BiologyCBSE Class 12 · Biotechnology and its Applications

Bt Cotton and Pest-resistant Plants

Updated 15 October 2026 · Fact-checked

Bt cotton carries cry genes from Bacillus thuringiensis that make an inactive protoxin. In the alkaline gut of an insect larva the protoxin becomes active, binds the midgut cells and makes pores, killing the insect. Tobacco plants are protected from the nematode Meloidogyne incognita by RNA interference, which silences an essential nematode mRNA.

Understand Bt Cotton and Pest-resistant Plants

Genetically modified (GM) crops. Plants, bacteria, fungi and animals whose genes have been altered by manipulation are genetically modified organisms (GMOs). GM plants have:
1. increased tolerance to abiotic stresses (cold, drought, salt, heat);
2. reduced reliance on chemical pesticides (pest-resistant crops);
3. helped reduce post-harvest losses;
4. increased the efficiency of mineral use by plants (preventing early exhaustion of soil fertility);
5. enhanced nutritional value, e.g. golden rice (vitamin A-enriched).
They can also be tailor-made to supply starches, fuels and pharmaceuticals to industry.

Bt toxin. Some strains of *Bacillus thuringiensis* (Bt) make proteins that kill certain insects: lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes). During a particular growth phase the bacterium forms protein crystals containing a toxic insecticidal protein.

Why the toxin does not kill Bacillus, and how it kills insects.
1. The toxin exists as an inactive protoxin.
2. An insect larva eats it; the alkaline pH of the insect gut solubilises the crystals and converts the protoxin into the active toxin.
3. The active toxin binds to the surface of midgut epithelial cells and creates pores.
4. The cells swell and lyse, and the insect dies.
The bacterium itself is not harmed because inside it the toxin stays inactive.

Bt cotton. Specific Bt toxin genes are isolated and put into crop plants such as cotton. The toxin is coded by cry genes; the choice of gene depends on the crop and the target pest, as most Bt toxins are insect-group specific:

cryIAc and cryIIAb control cotton bollworms;
cryIAb controls the corn borer.

So the plant makes its own insecticide, reducing pesticide use.

Pest-resistant tobacco by RNA interference (RNAi).

The nematode *Meloidogyne incognita* infects the roots of tobacco and greatly reduces yield.
RNAi occurs in all eukaryotes as a method of cellular defence: a complementary double-stranded RNA (dsRNA) binds a specific mRNA and prevents its translation (silencing). Natural sources of such RNA are viruses with RNA genomes and mobile genetic elements (transposons) that replicate through an RNA intermediate.
Using *Agrobacterium* vectors, nematode-specific genes were introduced into tobacco so that the host cells produced both sense and anti-sense RNA.
These two complementary RNAs formed dsRNA, which started RNAi and silenced a specific mRNA of the nematode.
The parasite could not survive in the transgenic host, so the plant was protected.

Flow chart (Bt): cry gene in cotton → protoxin in plant cells → larva eats leaf/boll → alkaline gut activates toxin → binds midgut epithelium → pores → swelling, lysis → larva dies.
Flow chart (RNAi): nematode gene inserted via Agrobacterium → sense + anti-sense RNA in plant → dsRNA → nematode feeds → its specific mRNA silenced → nematode dies.

Draw and label: RNAi protection of tobacco (schematic). Checklist: a plant cell making two complementary RNA strands that pair into dsRNA; a nematode feeding on the root; inside the nematode, its mRNA bound by the dsRNA with a cross over the ribosome. Labels: transgene (via Agrobacterium), sense RNA, anti-sense RNA, dsRNA, nematode mRNA, no translation, Meloidogyne incognita, tobacco root.

Bt Cotton and Pest-resistant Plants: key facts

Bt activation: $protoxin (inactive) alkaline gut pH active toxin \to pores in midgut cells \to death$; The bacterium is safe because the toxin is inactive inside it.
cry genes and targets: $cr y I A c, cr y I I A b \to cotton bollworms; cr y I A b \to corn borer$; Bt toxins are insect-group specific.
Insect groups killed by Bt: $lepidopterans, coleopterans, dipterans$; Tobacco budworm, armyworm; beetles; flies, mosquitoes.
RNAi: $sense RNA + anti-sense RNA \to d s R N A \to target mRNA silenced$; Used against Meloidogyne incognita in tobacco.
GM crop benefits: $stress tolerance; ↓ pesticides; ↓ post-harvest loss; ↑ mineral use; ↑ nutrition$; Golden rice: vitamin A.

