RNA interference and pest resistant plants

A worm in the roots

Some nematodes, which are small round worms, live as parasites in plants and animals. One of them, Meloidogyne incognita, infects the roots of tobacco and cuts the yield. (NCERT prints the species name as incognitia; the accepted spelling is incognita.) Spraying does not always reach it, because it lives in the soil and in the root.

Scientists used a different idea. They did not poison the worm. They switched off one of its genes, from inside the plant.

This part is not in the NEET syllabus text, but NCERT teaches it, and A-Level and IB ask about RNA interference. It is kept out of the NEET format paper of this course.

RNA interference

RNA interference, or RNAi, takes place in all eukaryotic cells as a method of cellular defence. It works in this way.

• A double stranded RNA (dsRNA) is present that is complementary to a particular messenger RNA.
• The dsRNA binds to that mRNA, and the mRNA is silenced. It cannot be translated, so the protein is not made.

Where does such a dsRNA come from? It can come from an infection by a virus that has an RNA genome. It can also come from mobile pieces of DNA called transposons, which copy themselves through an RNA stage.

A reminder of base pairing in RNA: A pairs with U, and G pairs with C. Two strands that pair along their whole length are complementary.

The tobacco experiment

1. Agrobacterium was used as a vector. It carried nematode-specific genes into the tobacco cells.
2. The DNA was put in so that it made both sense and antisense RNA in the host cell. The sense strand has the same sequence as the nematode mRNA. The antisense strand is complementary to it.
3. The two RNA strands are complementary to each other, so they formed a double stranded RNA.
4. This dsRNA started RNA interference, and it silenced the specific mRNA of the nematode.
5. The parasite could not survive in a transgenic host that makes the specific interfering RNA. The plant was protected.

Try it in Python

The program builds the antisense strand of an RNA and counts the hydrogen bonds in the double strand.

Practise with new numbers

Common mistakes

• Saying that the plant kills the worm with a poison. The dsRNA silences one gene of the worm.
• Mixing up sense and antisense. The sense strand has the same sequence as the mRNA. The antisense strand is complementary to it.
• Writing T in RNA. RNA has U. A pairs with U.
• Saying that the dsRNA is destroyed by the host. It is the mRNA of the nematode that is silenced.
• Thinking that RNAi is found only in plants. It occurs in all eukaryotes.

Remember these

• RNAi: a dsRNA complementary to an mRNA silences it, and the protein is not made.
• Tobacco and Meloidogyne incognita: nematode genes carried by an Agrobacterium vector gave sense and antisense RNA, which formed dsRNA.
• The parasite could not survive on the transgenic plant.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is RNA interference?

2. Name the parasite of tobacco in the experiment, and the part of the plant that it infects.

3. Which vector carried the nematode-specific genes into the tobacco cells?

4. Why did the plant make both sense and antisense RNA?

5. A sense RNA reads AUGCC. Write the antisense RNA.

References

• OpenStax Biology 2e, chapter 16, Gene Expression, section 16.5 Eukaryotic Post-transcriptional Gene Regulation (microRNAs and RNA stability).
• NCERT (2023). Biology, Textbook for Class XII, chapter 10, Biotechnology and Its Applications.