Mechanisms of Pathogen-Derived Resistance to Viruses in Transgenic Plants.

lhe Sainsbury Laboratory, John lnnes Centre, Colney, Norwich NR4 7UH, United Kingdom INTRODUCTION In 1985, Sanford and Johnston developed the simple and ele- gant concept of parasite- or pathogen-derived resistance (Sanford and Johnston, 1985). Subsequently, there have been numerous attempts to generate virus resistance in transgenic plants based on this concept through the expression of virus- derived genes or genome fragments (Beachy, 1993; Wilson, 1993; Baulcombe, 1994b; Lomonossoff, 1995). Many of these attempts have been successful, and some have led to the de- velopment of virus-resistant potato and squash cultivars for commercial application. In this article, the major emphasis is on the mechanisms of pathogen-derived resistance rather than on the practicali- ties of using this technology for crop improvement. These mechanisms have proven difficult to unravel largely because a resistance mechanism related to transgene silencing can override the direct phenotype of virus-derived transgenes. lhe examples discussed first are those in which it is clear whether the mechanism involves gene silencing. In later sections, some less well understood examples are reviewed. The final sec- tion anticipates likely future developments in pathogen-derived resistance.