Why Bt Toxin Kills Caterpillars but Spares the Bacterium That Makes It

Feeding a growing population means producing more food on limited land without harming soil and water. Biotechnology offers crops that resist pests, tolerate stress and carry better nutrition, alongside farming with chemicals or organic methods.

This part covers options for increasing food production, the benefits of genetically modified plants, Bt crops, and pest resistance through RNA interference.

Food production can be raised through agrochemical-based agriculture, organic agriculture or genetically engineered crop-based agriculture; the Green Revolution greatly increased yields with improved crop varieties, fertilisers and pesticides, but those gains were not enough for a growing population and the chemicals are costly and harmful.

Agrochemical-based agriculture:

- Relies on chemical fertilisers and pesticides
- Drove the yield increases of the Green Revolution, together with improved varieties
- The chemicals are too expensive for many farmers and harm the environment

Organic agriculture:

- Uses natural methods such as biofertilisers and biocontrol
- Kinder to the environment, though yields may be lower

Genetically engineered crop-based agriculture:

- Uses genetically modified organisms (GMOs) — plants, bacteria, fungi or animals whose genes have been altered by manipulation
- Aims to raise yields while reducing chemical use

Limits of the Green Revolution:

- Increased yields were not enough to keep pace with the growing population
- Further gains from breeding and chemicals alone are increasingly hard to achieve

An everyday example. Wheat fields in Punjab planted with high-yielding varieties, fertilisers and irrigation show how the Green Revolution raised output.

The substance. More chemicals do not keep raising yields forever — returns fall while environmental damage grows, which is why new approaches are needed.

Genetic modification has made crops more tolerant of abiotic stresses, less dependent on chemical pesticides, less prone to post-harvest losses, more efficient at using soil minerals, and more nutritious.

Benefits of GM plants:

- Tolerance to abiotic stresses — cold, drought, salt and heat
- Reduced reliance on chemical pesticides — through pest-resistant crops
- Reduced post-harvest losses
- More efficient use of minerals — preventing early exhaustion of soil fertility
- Enhanced nutritional value — for example, rice engineered to be rich in vitamin A
- Alternative resources for industries, such as starches, fuels and pharmaceuticals

Why vitamin A-rich rice matters. Vitamin A deficiency can cause night blindness, so a staple food carrying extra vitamin A could help people who depend heavily on rice.

An everyday example. Drought-tolerant crops could help farmers in rain-fed regions of India through a weak monsoon.

The substance. A GM crop changes a specific trait, not every characteristic of the plant — the rest of its genes remain the same.

The soil bacterium Bacillus thuringiensis makes insecticidal proteins coded by cry genes; these genes are transferred into cotton and corn so the plants make their own toxin, which exists as an inactive protoxin until the alkaline gut of an insect activates it and kills the insect — so the bacterium itself is never harmed.

The Bt toxin:

- Bacillus thuringiensis forms protein crystals containing a toxic insecticidal protein
- It kills certain lepidopterans such as the tobacco budworm and armyworm, coleopterans such as beetles, and dipterans such as flies and mosquitoes

Why it does not harm the bacterium:

- Inside the bacterium, the protein exists as an inactive protoxin
- In the alkaline pH of the insect gut, the crystals dissolve and the protoxin becomes active
- The active toxin binds to the epithelial cells of the midgut and creates pores
- The cells swell and burst, and the insect dies

Making Bt crops:

- The toxin is coded by genes named cry
- Proteins from cryIAc and cryIIAb control cotton bollworms
- The protein from cryIAb controls the corn borer
- The genes are expressed in the crop, so the plant protects itself without spraying

An everyday example. Bt cotton grown across India cuts the need to spray insecticides against bollworms.

The substance. The toxin acts only where it is activated — in the alkaline insect gut, which is why the protoxin is harmless inside the bacterium.

In RNA interference, double-stranded RNA silences a specific mRNA so its protein is not made; nematode-specific genes introduced into tobacco with Agrobacterium vectors make the plant produce both sense and antisense RNA, which pair into double-stranded RNA that silences an essential nematode mRNA, so the parasite cannot survive in the roots.

The pest. Meloidogyne incognita is a nematode that infects the roots of tobacco plants and greatly reduces yield.

RNA interference (RNAi):

- Occurs naturally in all eukaryotic organisms as a method of cellular defence
- A complementary double-stranded RNA binds to a specific mRNA and prevents its translation — this is silencing
- The double-stranded RNA can come from an infection by viruses with RNA genomes or from mobile genetic elements called transposons

Making resistant tobacco:

- Nematode-specific genes are introduced into the host plant using ***Agrobacterium* vectors
- The introduced DNA produces both sense and antisense RNA in host cells
- The two complementary RNAs form double-stranded RNA, which starts RNAi
- The specific mRNA of the nematode is silenced, and the parasite cannot survive in the transgenic host

An everyday example. A mute button on a remote silences one channel's sound without switching off the television — RNAi silences one gene's message without shutting down the cell.

The substance. RNAi acts after transcription** — the target gene is still transcribed, but its mRNA is blocked from being translated.