Last week I wrote about the anti-science campaign being waged by opponents of the use of genetically modified organisms in agriculture. In that post, I promised to address a series of questions/fears about GMOs that seem to underly peoples’ objections to the technology. I’m not going to try to make this a comprehensive reference site about GMOs and the literature on their use and safety. I’m compiling some general resources here, and a list of all FAQs here.
Question 2) Maybe GMOs aren’t automatically bad, but isn’t it obvious that it’s dangerous to consume crops that produce their own pesticides and can tolerate high doses of herbicides?
Approximately 90% of soybeans, maize, cotton and sugar beets grown in the US are have been genetically modified to produce a protein that kills common insect pests or to make them highly tolerant of an herbicide used to control weeds, or in some cases both. To make a rational judgment about whether these specific GMOs are good or bad, it’s important to understand exactly what they are and how they work.
Bt soy and corn
The pesticide resistant plants have been engineered to produce a protein isolated from the bacterium Bacillus thuringiensis (known generally Bt). Each strain of Bt produces a different version of the protein, known as Cry, each highly specific to a limited number of related species. Bt has evolved these proteins as a key part of a reproductive strategy in which they kill insects that ingest them and then eat nutrients released by the dying host. The Cry protein found in Bt spores must be activated by a protein-cleaving enzyme found in the host gut and then bind to a specific protein on the surface of cells in the digestive system, which Cry then destroys. Insects, who are not huge fans of this strategy, eventually evolve resistance by modifying one or both of these proteins. Bt stains that rely on this insect adapt in turn, creating highly-specific strain-insect relationships.
The irony of Cry becoming a major bugaboo of the anti-GMO movement is that, until the gene that produces it was inserted into corn, it was the poster-child of a “natural” insecticide, preferred over chemical pesticides because of the potential for extreme host specificity and complete biodegradability. Bt spores were sprayed on crops for decades, and are still widely used to control pests by organic farmers. But the effectiveness of Bt as an insecticide is limited because it degrades in the matter of days – more rapidly when it rains. This led agricultural biotechnology companies to try and insert Cry genes directly into the plants, and there are now many varieties on the market, each targeting pests that are a particular problem for a given crop (some varieties of Bt corn, for example, targets the European corn borer).
Given what we know about Cry proteins, there is very little reason to be concerned about the safety of eating it. These are proteins that have evolved to kill insects – and not just insects in general, but very specific subsets of insects. And humans are not insects. Regulatory agencies in the US and Europe have consistently rejected claims that plants that produce their own Cry cause problems in either humans or farm animals.
Nonetheless, anti-GMO activists continually raise the spectre of “plants that make their own pesticide” as if this alone was sufficient reason to not only avoid them, but to ban them. Here is a banner running on the website of one of the organizations pushing the CA GMO-labeling initiative:
If you don’t know a lot about plants, I can see how this would seem threatening. But this picture and the anti-GMO campaign it accompanies are based on the flawed premise that ”normal” plants are pesticide free. This could not be farther from the truth. Almost since they first appeared on Earth, plants have had to reckon with a diverse array of animals determined to eat them. And this is a battle that continues today, as anyone who has tried to garden, or wandered through a forest, can attest. To fight off these pests, plants have evolved a dizzying array of defense mechanisms, including the production of a diverse arsenal of chemicals targeted at the insects and other pests that afflict them.
As far as I know, natural pesticides have been found in every plant in which they have been sought, including all conventionally grown crops. Wheat makes a family of proteins lethal to hessian flies, peas contain the insecticidal protein PA1b, tomatoes tomatine, and so on. And even if the corn in that picture was not genetically modified, that cute little girl is about to get a mouthful of the insecticide maysin. Indeed almost any mouthful of unprocessed plants from any source will likely contain some kind of natural pesticide that is inert in humans. There is nothing at all unusual, or particularly worrisome, about eating plants that contain the Bt Cry protein as we’ve been eating insecticides for eons.
