Control of Plant Virus Diseases by Pathogen-Derived Resistance in Transgenic Plants.

Plant viruses have an enormous negative impact on agricultural crop production throughout the world, and, consequently, agronomists and plant pathologists have devoted considerable effort toward controlling virus diseases during this century. Prior to the advent of genetic engineering, traditional plant breeding methodology was sometimes successfully applied to develop resistance to viruses of agronomically important crops. In addition, standard techniques of plant pathology, including quarantine, eradication, crop rotation, and certified virus-free stock, have been important tools to control virus diseases, although each has disadvantages, such as expense, questionable effectiveness, and lack of reliability on a yearly basis. The prospects for pathogen-mediated intervention in virus disease development were first realized in 1929 when H.H. McKinney demonstrated that tobacco could be protected from infection by a severe strain of TMV by prior inoculation with a milder strain of TMV (see refs. in Gadani et al., 1990). This type of protective measure, known as cross-protection, has since been employed throughout the world on several important crops, including tomato, papaya, and citrus (see refs. in Beachy et al., 1990; Gadani et al., 1990; Hull and Davies, 1992). However, this labor-intensive type of protection is expensive and it necessitates the use of an infectious virus as a control measure. A problem of particular concern is that strains of viruses vary in their virulence on different crops and even within varieties of the same crop. A virus used to protect one crop could potentially cause serious diseases on alternate crops or varieties growing nearby. Therefore, it would be preferable to have a better understanding of cross-protection in order to employ a carefully regulated mechanism that could protect plants from the deleterious effects of virus infections and in order to derive strategies to prevent the cross-protecting virus from escaping to adjacent crops. Over the past decade, the tools of molecular biology have permitted rapid advances in our understanding of plant viruses and their replicative strategies, yet we know little about