Gene expression can be controlled with the help of  $\rm {RNA}$ . Explain the method with an example.

Several nematodes parasitise a wide variety of plants and animals including human beings. A nematode Meloidegyne incognitia infects the roots of tobacco plants and causes a great reduction in yield. A novel strategy was adopted to prevent this infestation which was based on the process of $RNA$ interference $(RNAi)$. 

$(RNAi)$ takes place in all eukaryotic organisms as a method of cellular defense. This method involves silencing of a specific $mRNA$ due to a complementary $dsRNA$ molecule that binds to and prevents translation of the $mRNA$ (silencing). The source of this complementary $RNA$could be from an infection by viruses having $RNA$ genomes or mobile genetic elements (transposons) that replicate via an $RNA$ intermediate.
          Using Agrobacterium vectors, nematode-specific genes were introduced into the host plant (Figure). The introduction of $DNA$ was such that it produced both sense and anti-sense $RNA$in the host cells.

These two $RNA's$ being complementary to each other formed a double stranded $(dsRNA)$ that initiated $RNAi$ and thus, silenced the specific $mRNA$ of the nematode. The consequence was that the parasite could not survive in a transgenic host expressing specific interfering $RNA$. The transgenic plant therefore got itself protected from the parasite