Host-dependent differences during synergistic infection by Potyviruses with potato virus X

SUMMARY A comparative analysis of the synergistic interaction between PVX and either PVY or TEV potyviruses was performed in Nicotiana benthamiana and N. tabacum plants. In each PVX/potyvirus combination, doubly infected plants developed much more severe symptoms than singly infected ones. However, while PVX accumulation increased in doubly infected N. tabacum plants compared with singly infected plants, the accumulation of PVX did not vary drastically in doubly infected N. benthamiana plants with respect to single infected ones. These findings suggest that the relationship between viral titre enhancement and synergism in PVX/potyvirus infections is host dependent. Since PVX and potyviruses contain suppressors of a plant antiviral defence system mediated by gene silencing, differences observed in the response of these two related hosts to PVX/potyvirus interactions might reflect the effect of these viruses on host specific antiviral defences.

Transient expression of homologous hairpin RNA causes interference with plant virus infection and is overcome by a virus encoded suppressor of gene silencing

Specific post-transcriptional gene silencing (PTGS) of target genes can be induced in a variety of organisms by providing homologous double-stranded RNA (dsRNA) molecules. In plants, PTGS is part of a defense mechanism against virus infection. We have previously shown and patented that direct delivery to nontransgenic plants of dsRNA derived from viral sequences specifically interfere with virus infection. Here, we show that transient expression of constructs encoding hairpin RNA homologous to a rapidly replicating plant tobamovirus also interferes with virus multiplication in a sequence-dependent manner. A three-day lag period between delivery of hairpin RNA and virus into the same tissues completely block virus infectivity. Several hallmarks characteristic of PTGS were associated with viral interference mediated by hairpin RNA: high level of sequence identity between the hairpin RNA and the target RNA, presence of siRNAs in extracts derived from leaves infiltrated with hairpin RNA, and helper component-proteinase (HC-Pro) of potyviruses, a suppressor of PTGS, overcame interference. No evidence for a mobile silencing suppression signal induced by transient expression of HC-Pro was observed. The approach described here has the potential to be used as a versatile tool for studying the onset of PTGS in cases involving virus infection, in opposition to dsRNA-transgenic plants, which allow primarily for the study of PTGS maintenance.