John Innes Centre, Department of Virus Research, Norwich NR4 7UH, U.K.
The use of viral sequences to produce virus-resistant plants is now almost a standard technique. A variety of sequences from a large number of viruses have been used but the mechanisms remain largely unknown. There are probably at least two distinct types of mechanisms operating: one requiring the expression of the viral protein(s) and the other dependent only on the presence of transgene-derived RNA. In this review, we will discuss this RNA-mediated resistance and its similarities with cosuppression, a recently described phenomenon leading to suppression of transgene expression and homologous sequences. We present a critical view of the current models available to explain this type of resistance.