manipulated in transgenic plants to confer extreme immunity against viruses (Baulcombe 1996).
Plant biologists and breeders have developed a number of plants that have pest-protection conferred by transgenes. Transgenic pest-protection strategies generally depend on expression of novel genetic resources or transfer of natural plant resistance or defense genes. Transgenic pest-protection based on novel genetic traits involves the introduction of genes that interfere with a specific pest but that are derived from organisms in which the gene's natural function is not that of plant protection. The application of transgenic resistance should be most useful where natural conventional breeding has failed due to lack of resistance genes in sexually compatible plants or due to undesirable agronomic traits in conventional pest-protected crops. For example, the oat Pc-2 resistance gene which controls crown rust disease caused by the fungus Puccinia coronata is coinherited with a trait that confers sensitivity to an unrelated fungal pathogen, Cochliobolus victoriae, so it would not be useful to deploy this gene in oat cultivars by conventional breeding methods (Walton 1996). Transgenic pest-protection can also reduce the time required for cultivar development in some crops. Release of conventionally bred varieties of winter wheat that have the eyespot-disease-resistance gene Pch1 required 13 years from the initial crosses, mainly because of time-consuming selections of lines with acceptable agronomic and disease-resistance characters (Jones et al. 1995).
It is important to recognize that transgenic resistance programs do not displace traditional breeding because transgenes alone cannot currently provide the full spectrum of agronomic traits necessary in commercial varieties. Furthermore, use of transgenes for resistance does not circumvent the normal process of agronomic quality assurance and testing that occurs throughout a well-managed breeding program.
The most publicized examples of engineered resistance based on novel genetic resources involve use of Bacillus thuringiensis (Bt) delta endotoxins (Estruch et al. 1997). Specific Bt endotoxin proteins are toxic to lepidopteran or coleopteran insects—many of which are destructive plant pests (such as the corn earworm and the tobacco budworm on cotton). Bt proteins in fermentation mixtures and spores have been used for decades in microbial formulations and by fermentation of B. thuringiensis strains that produce Bt