One of the major goals of plant genetic engineering is to protect plants from diseases. There are many examples of the introduction of chitinase genes into plants resulting in an enhancement of resistance of the host plant to fungal pathogens (Broglie et al. 1993; Lin et al. 1995; Vierheilig et al. 1993). However, the desired levels of resistance for successful commercial application have not yet been reached. The use of potent chitinases with proven antifungal activity is thus an attractive alternative. Because Ech42 from T. atroviride fulfills these criteria, the corresponding gene was introduced into tobacco and potato (Lorito et al. 1998). High expression levels of the fungal gene were obtained in different plant tissues, with no visible effect on plant growth and development. Substantial differences in endochitinase activity were detected among different transformed lines. Transgenic lines were highly tolerant or completely resistant to the foliar pathogens Alternaria alternata, Alternaria solani, B. cinerea, and the soilborne pathogen R. solani. Interestingly, the levels of tolerance reached in these experiments were higher than those previously achieved by expression of bacterial or plant chitinases (Lorito et al. 1998).
A similar strategy was used to improve scab resistance of apple (Bolar et al. 2000). The endochitinase gene (ech42), as cDNA and genomic clones, was transferred into apple cv. Marshall McIntosh. Eight lines propagated as grafted and self-rooted plants were inoculated with Venturia inaequalis. Six transgenic lines expressing the endochitinase were more resistant than controls. Disease severity in the transgenic lines tested compared with nontransformed controls was reduced, as well as the number of lesions and the leaf area infected. However, in contrast with the results previously reported (Lorito et al. 1998), expression of the endochitinase also had negative effects on the growth of both inoculated and uninoculated plants (Bolar et al. 2000). In a more recent investigation the same group introduced either an endo-chitinase or an exochitinase, both from Trichoderma, into apple plants (Bolar et al. 2001). In agreement with their previous results resistance to V. inaequalis correlated with the level of expression of either enzyme. Plants expressing both enzymes simultaneously were more tolerant that plants expressing either enzyme alone. Their results indicate that the two enzymes acted synergistically to limit disease development.
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