Transgenic approaches to disease resistant plants as exemplified by viruses
The advent of plant transformation has provided the opportunity to capitalise on existing conventional forms of plant resistance and to develop new and novel ones. In a review chapter on viral ‘cross protection′, Hamilton (1980) suggested that if complementary deoxyribonucleic acid (cDNA) of mild virus strains could be transferred to plants, these might provide protection against other strains of the virus. This was taken a step further by Sanford and Johnston (1985), who suggested the possibility of inducing resistance (to viruses) by transforming a susceptible plant with genes derived from the pathogen itself. This suggested form of resistance was termed ‘parasite-derived resistance’ and was subsequently called ‘pathogen-derived resistance′. The first demonstration of this concept was described the following year by Powell Abel et al. (1986). Since then, there have been many more examples and transgenic crops with pathogen-derived resistance are now grown commercially in a number of countries. The first virus-resistant transgenic crop plant commercially released in the USA was a squash Fi hybrid cultivar that was highly resistant to zucchini yellow mosaic virus (ZYMV) and watermelon mosaic virus 2 (Fuchs and Gonsalves, 1995). Subsequently, a transgenic squash line with resistance to the same two viruses plus cucumber mosaic virus (CMV) (Tricoli et al., 1995) has been deregulated and a transgenic papaya line with resistance to papaya ringspot type p virus (PRSV) (Gonsalves, 1998) has been commercialised. Tobacco made from virus-resistant transgenic plants is reported to be commercially available in China (Plafker, 1994; Birch, 1997). A number of excellent reviews on pathogen-derived resistance have appeared in recent years (Beachy, 1988; Wilson, 1993; Baulcombe, 1994a; Grumet, 1994; Lomonossoff, 1995; Malpica et al., 1998). Some of these reviews also cover other transgenic forms of resistance not dependent on pathogen-derived sequences. The ability to identify and clone natural plant resistance genes to pathogens has now provided further opportunities to confer plants with new sources of resistance, facilitating interspecific transfer of genes.