I’m sure some people will say that we may have been eating insecticides all along, but we haven’t been eating Bt Cry protein and, under the “you never know” principle, should just avoid it. This would all be fine and good if there weren’t strong evidence supporting the value of Bt corn and soy in reducing pesticide use on farms and limiting collateral damage to insects that are in the vicinity of, but not eating, the relevant crop. As a panel of the US National Academies of Science reported in a 2010 study of GMOs:
The evidence shows that the planting of GE crops has largely resulted in less adverse or equivalent effects on the farm environment compared with the conventional non-GE systems that GE crops replaced. A key improvement has been the change to pesticide regimens that apply less pesticide or that use pesticides with lower toxicity to the environment but that have more consistent efficacy than conventional pesticide regimens used on non-GE versions of the crops.
To me, the demonization of Bt in anti-GMO rhetoric is a emblematic of everything that is wrong with the GMO debates. The producers of Bt crops have done a horrible job of explaining why plants expressing a single insecticidal protein should not – and do not – harm humans. And the anti-GMO advocates either have not bothered to understand the science behind their activity, or (worse) are cynically exploiting peoples’ fears of pesticides to promote their cause.
Glyphosate tolerant crops
The second major class of GMOs (mostly soy) have been engineered to be tolerant of the herbicide glyphosate (Roundup). Glyphosate is a small molecule that inhibits an enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which catalyzes an essential step in the biosynthesis of the amino acids phenylalanine, tyrosine and tryptophan. By denying rapidly growing plants these amino acids, it is able to rapidly inhibit grown of plants onto which it has been sprayed. Glyphosate is generally considered to be inert in humans, who get these amino acids from their food, and do not have an EPSPS.
The obvious problem with using glyphosate to control weeds is that it will, under normal circumstances, also kill crop plants. However, plants that have been engineered to express an alternative form of EPSPS that functions normally even in the presence of glyphosate. These plants are thus “Roundup Ready“, and will survive doses of glyphosate used to kill weeds in the field.
Although the EPSPS gene used in Roundup Ready plants comes from a bacterium, the necessary changes could now easily be made to the plant’s own copy of EPSPS. Thus Roundup Ready crops, which produce no new proteins not found prior to genetic manipulation, shouldn’t really be places in the same class of GMOs as Bt expressing plants, which are expressing a new protein. And there is absolutely no reason to expect that there are any health risks associated with eating the altered form of EPSPS found in glyphosate resistant transgenic plants.
Concern about Roundup Ready plants focuses instead on the adverse effect of glyphosate on people and the environment. There are some suggestions that high doses of glyphosate are bad for humans, though these studies are hotly contested (note this was a Monsanto-funded study and must be assessed with that in mind). But the more important question is whether the use of glyphosate in conjunction with Roundup Ready crops is better for humans and the environment than the alternatives. Here, the aforementioned NRC report concluded that:
GE soybeans, corn, and cotton are designed to be resistant to the herbicide glyphosate, which has fewer adverse environmental effects compared with most other herbicides used to control weeds.
This does not argue that glyphosate is safe. However, it suggests that the net effect of the GMO – Roundup Ready- has been positive. There is a bigger discussion to be had about the role of herbicides in farming – but this is really orthogonal to issues of genetic modification.
NEXT: Question 3) Why should I trust the big companies that develop these crops? Didn’t it take years to realize PCBs, DDT etc. were bad for us?
I am a molecular biologist with a background in infectious diseases, cancer genomics, developmental biology, classical genetics, evolution and ecology. I am not a plant biologist, but I understand the underlying technology and relevant areas of biology. I would put myself firmly in the “pro GMO” camp, but I have absolutely nothing material to gain from this position. My lab is supported by the Howard Hughes Medical Institute, the National Institutes of Health and the National Science Foundation. I am not currently, have never been in the past, and do not plan in the future, to receive any personal or laboratory support from any company that makes or otherwise has a vested interest in GMOs. My vested interest here is science, and what I write here, I write to defend it.