Local infection confers systemic viral immunity, implying that an inducing signal is propagated throughout the plant. This isn’t too surprising, since viruses encode a movement protein that promotes their systemic propagation.
1. In 1997, Baulcombe’s and Vaucheret’s labs reported the remarkable finding that gene silencing could spread throughout a plant from a local site of DNA introduction. This is shown here for a GFP gene taken from a Baulcombe’s initial report. In this experiment, the GFP gene was introduced on an Agrobacterium T-DNA vector on a lower leaf, indicated by the arrow, and silencing of a GFP gene already in the plant was induced in other parts of the plant, as evidenced by the red fluorescent sectors in these leaves. On the right are control leaves and leaves taken from different parts of a plant in which gene silencing had been initiated at a distant site.
2. Vaucheret’s lab used different transgenes, an NIA and a GUS gene, with similar results. And they also reported that silencing is graft transmissible, more evidence for a transmitting signal.
3. There is evidence that the signal moves both symplastically through plasmodesmata, as well as systemically through the phloem.
4. These are curious observations, indeed. Here is evidence that some sort of signal originates from a cell that receives a foreign gene and replicates and spreads much like a virus!