How to answer Bt Cotton and Pest-resistant Plants questions

Questions ask how Bt works, why it is safe for the bacterium, which cry gene fits which pest, or how RNAi protects plants.

1For Bt, write the four-step chain: protoxin → alkaline gut activation → binding to midgut epithelium and pores → swelling, lysis, death.
2For 'why not the bacterium', say: the toxin exists as an inactive protoxin and is activated only in the alkaline insect gut.
3For cry genes, match the gene to its pest and stress that toxins are insect-group specific.
4For RNAi, give the organism and host (Meloidogyne incognita, tobacco roots), the vector (Agrobacterium), sense and anti-sense RNA, dsRNA, and silencing of nematode mRNA.
5For GM crop advantages, list at least three with one example (golden rice for nutrition).
6In assertion-reason items, check whether the reason explains the mechanism (pH activation, pore formation, silencing).

Bt Cotton and Pest-resistant Plants in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Specificity of Bt toxins

Suppose a farmer grows Bt cotton containing cryIAc. Bollworm damage drops sharply, but aphids (sap-sucking insects not in the bollworm group) still attack the crop, and the farmer asks why the 'insect-proof' cotton failed.

Explain why Bt cotton controls bollworms but not aphids, and suggest what the farmer can do about the aphids without broad-spectrum sprays.

Try it first, then show the solutionSolution

cryIAc codes for a toxin specific to cotton bollworms (a lepidopteran group); most Bt toxins are insect-group specific.
Bollworm larvae eat the plant tissue; the protoxin is activated in their alkaline gut, binds midgut cells, forms pores and kills them.
Aphids belong to a different group and are not targets of this toxin, so they are unaffected.
The farmer can use biocontrol such as ladybirds, which eat aphids, as part of integrated pest management.

Answer: cryIAc toxin is specific to bollworms; aphids are not targeted. Use ladybirds (biocontrol) for aphids.

Memory hook: One cry gene, one pest group.

Tricks to remember Bt Cotton and Pest-resistant Plants

Short memory aids. Say each one aloud once, then use it on a question the same day.

Cry for the bollworm
cryIAc and cryIIAb make bollworms cry; cryIAb hits the corn borer.
Use it when: Matching cry genes.
Pro to Pore
PROtoxin is activated, then makes PORES.
Use it when: Bt mechanism.
Two strands silence
Sense + anti-sense = dsRNA = silence.
Use it when: RNAi.

Bt Cotton and Pest-resistant Plants: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Bt mechanism · 3 marks · Short answerBoard level

How does Bt toxin kill insects but not the bacterium that makes it?

Try it first, then show the solutionSolution

In Bacillus thuringiensis the toxin exists as inactive protoxin crystals, so the bacterium is not harmed (1 mark).
When an insect eats it, the alkaline pH of the gut solubilises the crystals and converts the protoxin into active toxin (1 mark).
The active toxin binds to midgut epithelial cells and creates pores, causing swelling and lysis, and the insect dies (1 mark).

Answer: Inactive protoxin in bacterium; alkaline gut activates it; pores in midgut cells kill the insect.

Example 2: cry genes · 2 marks · Very short answerBoard level

Name the genes that code for Bt toxin against (a) cotton bollworms and (b) corn borer. Why is the choice of gene important?

Try it first, then show the solutionSolution

(a) cryIAc and cryIIAb; (b) cryIAb (1 mark).
Most Bt toxins are insect-group specific, so the gene must match the crop's target pest (1 mark).

Answer: cryIAc, cryIIAb (bollworms); cryIAb (corn borer); toxins are pest-specific.

Example 3: RNAi in tobacco · 5 marks · Long answerHarder board level

Explain how RNA interference has been used to protect tobacco plants from the nematode Meloidogyne incognita.

