Plant Insect Resistance
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Plants have evolved many defences against herbivores (when I say herbivores, I mean insects in this case, and not cows or sheep). Plants may localise toxins in specific organs, such as young leaves and seeds. It has been suggested that these toxins have evolved for some physiological function, rather than for defence. The best evidence that indicates these chemicals are primarily for defence, is found in Acacia, a family of neo-tropical trees. These trees have cyanide-releasing glycosides, which is toxic to many insects. Another example, is the Ginkgo tree. It's leaves are very acidic, containing malic acid and oxalic acid, and therefore may prevent most insect pests from feeding on them. When damaged, leaves release 2-hexenal -- a pest repellent. Also, it's roots have been found to inhibit larval growth. The Ginkgo rarely has a pest problem. Herbivores have evolved ways of countering plant defence systems, such as using enzymes that inactivate or degrade plant toxins. For example, cabbage white butterfly larvae may be stimulated to eat by chemicals that would kill other types of insects. And so the coevolutionary "arms race" goes on. Pest control by humans is vital for agriculture. Conventional means of eradication and control is by chemical pesticides. The problems, of course, are environmental damage and effects on non-target organisms. Bacillus thurigiensis (Bt) has been used commercially in producing conventional pesticides since the 1930s, although B. thuringiensis had been first described in 1911. It is a soil bacterium that undergoes sporulation -- i.e. it becomes a spore, during which it produces a toxin. Bt-pesticides have been used primarily for control of Lepidopterans (butterflies and moths) all over the world. Bt-toxins are specific for certain insect pests -- that is, they have no harmful effects on beneficial insects (such as bees) or humans. Among the many problems with pesticides, standardization of toxicities is a main one, since crystal count is independent of the number of spores present. This is a problem, because you need to know what the toxicity level of the pesticide is, in order for it to be effective. B. thurigiensis produces a crystal protoxin (or a non-toxic form of the crystal). When the crystal is ingested by an insect larva, the protoxin dissolves readily in the midgut (at pH 10) and is cleaved by proteases which generates the toxic fragments. There are three types of crystal toxins made by B. thurigiensis: pathotype A, which is specific for Lepidopterous larvae, pathotype B, specific for Dipteran larvae (flies), and pathotype C, specific for Coleopterous larvae (beetles). How does this toxin work? Bt-toxins bind to epithelial receptors in the larval midgut. There is a receptor-binding domain on the active fragment that determines its specificity, and therefore determines the host range. The Bt-toxins allow for the formation of small pores in the plasma membrane which causes rapid swelling and cell lysis. This in turn, causes a disruption in the gut-epithelium and the larva dies from starvation. In other words, cells in the gut lining burst, thereby leaving holes in the intestine preventing proper food digestion. The midgut then has a subsequent decrease in pH, which favors Bt-spore germination in the nutrient-rich, dead larva. Mammalian cells do not have recptors for the insect specific toxins, and so the insect-specific Bt-toxins are non-toxic to humans. However, a crystal fragment in pathotype B has been found to be cytotoxic to mammalian cells, but non-toxic to insects. Recently, scientists have generated transgenic agricultural plants using B. thurigiensis. There has been a some concern in the public sector over the use of transgenic plants and animals. Before I deal with these issues, let me summarize the use of pesticides vs. transgenics: Problems with pesticide use:
Problems with transgenics:
Why all the interest in transgenics, if the public is so apparently concerned over their use? For one thing, more than 500 insect species have been identified as having evolved some resistance to all registered pesticides. This is one reason for using transgenic plants. Pesticides don't work in the long term, since insects will evolve resistance and come back in much greater force, causing even greater crop damage than the initial infestation. Everyone knows about the damaging effects of DDT: pesticides are not good for life in general or for the environment. So, the primary concerns with pesticides are evolved insect resistence and environmental damage. How are transgenics any better? As noted above, transgenics do have a few associated problems. There are, however, solutions to these problems that are not really feasible with pesticides, but let me first describe insect resistance to Bt-toxins. Bt-resistance is in all cases, inherited as fully or partially recessive. The frequency of initial resistance (i.e. before presenting the Bt-toxin/pesticide) has usually been estimated to be in the range of 10^-2 to 10^-13 for the recessive allele. That is a very wide estimate, meaning we don't actually know what the initial resistance is to the toxins until after we've used them, although there usually is some level of resistance. Resistance has been correlated with a reduction in affinity between the midgut-epithelial membrane receptors and the Bt-toxin. In other words, the receptors are less effective at binding the toxins, and therefore the toxins cannot disrupt the intestinal cells. One concern with transgenics, is their effects on local flora. There is the question of whether the engineered plants will pass on the resistance genes to wild type relatives in the surrounding regions. Whether this is a problem really depends on what the trait is: is it an insect resistance trait or herbicide tolerance trait? In either case, will a hybrid "super-weed" be created and cause problems for agriculture or the environment? The incidence of gene transfer is