Remarkably, in at least plants and worms, if gene silencing is initiated in one tissue it can be systemically transmitted throughout the organism. A mobile, sequence-specific, silencing signal is responsible for triggering gene silencing in plants, but its molecular nature remains to be elucidated. We have been using Green Florescent Protein (GFP) as a reporter to study the mechanisms of induction and systemic spreading of gene silencing in Arabidopsis. In earlier work using grafting and a reverse genetics approach, we identified a collection of genes to be required for systemic gene silencing (Brosnan et al., 2007). All of these genes were required only for receiving the silencing signal in newly formed shoot tissue and not for sending the silencing signal from the rootstock (Brosnan et al., 2007).
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