Question

How can one do plant breeding for developing resistance to insect pests? Give a few examples to insect resistance characteristics. Also mention the source of pest resistance genes.

Answer 1

The concept of Bt

Bacillus thuringiensis, commonly known as Bt, is a bacterium that occurs naturally in the soil. For years, bacteriologists have known that some strains of Bt produce proteins that kill certain insects with alkaline digestive tracts. When these insects ingest the protein produced by Bt, the function of their digestive systems is disrupted, producing slow growth and, ultimately, death.

Bt is very selective - different strains of the bacterium kill different insects and only those insects. Strains of Bt are effective against European corn borers and cotton bollworms (Lepidoptera), Colorado potato beetles (Coleoptera), and certain flies and mosquitos (Diptera). Bt is not harmful to humans, other mammals, birds, fish, or beneficial insects.

Bt was first identified in 1911 when it was discovered that it killed the larvae of flour moths. Bt was registered as a biopesticide in the U.S. in 1961. Today it is used in insecticide sprays sold to home gardeners and others worldwide.

In 1983, the World Health Organization used Bt in West Africa to control disease-carrying blackflies. In the U.S., various strains of Bt are used to control spruce budworms and gypsy moths in forests, cabbage worms in broccoli and cauliflower, loopers or budworms in cotton and tobacco, and leaf rollers in fruits.

However, less than one percent of all pesticides used in the U.S. each year contain Bt (Monsanto). As an ingredient of commercial sprays, Bt is relatively expensive and has some drawbacks. Although some pesticides kill on contact, Bt must be eaten by insects to be effective. Sunlight breaks down Bt, and rain washes it from the plants. Therefore, Bt must be applied exactly where and when the target insects are feeding and they must consume it quickly before it disappears.

Bt and Biotechnology

Today, plants can be genetically engineered to produce their own Bt. Genetic (recombinant DNA) engineering is the modification of DNA molecules to produce changes in plants, animals, or other organisms. DNA (deoxyribonucleic acid) is a double-stranded molecule that is present in every cell of an organism and contains the hereditary information that passes from parents to offspring. This hereditary information is contained in individual units or sections of DNA called genes. The genes that are passed from parent to offspring determine the traits that the offspring will have.

In the last twenty years, scientists made a surprising discovery - DNA is interchangeable among animals, plants, bacteria or any organism! In addition to using traditional breeding methods of improving plants and animals through years of crossbreeding and selection, scientists can now isolate the gene or genes for the traits they want in one animal or plant and move them into another. Of course, when a trait is controlled by several genes, the transfer process is more difficult. The plants or animals modified in this way are called transgenic.

Out of Bt Into the Plants

DNA technology makes it possible to locate the gene that produces Bt proteins lethal to insects and transfer the gene into crop plants.

First, scientists identify a strain of Bt that kills the targeted insect. Then they isolate the gene that produces the lethal protein. That gene is removed from the Bt bacterium and a gene conferring resistance to a chemical (usually antibiotic or herbicide) is attached that will prove useful in a later step.

The Bt gene with the resistance gene attached is inserted into plant cells. At this point, scientists must determine which plant cells have successfully received the Bt gene and are now transformed. Any plant cell that has the Bt gene must also have the resistance gene that was attached to it. Researchers grow the plant cells in the presence of the antibiotic or herbicide and select the plant cells that are unaffected by it. These genetically transformed plant cells are then grown into whole plants by a process called tissue culture. The modified plants produce the same lethal Bt protein produced by Bt bacteria because the plants now have the same gene.

Research to transfer insect resistance genes from Bt to crop plants is well under way. Corn, cotton, and potatoes are three of the many commercial crops targeted for Bt insect resistance.

Bt in Corn Confers Resistance to European Corn Borers

Modern corn hybrids have some resistance to European corn borers (ECB), but they still sustain damage from moderate and high levels of infestation. Genetically engineered insect-resistant corn is providing a new defense against an old enemy.

The Bt Solution

Scientists know that a family of insecticidal proteins from Bt kills ECB. The bacteria produce the protein as an insoluble crystal. When a susceptible caterpillar, like the corn borer, eats the crystal, part of it binds to, penetrates, and collapses the cells lining its gut, causing death.

Although Bt genes have been introduced into tobacco, tomatoes, cotton, and other broadleaf plants, gene transfer technology for corn is a recent achievement. The development of corn plants expressing Bt proteins requires substantial changes in the Bt genes, including the creation of synthetic versions of the genes, rather than the microbial Bt gene itself.

Agronomic and cultural practices

Certain agronomic practices in cotton agroecosystem may change the characteristics of the crop plant and crop environment, thus reducing insect pest levels. Such practices may include planting date, crop rotation, plant density, eradication of alternate host plants and other sanitary measures. Examples are the successful control of pink bollworm in Texas (U.S.A) by adopting certain cultural practices, and the increase of mortality rate of leafworm in iugypt by not irrigating clover after the first of May. The presence or alternate host plants near cotton fields may be harmful or beneficial, because it was found that lygus bugs were attracted to and remained in alfalfa strips interplanted in cotton fields, whereas in ïanzania growing maize (Zea maize) with cotton apparently increased Heliothis damage to cotton. In the Sudan, large areas of dura (Sorghum vulgäre) and groundnuts (Arachis hypogae) grown before cotton supported the populations of Heliothis. It was also found that lubia (Dolichos lablab) was very attractive to moths and "breeding of Whitefly. In southern U.S.A., prior to 1900, cotton cultivars were susceptible to boll Weevil and the breeders found that rapid fruiting and early maturing cultivars could circumvent Weevil attack.

Breeding resistant cultivars

Breeding crop plants resistant to insects is a complex problem, and in most cases the knowledge of the mechanisms underlying plant resistance are inadequate. More information about the interaction between the host and insect are required to allow for a better knowledge for both breeders and entomologists to enable them to cooperate and carry out effecient breeding programmes. It is well known that plant resistance to insects is the more economical and highly effective means of reducing damage to crop plants, and minimizing hazards to humans, animals and natural predator». In breeding for resistant cultivars the breeder should take into consideration the life-cycle of the insect, the infesting stage, the relationships between the insect and the crop plant together with the morphology, physiology and the genetic make up of the plant, and the insect« It is also worth mentioning that resistance developed to a particular insect may not be permanent or may be beneficial to other insects. Example of this is the hairy cotton cultivar synthesized in the Sudan for jassid, resistance and found to be harbouring Whitefly. Criteria to screen effeciently the breeding material and the knowledge concerning the complex interactions between insects and their host plants are important for a successful breeding programme. Plant resistance is the collective heritable characteristics by which a plant species, race, clone or individual may reduce the probability of successful utilization of that plant as a host by an insect pest. I believe that resistance should also be looked upon as the degree of inter-action between the insect and its host plant under certain physiological, genetical and environmental conditions affecting both the insect and its host plant. The mechanisms of resistance include oviposition, feeding habit of insect, morphological and biochemical characteristics of the host plant. However, such characteristics will be utilized by the breeder in surveying his germ plasm for insect resistance.