INITIATION AND MAINTENANCE OF VIRUS-INDUCED GENE SILENCING

The phytoene desaturase (PDS) gene of Nicotiana benthamiana was silenc ed in plants infected with potato virus X (PVX) vectors carrying PDS i nserts, and a green fluorescent protein (GFP) transgene was silenced i n plants infected with PVX-GFP. This virus-induced gene silencing (VIG S) is post-transcriptional and cytoplasmic because it is targeted agai nst exons rather than introns of PDS RNA and against viral RNAs. Altho ugh PDS and GFP RNAs are most likely targeted through the same mechani sm, the VIGS phenotypes differed in two respects. PDS mRNA was targete d by VIGS in all green tissue of the PVX-PDS-infected plant, whereas P VX-PDS was not affected. In contrast, VIGS of the GFP was targeted aga inst PVX-GFP. Initially, VIGS of the GFP was initiated in all green ti ssues, as occurred with PDS VIGS. However, after 30 days of infection, the GFP VIGS was no longer initiated in newly emerging leaves, althou gh it was maintained in tissue in which it had already been initiated. Based on these analyses, we propose a model for VIGS in which the ini tiation of VIGS is dependent on the virus and maintenance of it is vir us independent.