Bt cotton and the cry proteins · Biology Class 11–12 | StemPy Lab
Course 28.1·Lesson 2 of 3
Bt cotton and the cry proteins
A soil bacterium makes a protein that kills certain insects. Its gene was cloned and put into cotton, so the plant makes its own insecticide. This lesson follows the gene into the plant and the toxin into the insect, and lists the cry genes and the pests that each one controls.
about 25 min1 simulation1 practice question
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A plant that protects itself
Cotton is eaten by caterpillars called bollworms. They bore into the cotton bolls and spoil the fibre. Farmers once sprayed the field again and again. Then a gene from a bacterium was put into the cotton, and the plant began to kill the bollworms by itself.
The bacterium is Bacillus thuringiensis, called Bt for short. The plant that carries its gene is Bt cotton.
The bacterium and its crystals
Some strains of Bacillus thuringiensis make proteins that kill certain insects. Which insects? These groups:
Lepidopterans: moths and butterflies, for example the tobacco budworm and the armyworm.
Coleopterans: beetles.
Dipterans: flies and mosquitoes.
At one stage of its growth, the bacterium forms protein crystals. They hold a toxic, insecticidal protein, the cry protein (cry for crystal). The gene that codes for it is the cry gene.
You may ask a fair question. Why does this toxin not kill the bacterium that makes it? The answer is that the crystals hold the toxin as an inactive protoxin. It does nothing until it reaches the right place.
How the toxin kills
Five panels in a row that show how the Bt toxin kills a caterpillar. Panel 1: a bacterium holds a crystal of inactive protoxin. Panel 2: a caterpillar eats a leaf that carries the crystals. Panel 3: in the alkaline gut the crystals dissolve and the protoxin becomes the active toxin. Panel 4: the active toxin binds to the surface of the cells that line the midgut. Panel 5: the toxin makes pores in these cells, they swell and burst, and the caterpillar dies.
Follow the five steps in the picture.
A caterpillar eats a leaf that carries the protoxin.
The alkaline pH of the gut dissolves the crystals.
The protoxin is changed to the active toxin.
The active toxin binds to the surface of the cells that line the midgut, the epithelial cells.
It makes pores in these cells. The cells swell and burst (this is called lysis), and the insect dies.
NCERT states only the mechanism in the insect. Biology books explain the safety for people in this way: the human stomach is acidic, so the crystals do not dissolve, and the human gut cells lack the binding sites for the toxin.
Making Bt cotton
The steps follow what you learnt about gene transfer in the previous chapter. The cry gene is cut out of the bacterium, joined into a vector and carried into a cotton cell. A whole plant is then grown from that cell. Every cell of the plant makes the toxin, so the plant is protected without a spray. In effect, it is a biopesticide.
Four boxes in a row joined by arrows. Box 1: a cell of the soil bacterium Bacillus thuringiensis with protein crystals, from which the cry gene is cut out. Box 2: the cry gene is joined into a vector. Box 3: the vector carries the cry gene into a cotton cell. Box 4: a whole cotton plant grows from that cell and makes the toxin in its own tissues, and this is Bt cotton.
The choice of gene depends on the crop and on the pest, because most Bt toxins act on one group of insects only.
Gene
Pest that it controls
cryIAc
cotton bollworms
cryIIAb
cotton bollworms
cryIAb
corn borer
Bt toxin genes have been put into cotton and also into corn, rice, tomato, potato and soyabean.
Try it in Python
The program counts the larvae that live on Bt cotton and on ordinary cotton.
Practise with new numbers
Common mistakes
Saying that the toxin is active inside the bacterium. The crystal holds an inactive protoxin.
Placing the activation in the stomach of the insect. It happens in the alkaline gut, and the active toxin acts on the midgut cells.
Mixing up the genes.cryIAc and cryIIAb are for the cotton bollworms, and cryIAb is for the corn borer.
Saying that Bt cotton kills all insects. Most Bt toxins act on one group of insects only.
Saying that the gene came from a plant. It came from a bacterium.
Remember these
Bacillus thuringiensis forms protein crystals with an inactive protoxin.
Alkaline gut: crystals dissolve, protoxin becomes toxin, it binds to midgut cells, pores form, cells swell and burst.
cryIAc and cryIIAb: cotton bollworms. cryIAb: corn borer.
Bt toxin genes are used in cotton, corn, rice, tomato, potato and soyabean.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the bacterium that gives the Bt toxin. What does it form at one stage of its growth?
Why does the toxin not kill the bacterium?
Why does the toxin become active in the insect?
What does the active toxin do to the midgut cells?
Which gene controls the corn borer? Which two control the cotton bollworms?