I was reading about using RNA-interference as a method of introducing pest-resistance in plants. In tobacco plants, using Agrobacterium vectors, genes specific to the nematode Meloidegyne incognitia were introduced. This was done in such a way that it produced both sense and anti-sense RNA in the host cell, forming dsRNA and ultimately silencing specific mRNA of the nematodes.
My question is, how can every single cell, or a proportion of cells sufficiently large so as to confer pest-resistance, of an already mature plant be transformed? Are a large number of vectors introduced, and as the infection gradually progresses, the immunity spreads throughout? Sorry if it's a stupid question, I've been out of touch with biology for a while.