INSECT RESISTANCE

Several microbes & plants produce proteins toxic to specific insects, pests that feed on plants.

One goal of plant genetic engineering is to transfer the genes encoding these protein toxins to agronomically important plants with the hope that expression of the toxin genes in these plants will provide biological control of some serious insect pests.

Currently insect pests are controlled by the use of broad spectrum chemical insecticides.

However there is reason for concern about the potential damage to ecosystems & pollution of ground water that might result from the wide spread use of these chemicals on agricultural crops.

The best known example of the use of natural gene products to control plants pests or the insects are toxin of Bacillus thuringiensis.

1. B.thuringiensis produces endotoxin called (toxin of Bacillus thuringiensis) Bt toxin.

Each of the B.thuringiensis toxin genes encodes a large protein that aggregates to form protein crystals in spores & the protein crystals are highly toxic to certain insects.

Some of the insects that are killed by the protein are plant pests of major economic importance.

Different subspecies of B.thuringiensis produce toxins that kill different insects.

For example, the toxin produced by B.thuringiensis subspecies (krustak), kills lepidopteran larva such as the tobacco hornworm.

The gene that encodes this toxin has been isolated & shown to synthesize a functional toxin in E.coli . the gene is put under the control of a 35s promoter of CaMV & introduced into plants through disarmed plasmid.

Transgenic tomato plants were regenerated & shown to express the gene.

The toxicity of the gene – product synthesized in the transgenic plants was tested by allowing tobacco hornworm larvae to feed on the transgenic plants died within a few days.

Larvae applied to control plants remained healthy & eventually consumed the entire plant.

Transgenic tomatoes with Bt gene are resistant to tobacco hornworm (Manduca sexta) tomato fruit worm,(Heliothis zea) & tomato pinworm (Keferia lycopersicella ).

The Monsanto company (US) has chemically synthesized a powerful Bt gene that introduced it into cotton through disarmed Ti plasmid.

The transgenic cotton is called killer cotton.

It is resistant to attack by tobacco budworm (Heliothis virescens), cotton bollworm, pink bullworm, beatarmy worms.

2. TRANSGENIC PLANTS WITH Bt TOXIN GENE & SERINE PROTEASE INHIBITOR GENE.

Bt toxin produced in transgenic plants is not effective against a broad range of insect pests, as it is present in low concentration. for enhancing the insecticidal activity a serine protease inhibitor gene (SPI Gene) is fused with a Bt toxin gene & CaMV promoter.

The DNA construct is introduced through disarmed Ti plasmid.

The transgenic plant produce Bt toxin as well as serine protease inhibitor, pesticidal activity of such transgenic plant is twenty times greater than that of transgenic plant having Bt toxin alone.

NOTE : plants contain peptides acting as protease inactivating protein (PIPts). The different proteinases are serine, cysteine, aspartic acid & metallo proteinases. They catalyze the release of aminoacids from dietary protein thereby providing the nutrients crucial for normal growth & development of insects. The proteinase inhibitor deprive the insect of nutrients by interfering with digestive enzymes of the insect.

3. TRANSGENIC PLANTS WITH COWPEA TRYPSIN INHIBITOR

Cow pea is resistant to many storage insects in grainaries. It has a protein called cowpea trypsin inhibitor (CPTI)

CPTI found in cowpea is the most active inhibitor upto date.

This inhibitor gene produces antimetabolite substances that provide protection against major storage pest Brichid beetle (Callosobrichus maculatus).

The gene is also harmful to various Lepidoptera insects (Heliothis verescens), spodopteran insects (Manduca sexta), colepteron insects (callosobruchus, anthonomus grandis), orthopteran insects (Locusta migratoria), but is not harmful to mammals.

The gene coding for CPTI was transferred to tobaccp through a binary vector system made from Ti plasmid. The transgenic tobacco has been resistant to many insect pests including tobacco budworm.

It produced 2 microgram of CPTI per mg, which is not toxic to man & animals.

4. TRANSGENIC PLANTS WITH Nicotiana alata PROTEINASE :

Nicotiana alata (an ornamental plant) has insecticidal property because of presence of a proteinase inhibitor called Nicotiana alata proteinase inhibitor. (NAPI).

It is encoded by NAPI gene.

Transgenic tobacco, clovers & peal have been developed by introducing cDNA of NAPI genes.

The transgenic plants produce NAPI & hence resist several insect pests.