CAN WE EXPLAIN RNA-MEDIATED VIRUS-RESISTANCE BY HOMOLOGY-DEPENDENT GENE SILENCING

The use of viral sequences to produce virus-resistant plants is now al most a standard technique. A variety of sequences from a large number of viruses have been used but the mechanisms remain largely unknown. T here are probably at least two distinct types of mechanisms operating: one requiring the expression of the viral protein(s) and the other de pendent only on the presence of transgene-derived RNA. In this review, we will discuss this RNA-mediated resistance and its similarities wit h cosuppression, a recently described phenomenon leading to suppressio n of transgene expression and homologous sequences. We present a criti cal view of the current models available to explain this type of resis tance.