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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.

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    $\begingroup$ Can you provide a citation for this method? Most likely they are introducing the genes in seeds, not in mature plants. $\endgroup$ – Bryan Krause Mar 11 at 21:28
  • $\begingroup$ @BryanKrause not necessarily, see my answer. $\endgroup$ – NatWH Mar 12 at 12:45
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    $\begingroup$ @NatWH Very nice, thank you; I had initially searched the technique and found the first couple papers were working with embryos so I'm glad your expertise was able to provide a better answer, and also introduce me to the term in planta. $\endgroup$ – Bryan Krause Mar 12 at 17:18
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Your inference is basically correct. The Agrobacterium acts as a vector for a plasmid containing the genetic construct one wishes to introduce into the plant genome. A medium containing a large number of bacterial cells transformed with this plasmid is injected into plant tissue, and the plasmids enter plant cells where parts of their DNA are integrated into the host genome. The overall effect of this is that there is localized integration in the areas where the Agrobacterium was introduced. A nice visual illustration of this comes from a paper from our lab where we introduce the genes for betalain biosynthesis into the tissue of Nicotiana benthamiana: in Figure 3 you will see only a localized area of cells around the injection site gain the ability to produce betalains.

You can find a nice review of the whole process of transformation with Agrobacterium here.

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