NCERT grounding
NCERT Class 12 Biology, Chapter 10 (Biotechnology and its Applications), section 10.1 places Bt cotton at the head of the discussion of pest-resistant transgenic plants. The chapter introduces Bacillus thuringiensis as the source of the toxin, explains why the bacterium is unharmed, and lists cryIAc, cryIIAb and cryIAb as the genes that the NEET syllabus expects you to know by name.
“Bt toxin gene has been cloned from the bacteria and been expressed in plants to provide resistance to insects without the need for insecticides; in effect created a bio-pesticide.”
NCERT Class 12 · Biotechnology and its Applications · §10.1
NIOS Senior Secondary Biology, Lesson 30 (Biotechnology), supplements this with the vector route — Agrobacterium tumefaciens and its Ti plasmid — under the heading of transgenic plants, and explicitly lists “cotton which can resist attack by worms” as a flagship example. Read this subtopic alongside the sibling pages on biotech in agriculture and RNA interference to see how cry-protein engineering and RNAi sit side by side as two distinct routes to pest resistance.
How Bt toxin kills the bollworm
The bacterium and its crystal
Bacillus thuringiensis is a rod-shaped, gram-positive, spore-forming soil bacterium. Different strains target different insect groups: certain strains produce proteins toxic to lepidopterans (tobacco budworm, armyworm, bollworms), some to coleopterans (beetles) and some to dipterans (flies and mosquitoes). The toxin is not secreted into the medium during normal growth. Instead, when the bacterium enters sporulation, it deposits a dense parasporal crystal alongside the spore. The crystal is built from cry proteins — the insecticidal proteins that give the bacterium its commercial value.
A natural question that NCERT poses directly is: why does the crystal not kill the Bacillus itself? The answer is two-fold. First, the protein is stored as an inactive protoxin, not as the lethal active toxin. Second, the crystal is insoluble at the near-neutral pH of the bacterial cytoplasm; the protein remains packaged and inert as long as it stays inside the cell or in moist soil. Only when an insect ingests the crystal does the local environment change enough to unlock the toxin.
From ingestion to gut lysis
Once a chewing larva — say, a young cotton bollworm — eats plant tissue contaminated with Bt crystals, the crystals travel with the food bolus into the midgut. Lepidopteran larvae maintain a strikingly alkaline midgut, with pH in the range of 9.5 to 10.5. This high pH solubilises the crystal lattice and releases the protoxin into solution. Gut proteases then cleave the protoxin into a smaller, stable active toxin fragment.
The active toxin diffuses through the peritrophic membrane and reaches the brush border of the midgut epithelium. There it binds with high specificity to receptor proteins, most notably cadherin-like receptors and aminopeptidase N, on the apical membrane of columnar midgut cells. Receptor binding triggers oligomerisation of the toxin and insertion of a pore-forming complex into the membrane. Pores open, ions and water rush in, the columnar cells swell, and the midgut epithelium undergoes osmotic cell lysis. The larva stops feeding within hours, the gut barrier collapses, and septicaemia from gut microbes — combined with starvation — kills the insect over the next one to three days.
- Crystal forms during sporulation, not vegetative growth.
- Protoxin is inactive at neutral pH; alkaline midgut solubilises it.
- Active toxin binds midgut epithelial receptors.
- Pores cause cell swelling and lysis.
- Bacterium itself is unaffected because the toxin is immature inside it.
- Specificity is set by which insect group has matching gut receptors.
Figure 1. The five-stage path from Bacillus thuringiensis crystal to bollworm death: sporulation-stage crystal, larval ingestion, alkaline solubilisation, protease activation, and pore-mediated lysis of the midgut epithelium.
Bt mechanism — single-line summary
-
1
Crystal forms
During sporulation, B. thuringiensis packs cry protein into a parasporal crystal.
protoxin · inactive -
2
Larva ingests
Chewing larva eats transgenic plant tissue containing the cry crystal.
midgut lumen -
3
Alkaline pH
Midgut pH ≈ 9.5–10.5 solubilises the crystal; proteases trim protoxin.
activated toxin -
4
Receptor binding
Active toxin binds midgut epithelial cell receptors with insect-group specificity.
cadherin / APN -
5
Pore + lysis
Pores open; columnar cells swell and lyse; larva stops feeding and dies.
death in 1–3 days
Cry genes, crops and target pests
The toxin is encoded by a family of genes named, naturally, cry. Each cry gene yields a protein that hits a different gut-receptor repertoire, so cry genes are insect-group specific. The NEET-relevant trio is small and worth memorising in exactly the form NCERT uses:
“Bollworm” in NEET stems usually means Helicoverpa armigera, the American or gram bollworm — a chewing lepidopteran that bores into the developing cotton boll and devastates yield. Two other species are clubbed under cotton bollworms: Pectinophora gossypiella (pink bollworm) and Earias vittella (spotted bollworm). NCERT names the gene pair cryIAc and cryIIAb together against “cotton bollworms”; the pyramid of two genes in a single Bt cotton line is a deliberate strategy to delay resistance evolution.
cry IAc
Cotton
crop
- Target: cotton bollworm (H. armigera)
- NCERT line 141 pairs it with cry IIAb in Bt cotton
- NEET-2024 match: cry IAc ↔ cotton bollworm
cry IAb
Corn
crop
- Target: corn borer (Ostrinia)
- NCERT line 142 names it for corn borer
- NEET-2024 match: cry IAb ↔ corn borer
Three other crops that NCERT lists as already engineered with cry genes are rice, tomato, potato and soyabean. The mention is brief but examinable, because question stems sometimes ask which of the listed crops is not a Bt example. Note that golden rice, in contrast, is a Vitamin-A enriched transgenic — it carries a daffodil-derived phytoene synthase gene, not a cry gene; mixing the two is a classic NEET trap (see NEET 2019, Q.62).
