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

(A) Several nematodes parasitize a wide variety of plants and animals,including human beings. $A$ nematode $Meloidogyne \text{ } incognitia$ 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. The introduction of $DNA$ was such that it produced both sense and anti-sense $RNA$ in the host cells.
These two $RNA$ molecules,being complementary to each other,formed a double-stranded $RNA$ $(dsRNA)$ 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,became protected from the parasite.