currently being researched. It may turn out that there is a minimal effect on local flora. On the other hand, if there are wild type relatives in the region, certain transgenic crops may be banned from those regions to prevent any gene transfer. As with insects evolving resistence to pesticides, insects may evolve resistence to the transgenics. Will the insects simply adapt to the new toxins, creating the need for more and/or "improved" pesticides/transgenics? The solution to this is described below. One benefit in using transgenics over pesticides is that in some species, insect larvae live and feed on the inside of the plants (e.g. in the stem, such as the yellow stem borer, Scirpophaga incertulas). Certain pesticides can only act when directly sprayed onto the insects, and so may be useless with stem borers. In such cases, transgenics are more effective, because the insect toxins are present in the plant tissue that is eaten by the larvae. Several governmental and administrative groups have been created to deal with these problems and risk assessment. Groups such as the:
Regulatory approval is required before companies can legally sell transgenic crops or seeds. The first transgenic to be approved was Bt-cotton in 1996 by the EPA; corn was next, also in 1996. These approvals are conditional, and approvals are given only after appropriate field-testing has been done, with encouraging results. One of the strategies of planting transgenics, has been to use refuges. Refuges were mandated by the EPA under certain guidelines -- i.e. Resistance Manage Plans (RMPs). The idea behind refuges is that a large portion of the insect population is maintained as non-resistant to the toxin. This is done by planting areas of non-transgenic plants with the transgenics. These non-resistant insects are allowed to interbreed with the resistant individuals, so that the effectiveness of the transgenics may be prolonged. The effect is to "drown" the resistance genes in a larger, non-resistance gene pool. So far, there is one of two strategies that have been implemented:
The 4% strategy might work under ideal conditions, but in reality, it may not be enough. Refuge strategies are intended to solve the problem of insects evolving resistence to the toxins. There are problems with the refuge strategy, in that more information needs to be known on the mating patterns and movements of insects. Some species may not move very far and stay within the confines of their refuges, so they end up not interbreeding with the resistant individuals. Thus, refuges may be ineffective for certain species. There is some research going on to answer these questions. Then, there is the public perception: are we moving too fast with biotechnology? The public may not want to buy transgenic products, because moral issues and of possible, unknown side effects. Transgenics (of yeast, bacteria, and tobacco) with scorpion (Buthus eupeus) insectoxin had been attempted in 1992. The problem was that -the transgenics lacked toxicity, due to improper protein folding. However, the scorpion toxin was specific to insects (i.e. no effects on humans), and they were able to get a product, albeit inactive. The point is that there are technical problems, such as the improper folding of proteins (due to differences between the "host" species and "donor" species) that may cause ineffective products, in some cases. The question is: would you buy and use a transgenic food product that contains scorpion toxins? Recently, the European public has been in uproar, because some companies were selling unadvertised transgenic products (that used scorpion insectoxin) along with non-transgenic products. Why is biotechnology frowned upon by the public? Humans have been selectively breeding agricultural crops and livestock since the dawn of civilization. Genetic engineering is simply a method that modifies organisms for particular uses (e.g. for higher seed yield, or insect resistance). Selective breeding essentially does the same thing -- it just takes a little longer to get the desired effects. Also with selective breeding, whole genomes are involved and therefore undesired traits are passed along with the desireable ones. Biotechnology has three important advantages over classical breeding: it is more precise (i.e. single genes may be transferred rather than whole genomes), it is much faster (i.e. you don't have to wait several seasons for good crop yields), and it allows gene transfer between very different organisms. This third point, in the public eye, is a matter of much concern. For example, transferring human growth genes into pigs to make them grow larger and faster. Is it really unethical to transfer genes from one species to another? In nature, gene transfer between different species (esp. between different plants or bacteria) occurs quite often. In fact, it may be a matter of survival for bacteria to have genes transferred from another bacterial species, or even from viruses. The difference is, that humans do it consciously: both selective breeding and genetic engineering are done for our own benefit. Just because biotechnology does the job faster and much more precisely, does that mean it is unethical or wrong? No, the technology in itself is not moral or immoral. It is how the technology is used by the companies or government that makes it moral or immoral. Eugenics is an obvious example of the immoral use of a science. Ethics may also come into play depending on the type of organism used for genetic engineering. Ethical issues seem to surround transgenic animals much more than transgenic plants or bacteria. That's because with animals, you have to worry about their lifestyle and well being more than you do with plants. In any case, it is not up to the scientists to decide whether the public will use transgenic products. It is the scientist's responsibility to give an assessment of their work, so the public may decide how or if the technology will be used. References:
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