9 min readPathway sheet2 or 3-mark question on RNAi in tobacco; assertion-reason and case-based items; also a NEET favourite

Pest-resistant plants by RNA interference

RNA interference silences a specific mRNA with complementary double-stranded RNA. Tobacco given nematode genes made sense and antisense RNA, silencing the Meloidogyne incognita mRNA so the worm died.

PestMeloidogyne incognita, a nematode (roundworm) that infects tobacco roots and greatly reduces yield

RNAiSilencing of a specific mRNA by a complementary dsRNA that binds it and prevents its translation

Natural sources of the RNAViruses with RNA genomes, or transposons that replicate through an RNA intermediate

Vector usedAgrobacterium carried the nematode-specific genes into tobacco

The problem

Many nematodes (roundworms) are parasites of plants and animals, including humans. The nematode Meloidogyne incognita (your textbook prints it as Meloidegyne incognitia) infects the roots of tobacco plants and causes a great loss of yield. A new strategy based on RNA interference (RNAi) was used to stop it.

What is RNA interference?

RNAi happens in all eukaryotic organisms, where it works as a cellular defence. A complementary double-stranded RNA (dsRNA) binds to a specific mRNA and prevents its translation. This is called silencing. The complementary RNA may come from viruses with RNA genomes. It may also come from transposons (mobile genetic elements) that replicate through an RNA intermediate. Sense RNA has the same sequence as the mRNA; antisense RNA is complementary to it.

Making a nematode-resistant tobacco plant

1
Nematode-specific genes are chosen.
2
They are introduced into tobacco cells using Agrobacterium vectors.
3
The DNA is arranged so the host cells make both sense and antisense RNA of the nematode gene.
4
The two complementary RNAs pair to form dsRNA.
5★
The dsRNA starts RNAi and silences the specific mRNA of the nematode.
6★
The nematode feeding on the transgenic roots cannot survive; the plant is protected.

★ key step

Sense RNA vs antisense RNA

Sense RNAHas the same base sequence as the target mRNA.

Antisense RNAIs complementary to the target mRNA, so it pairs with it.

Made together in one cell, they pair into dsRNA, which triggers RNAi.

Bt cotton vs RNAi tobacco

Feature	Bt cotton	RNAi tobacco
Pest	Bollworm (an insect)	Meloidogyne incognita (a nematode)
Where the gene comes from	Bacillus thuringiensis	The nematode itself
What the plant makes	A protein toxin (protoxin)	Double-stranded interfering RNA
How the pest dies	Its gut cells burst	An essential mRNA is silenced

RNAi protection at a glance

In real life

Root-knot nematodes like Meloidogyne also damage Indian vegetables such as tomato, brinjal and okra. RNAi offers a way to protect roots without chemical nematicides that harm soil life.

Exam angle

3 marks: “How has RNA interference been used to make tobacco plants resistant to nematodes?” Nematode-specific genes introduced into tobacco with Agrobacterium vectors; host cells make sense and antisense RNA; these pair into dsRNA; RNAi silences the nematode's specific mRNA; the parasite cannot survive in the transgenic host.

2 marks: “What is RNA interference? Name two natural sources of the complementary RNA.” Silencing of a specific mRNA by a complementary dsRNA that blocks its translation. Sources: viruses with RNA genomes; transposons that replicate via an RNA intermediate.

1 marks: “Name the nematode that infects the roots of tobacco plants.” Meloidogyne incognita.

Remember it

Sense plus antisense equals silence: two complementary RNAs pair, and the pest's mRNA goes quiet.