Constructing a Bt plant
Pest-resistant transgenic crops are built in five well-defined operations. The molecular toolkit — restriction enzymes, ligase, Ti plasmid, regeneration in tissue culture — is exactly the toolkit you met in the previous chapter on biotechnology principles, applied here to a specific cargo gene.
Figure 2. Engineering Bt cotton — isolate the cry gene from Bacillus thuringiensis, ligate it into a disarmed Ti plasmid carried by Agrobacterium tumefaciens, infect cotton explants in tissue culture, and regenerate transgenic plants that synthesise the cry protein in their own tissues.
The five operations are mechanical and well known; what NEET tests is the right vector and the right host.
Isolate
Specific cry gene cut out of the Bacillus thuringiensis genome with restriction enzymes.
Clone
Ligated into the T-DNA region of a disarmed Ti plasmid of Agrobacterium with a strong promoter.
Infect
Agrobacterium infects cotton tissue; T-DNA integrates into the plant nuclear genome.
Regenerate
Transformed explants are selected on antibiotic medium and regenerated into whole plants by tissue culture.
Express
Transgenic plant synthesises cry protein in its own leaves, stems and bolls — every chewing bollworm gets a fatal dose.
Field impact and ongoing concerns
Because the plant itself produces the toxin, the farmer no longer needs to spray chemical insecticide against the targeted bollworms. NCERT highlights this directly: GM crops have “reduced the reliance on chemical pesticides (pest-resistant crops)”. In commercial Bt cotton fields the number of pesticide sprays for bollworm typically falls from six or more per season to one or two, with downstream gains in farm income, reduction in pesticide poisoning of farm workers, and lower contamination of soil and surface water.
Insecticide sprays per season (typical)
Conventional cotton needs repeated bollworm sprays; Bt cotton expressing cryIAc + cryIIAb usually needs one or none, because the toxin is present inside the plant throughout the boll-forming stage.
Three concerns are routinely raised in NCERT-style essays and short-answer questions, and they map cleanly to a memorisable triad:
- Resistance evolution. Continuous selection pressure favours rare resistant bollworm genotypes. Field cases of Pectinophora gossypiella (pink bollworm) developing resistance to cryIAc are documented in India. The standard mitigation is a non-Bt refuge — a buffer of non-transgenic cotton next to the Bt field, where susceptible insects survive and dilute resistance alleles.
- Non-target effects. Cry proteins are insect-group specific, but concerns persist about pollinators and soil arthropods. Independent studies are required by regulators before commercial release.
- Regulatory and ethical oversight. In India the Genetic Engineering Appraisal Committee (GEAC) is the statutory body that clears GM organisms for release; NEET-2018 Q.113 tests this name directly.
A final point that NCERT slips in but that students often miss: pest-resistant transgenics are not a universal solution. The cry strategy works against chewing insects with the right gut chemistry and the right receptors; it does not protect against fungal disease, viral infection, sap-sucking insects with neutral or acidic guts, or plant-parasitic nematodes. Nematode resistance in tobacco is achieved by a separate RNAi-based strategy that you study in the sibling RNA interference subtopic.
Worked examples
In a bollworm gut, what is the precise event that converts Bt protoxin into the killing form, and which insect property makes this conversion possible?
Solution. The crystal of cry protoxin enters the larval midgut with food. The lepidopteran midgut maintains an alkaline pH of about 9.5–10.5, which solubilises the crystal lattice and exposes the protoxin to gut proteases. Proteolytic cleavage of the N-terminal and C-terminal extensions trims the protoxin to a smaller, stable active toxin fragment that can bind midgut epithelial receptors. The insect property that matters is the alkaline pH of the gut; absence of this alkaline environment in mammals (whose stomachs are strongly acidic) is one reason the toxin is non-toxic to humans.
Match the cry genes with their target pests: (A) cryIAc, (B) cryIIAb, (C) cryIAb against (i) corn borer, (ii) cotton bollworm.
Solution. Both cryIAc and cryIIAb are used together in Bt cotton against the cotton bollworm complex (Helicoverpa armigera, Pectinophora gossypiella, Earias vittella). cryIAb is used in Bt corn against the corn borer (Ostrinia nubilalis). Mapping: A → ii, B → ii, C → i. This is essentially the NEET-2024 matching question rephrased.
A student claims that since Bt cotton produces cry toxin in its own cells, the Bacillus thuringiensis bacterium should also be killed by the toxin in nature. Identify the flaw.
Solution. Inside the bacterium, the cry protein is stored as an inactive protoxin packed into an insoluble parasporal crystal during sporulation. The cytoplasmic pH of the bacterium is near neutral, so the crystal does not dissolve and the protein cannot bind anything. Activation requires both (i) an alkaline pH high enough to solubilise the crystal and (ii) gut proteases to trim the protoxin — neither of which is present inside the bacterial cell. Therefore the bacterium remains unharmed; the option “toxin is inactive” is the correct NCERT answer to its own intext question.
Which Indian regulatory body must clear a new Bt crop for commercial release, and why is a non-Bt refuge planted alongside the Bt field?
Solution. Clearance for environmental release of GM crops in India is granted by the Genetic Engineering Appraisal Committee (GEAC) under the Ministry of Environment, Forest and Climate Change. A non-Bt refuge — a strip or block of conventional cotton planted next to the Bt field — provides a pesticide-free habitat where susceptible bollworms can survive. They mate with the rare resistant survivors emerging from the Bt field, producing heterozygous offspring that are still killed by the toxin. This high-dose/refuge strategy slows the spread of resistance alleles and extends the agronomic life of the Bt trait.