Annals of Botany

Poppy proves a big turn-off

Degradation of double-stranded RNA plays at least two roles in plants. The first is in silencing of endogenous genes via the phenomenon of RNA inhibition. The other is as a defence mechanism against viruses, where the relevant ribonuclease activity is directed against double-stranded viral RNA. By using viral genomes as carriers of particular gene fragments it is possible to induce silencing of those genes in host cells: the plant treats the genes as if they belong to the virus. This technique, known as virus-induced gene silencing (VIGS), pioneered by David Baulcombe and his colleagues at Norwich, has now been used successfully by Wege et al. (Bremen and Mainz, Germany, pp. 641–649) with California Poppy (Escholzia californica). The reason for doing the work is that the Papaveraceae is one of the basal angiosperm families and thus of great interest in relation to the evolution of flowering plants. The specific aim was to show whether VIGS would be effective in this species. The authors therefore selected a gene for which silencing would lead to an obvious phenotype, namely Phytoene Desaturase, the silenced phenotype of which is photobleaching. A section of this gene, along with the tobacco rattle virus genome, was introduced into E. californica using an Agrobacterium-based transformation system. Within 2 weeks over 90 % of the plants started to exhibit photobleaching, which at its most extreme resulted in white petals, fruit, shoots and leaves. Evidence that photobleaching was associated with silencing of Phytoene Desaturase was obtained by assaying transcripts of the endogenous gene (targetting that part of the sequence not present in the construct used for transformation): strong photobleaching was always correlated with complete or extensive reduction in transcript levels. The authors conclude that this system demonstrates the feasibility of using VIGS for study of gene function in E. californica, especially for those genes where the knockout phenotype is readily detectable.