Try it first, then show the solutionSolution

Meloidogyne incognita infects tobacco roots and greatly reduces yield (1 mark).
RNAi is a natural cellular defence in eukaryotes in which a complementary dsRNA binds a specific mRNA and prevents its translation (silencing) (1 mark).
Nematode-specific genes were introduced into tobacco using Agrobacterium vectors so that the plant produced both sense and anti-sense RNA (1 mark).
These complementary RNAs formed dsRNA, which initiated RNAi and silenced a specific mRNA of the nematode when it fed on the plant (1 mark).
The parasite could not survive in the transgenic host, so the plant was protected (1 mark).

Answer: Agrobacterium-delivered nematode genes → sense + anti-sense RNA → dsRNA → nematode mRNA silenced → nematode dies.

Example 4: GM crops · 2 marks · Very short answerHarder board level

List four ways in which genetically modified crops have been useful.

Try it first, then show the solutionSolution

Tolerance to abiotic stresses (cold, drought, salt, heat); reduced reliance on chemical pesticides (1 mark).
Reduced post-harvest losses; increased efficiency of mineral use; enhanced nutritional value such as vitamin A-rich golden rice (any two) (1 mark).

Answer: Stress tolerance, fewer pesticides, less post-harvest loss, better mineral use, better nutrition.

Example 5: If the gut were acidic · 3 marks · Short answerHOTS / competency

Suppose a new insect pest has an acidic midgut. Predict whether Bt cotton would control it, and explain using the mechanism of Bt toxin.

Try it first, then show the solutionSolution

Bt toxin is made as an inactive protoxin that becomes active only after alkaline pH solubilises the crystals (1 mark).
In an acidic gut the protoxin would stay inactive, so it could not bind the midgut cells and make pores (1 mark).
So the pest would probably survive; Bt cotton would not control it, and another strategy (another specific toxin, RNAi or biocontrol) would be needed (1 mark).

Answer: Probably not: the protoxin needs alkaline activation; without it, no pores and no killing.

Example 6: Sources of interfering RNA · 2 marks · Very short answerHarder board level

Where does the complementary RNA for RNAi come from in nature, and how was it supplied in transgenic tobacco?

Try it first, then show the solutionSolution

In nature: infection by viruses with RNA genomes, or transposons that replicate via an RNA intermediate (1 mark).
In tobacco: nematode-specific DNA introduced with Agrobacterium so that the plant made both sense and anti-sense RNA, which paired into dsRNA (1 mark).

Answer: Natural: RNA viruses, transposons. Transgenic: plant expresses sense and anti-sense RNA.

Where students lose marks in Bt Cotton and Pest-resistant Plants

The slips examiners see most often, and what to write instead.

Mistake: Saying the toxin kills by poisoning nerves
General idea of insecticides is used.
Wrong working
Bt toxin paralyses the nervous system of insects.
Right working
Activated toxin binds midgut epithelial cells and makes pores, causing swelling and lysis.
Do this: Learn the gut-pore mechanism.
Usually costs: 1-2 marks.
Mistake: Acidic gut
Students assume all guts are acidic like the human stomach.
Wrong working
Acidic pH of the insect gut activates the toxin.
Right working
Alkaline pH of the insect gut activates it.
Do this: Alkaline activates.
Usually costs: 1 mark.
Mistake: RNAi described as a protein toxin
Bt and RNAi are mixed.
Wrong working
Tobacco makes a toxin that kills the nematode.
Right working
Tobacco makes dsRNA that silences a nematode mRNA.
Do this: RNAi = silencing, not toxin.
Usually costs: 1-2 marks.
Mistake: Wrong cry gene for a pest
Gene names are similar.
Wrong working
cryIAb controls cotton bollworms only.
Right working
cryIAc and cryIIAb: bollworms; cryIAb: corn borer.
Do this: Learn the two pairings.
Usually costs: 1 mark.

Exam tips

Use the terms protoxin, alkaline pH, midgut epithelial cells, pores, lysis in Bt answers.
In RNAi answers, name Meloidogyne incognita, tobacco roots, Agrobacterium, sense and anti-sense RNA, dsRNA and silencing.
Write cry gene names exactly (cryIAc, cryIIAb, cryIAb).
List advantages of GM crops with examples when asked for applications in agriculture.
Mention that RNAi is a natural defence mechanism in all eukaryotes.

