell-wall of
various pathogenic fungi. Growth suppression of
fungi entails the occurrence of β-1,3 -glucanase and
chitinase in combination. Actions of enzymes are