Biomedical Engineering Reference
In-Depth Information
human use, diseases have caused major yield losses and have impacted
the well-being of humans worldwide. Nearly all agricultural crop cultivars
currently being used have some form of genetic resistance incorporated
involving single or multiple genes that are characterized as having
recessive or dominant effects against a number of diseases. Without the
incorporation of these resistance genes, crop productivity and yield would
be substantially reduced. The incorporation of disease resistance genes
into plants has been successfully achieved either by using conventional
breeding methods, or using genes of microbial origin effective against
phytopathogens (Crute and Pink 1996, Punja 2001).
Transgenic plants with tolerance to different pathogens can be
developed by transfer and expression of genes responsible for antimicrobial
activities from homologous or heterologous sources. One among the
several strategies used most widely is the over expression of plant genes
that are induced after biotic stresses, such as chitinases and glucanases
(Hong and Hwang 2006), peroxidases (Amaya et al. 1999), disease-related
R genes and ferritins (Deak et al. 1999); mitogen-activated protein kinases
(Xiong and Yang 2003) and specifi c transcription factors (Park et al. 2001).
However, constitutive over expression of plant genes may lead to various
developmental abnormalities compared to over expression of the same
genes from other sources that are inducible by pathogens (Kachroo et al.
2003). Therefore, alternative or complementary efforts had also been made
to reinforce the plant array of responsive genes by introducing heterologous
genes of well-known anti-pathogenic effect belonging to other phyla (Bolar
et al. 2001, Kunze et al. 2004). Most of the fungal pathogens of plants contain
chitin as structural component of their cell wall and the microbial or plant
originated chitinases have a signifi cant mycolytic effect on such fungal
pathogens. Chitinases are thought to play a dual role both by inhibiting
fungal growth, by cell wall digestion and by releasing the fungal cell wall
fragments as a result of extracellular chitinase activity that may in turn
induce further defense reactions in the host (Grison et al. 1996, Lorito et al.
1998). Transgenic plants over expressing chitinases of several origins have
been shown to exhibit enhanced levels of resistance to fungal infection
and delayed disease symptoms when challenged with fungal pathogens
(Lorito et al. 1998, Hong and Hwang 2006). We have tried to summarize
these aspects in this paper.
TRANSGENES OF FUNGAL ORIGIN
Microbes, including mycoparasitic fungi, represent rich sources of genes
that can be utilized to genetically engineer important crops for resistance
against plant pathogens (Punja 2001). The chitinase-encoding transgenes
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