Practice questions on Bt Cotton and Pest-resistant Plants

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The Bt toxin is activated in the insect gut because of:
(a) acidic pH (b) alkaline pH (c) high temperature (d) the insect's enzymes only
Hint
It solubilises the crystals.
Answer
(b) alkaline pH.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The cry gene put into a crop is chosen according to the crop and its target pest.
Reason (R): Most Bt toxins are insect-group specific.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
cryIAc for bollworms, cryIAb for corn borer.
Answer
(a). Because each Bt toxin acts on a particular insect group, the gene must match the pest (cryIAc, cryIIAb for cotton bollworms; cryIAb for corn borer).
Q3 · 2 marks · Very short answerBoard level
Name three groups of insects killed by Bt toxins, with one example each.
Hint
Moths, beetles, flies.
Answer
Lepidopterans (tobacco budworm, armyworm), coleopterans (beetles), dipterans (flies, mosquitoes).
Q4 · 3 marks · Short answerHOTS / competency
Suppose a biotechnologist wants to protect a rice variety from a root-feeding nematode without making any toxin protein. Outline the strategy, naming the vector, the RNA molecules involved and the final effect on the nematode.
Hint
Follow the tobacco model.
Answer
Use RNAi: identify an essential nematode gene; using an Agrobacterium vector, introduce nematode-specific DNA into rice so that the plant cells make both sense and anti-sense RNA; these pair into dsRNA. When the nematode feeds, the dsRNA silences the matching mRNA (prevents its translation), so the nematode cannot survive and the crop is protected, with no toxin protein produced.
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a seed company's brochure says: 'Our cotton contains a gene from a soil bacterium. The protein it makes is harmless until a bollworm eats it; then the worm's gut cells swell and burst.'
(a) Name the bacterium and the gene family. (1)
(b) Why is the protein 'harmless until eaten'? (1)
(c) Explain why the gut cells swell and burst, and give one environmental benefit of growing this cotton. (2)
Hint
Protoxin; pores.
Answer
(a) Bacillus thuringiensis; cry genes (e.g. cryIAc, cryIIAb). (b) It is made as an inactive protoxin and is activated only by the alkaline pH of the insect gut. (c) The active toxin binds to midgut epithelial cells and creates pores, so water and ions enter and the cells swell and lyse; the insect dies. Benefit: less use of chemical pesticides, reducing pollution.
Q6 · 1 mark · Very short answerHarder board level
What is golden rice, and which advantage of GM crops does it illustrate?
Hint
Vitamin.
Answer
Golden rice is vitamin A-enriched GM rice; it illustrates enhanced nutritional value of food.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which bacterium is the source of Bt toxin?Show answer
Bacillus thuringiensis.
2.Which genes control cotton bollworms?Show answer
cryIAc and cryIIAb.
3.Where does the active Bt toxin bind?Show answer
The surface of midgut epithelial cells.
4.Which nematode infects tobacco roots?Show answer
Meloidogyne incognita.
5.Which vector introduced nematode genes into tobacco?Show answer
Agrobacterium.
6.What is golden rice enriched with?Show answer
Vitamin A.

Bt Cotton and Pest-resistant Plants: frequently asked questions

What is Bt cotton?

Cotton carrying cry genes from Bacillus thuringiensis, so the plant makes a protein toxic to bollworms and needs less pesticide.

How does Bt toxin kill insects?

The inactive protoxin is activated by the alkaline pH of the insect gut, binds to midgut epithelial cells and makes pores, causing swelling, lysis and death.

Why doesn't Bt toxin kill the bacterium?

Inside the bacterium the toxin is an inactive protoxin; it becomes active only in the alkaline gut of an insect.

What is RNA interference?

A cellular defence in eukaryotes in which a complementary double-stranded RNA binds a specific mRNA and prevents its translation (silencing).

How is tobacco protected from nematodes?

Nematode-specific genes were introduced with Agrobacterium so that the plant makes sense and anti-sense RNA; the resulting dsRNA silences a nematode mRNA, and the nematode dies.