Enhanced Resistance to Fungal Pathogens in Plants through Selective Utilization of Useful Fungal Genes - Biotechnology of Fungal Genes - page 93

from certain fungi have been shown to improve plant defense against a
broad range of fungal pathogens. Rather disease resistance in transgenic
plants has been enhanced, for the fi rst time, by the insertion of a gene
from the biocontrol fungus Trichoderma harzianum that encodes a strong
antifungal endochitinase. Substantial differences in endochitinase activity
were detected among the transformants. Some transgenic lines were
highly tolerant or completely resistant to the foliar pathogens Alternaria
alternata , A. solani , Botrytis cinerea and the soilborne pathogen Rhizoctonia
solani . The high level and the broad spectrum of resistance obtained with
a single chitinase gene from Trichoderma overcame the limited effi cacy of
transgenic expression in plants of chitinase genes from plants and bacteria
(Lorito et al . 1998). The enzyme chitinase is capable of degrading chitin,
that constitutes 3-60% of the cell wall of most phytopathogenic fungi
(Collinge et al. 1993). In terms of antifungal activity, chitinase genes from
biocontrol fungi such as T. harzianum (Lorito et al. 1998) and Rhizopus
oligosporus (Terekawa et al. 1997), has proven to be more effective both
in terms of the level and in the spectrum of fungal pathogen resistance
compared to the plant-derived chitinases (Emani et al. 2003). In planta
expression of these fungal genes encoding for chitinolytic enzymes from
diverse fungal species can even reach the antifungal activity level of some
chemical fungicides based on ED50 values. Furthermore, extensive testing
in vitro has shown that there are virtually no chitinous pathogens resistant
to Trichoderma chitinases (Lorito et al. 1993, 1996). Therefore, it is expected
that the transgenic use of these enzymes should produce a high level of
resistance in crop plants against a variety of fungal pathogens and, in
contrast to plant genes, could be accomplished with a single fungal gene
(Lorito et al . 1998).
FUNGAL ENDOCHITINASES
Trichoderma species are the most explored economically important
fungi that have been exploited extensively as commercial biofungicide
(Mukhopadhyay and Mukherjee 1996; Howell 2003; Singh 2006). Several
direct and indirect modes of Trichoderma antagonism are postulated.
Some among them are that they can modify environmental conditions,
promote plant growth, induce plant systemic resistance response, alleviate
pathogen induced oxidative stress or act as mycoparasites (Benítez et
al. 2004, Singh et al. 2011). A recent study showed that mycoparasitism
by Trichoderma species is an ancestral life style (Kubicek et al. 2011).
Mycoparasitism of Trichoderma is a complex process that includes secretion
of cell wall-degrading enzymes. As mentioned earlier one of the most
abundant components of cell wall in many fungal phytopathogens is