3 Common traits introduced by GM
3.1 Insect resistance
We will now look briefly at the science underlying the traits introduced into commercial crops, which you explored in Activity 1; a useful place to start is by considering how the property of resistance to insects is acquired by crops.
Insect damage causes huge losses of agricultural crops each year. For example, without control measures it is estimated that over 35% of current global cotton production would be lost. Insect control by conventional means is big business, and the sale of insecticides generates many billions of dollars of revenue for multinational companies. Unfortunately, insects develop resistance to insecticides over time, and this can force farmers to use ever increasing amounts to achieve control. This increases the costs to the farmers, and deposits ever larger amounts of toxic chemicals into the environment.
If plants could be genetically engineered to produce their own insecticides, the costs and hazards of insecticide spraying might be reduced or removed altogether. A number of strategies to genetically modify plants in this way have been developed, but the only crops that have been grown commercially at the time of writing (2006) have been the so-called Bt crops. These have been modified so as to produce an insecticide derived from Bacillus thuringiensis (Bt for short), another common soil bacterium.
When growing conditions are not optimal, Bacillus thuringiensis forms spores that contain protein crystals toxic to insects. Bt comprises a large number of subspecies and each one produces its own particular toxin. So, for example, Bacillus thuringiensis subspecies kurstaki produces a toxin that kills the larvae of Lepidoptera (i.e. moths and butterflies) and a toxin from the subspecies israelensis is effective against Diptera such as mosquitoes and blackflies.
Spore preparations derived from Bacillus thuringiensis have been used by organic farmers as an insecticide for several decades. When the target insect ingests the Bt spore, the protein crystal dissociates into several identical subunits. These subunits are a protoxin, i.e. a precursor of the active toxin. Under the alkaline conditions of the insect's gut, digestive enzymes (proteases) unique to the insect break down the protoxin to release the active toxin. The toxin molecules insert themselves into the membrane of the gut epithelial cells, setting in motion a series of processes that eventually stop all the cell's metabolic activity. The insect stops feeding, becomes dehydrated and eventually dies. The protoxin requires both alkaline pH and specific proteases before it can be converted to its active form. It is considered unlikely that humans or farm animals would be affected by the protoxin, as initial digestion in mammals occurs under acidic conditions. In addition, there are no binding sites for the toxin on the surface of mammalian intestinal cells.
These properties of Bt spores do make them a particularly appropriate form of insecticide, but there are a number of limitations. The spores are not toxic on contact – they must be eaten by insects during the feeding stage of their development, i.e. when they are larvae. Spraying has to be carried out when most of the insect population are at this stage in the life cycle. In order to encourage insects to eat the spores, they have to be mixed with appropriate insect attractants. Another limitation is that once boring insects have penetrated into the stems or roots of the plants, any spraying will be ineffective, as the spores remain on the surface of the plant.
The toxin would be present in the plant throughout the growing season, protecting the plant at all times. If the toxin is expressed in all cells, the insects will be affected irrespective of whether they are feeding on the surface or have bored into a root or stem.
Several crops have been modified so as to be insect-resistant by incorporation of Bt genes. These include tobacco, tomato, potato, cotton and maize. The insertion of the Bt gene directly into the genome of the crop allows the plants to produce Bt protoxin in their own cells. In most instances, the transfer of the Bt gene into crops has been mediated by A. tumefaciens, but microprojectile bombardment (Box 1) has also been used.
However, initial attempts to introduce the gene into a variety of crops did not produce plants with an effective defence against insect attack. Initially, the levels of the protoxin produced in the plant cells were too low to be effective. The problem was that the protoxin genes were not well expressed in plant cells. A number of strategies were adopted to increase the levels of expression, including the following.
- A shortened version of the gene was used, producing a smaller, but equally toxic, protein. This seemed to make it easier for the plant cell's biochemical machinery to produce the protoxin, and increased levels of expression a little.
- A particularly powerful promoter sequence was incorporated into the plant genome, alongside the shortened gene. This increased the levels of expression such that an enhanced insecticidal effect was observed.
- A synthetic version of the gene was produced, containing the DNA triplets more commonly used in plants rather than those found in bacteria. This resulted in approximately 100 times more expression of the protoxin in the plant cells, and provided the plants with significant protection against insect attack.
Theoretically, producing Bt crops, which express high levels of the Bt protoxin in their cells, should confer constant insect resistance, and therefore remove the need for application of any insecticide dusts or sprays. In practice, the system is not completely effective, and the number of insecticide applications is reduced rather than eliminated altogether.