What exactly is a genetically modified plant?
Over the last few months I’ve been forced to think about the agricultural giant Monsanto. On the one hand I know someone who went to work there, someone I hold in high regard. And on the other hand I see repeated calls to action in the social media (by people I count among my closest friends) to boycott all Monsanto products.
Having done some research I have yet to come to a definitive conclusion myself. Monsanto has, in the past, been found guilty of serious environmental and ethical misconduct, and is currently being accused of aggressive enforcement of its patents amongst mid-western farmers. These things are obviously inexcusable. That being said, often the problems being voiced against the company are to do with the food their seeds ultimately produce. The goal appears to be to smear science when it is in fact human error that is to blame for Monsanto’s reputation.
But that’s as far as I will go with my opinion on the matter. My goal on this blog is to tackle hardcore science and explain it without any jargon, so that is what I will attempt to do. I’ll start with conventional plant breeding, move on to looking at how modern molecular biology has accelerated plant breeding processes, and finally take a look at how true genetically modified (a.k.a. transgenic) plants are made.
Conventional Plant Breeding
Humans have been breeding plants for around 10,000 years. The basic goal of this breeding was to improve certain plant characteristics. For example wheat, a common cereal crop, has been bred to produce more and larger seeds than its wild ancestor, and to fight off common infections. These traits were all present in the wild ancestor, but over time farmers encouraged the consolidation of these traits into better and better strains of wheat. The diagram below shows a brief schematic of how this is done:
Enter Molecular Biology
In the mid-19th century Gregor Mendel showed using pea plants that traits (or phenotypes) are inherited in a predictable manner. A century later, Watson and Crick showed that the molecular identity of this heritable material, genes, was DNA. From this discovery the field of molecular biology was born. New techniques started springing up left and right to manipulate DNA, to duplicate DNA, and, importantly, to sequence DNA. Knowing the sequence of an organism’s genome leads to a far greater understanding of the genes it contains, and therefore the day-to-day workings of its cells. It also allows scientists to track the movement of genes, a fact that is extremely useful in plant breeding. By using DNA sequencing technology, breeders can ensure that they select the plant that has gained a favorable gene without also assimilating any unwanted DNA.
We can therefore re-draw the above genetic cross as a molecular biologist would view it:
Vistive Brand Soybeans
Soybeans (as most other plants) produce fatty acids. These fatty acids come in a variety of different forms, but broadly speaking are long strings of carbon atoms decorated with hydrogen atoms. Soybeans contain linoleic acid, which contains two carbon-carbon double bonds. This renders it an “unsaturated fatty acid”. Saturated fatty acids, on the other hand, contain no such double bonds. The carbon atoms are “saturated” with hydrogen atoms. Chemical hydrogenation, a common facet of food processing, aims to remove these double bonds. But this process has the unfortunate side effect of producing trans fats, in which the carbon-carbon double bond assumes a different shape. This alternate conformation is rarely found in nature, hence the aptitude of trans fats for clogging up our arteries.
So Monsanto used molecular biology to speed up the conventional breeding process to vastly reduce the amount of linoleic acid their soybeans produced. They inserted no foreign DNA into the plants, they simply selected the plants that had the correct genomic sequence.
Generating a Transgenic Plant
Towards the end of the twentieth century, molecular biological techniques had become so advanced that scientists were able to move pieces of DNA, particularly interesting or useful genes, between organisms. In the case of agriculture, plant biologists started to insert genes into crop breeds that could impart resistance to the common pests that plagued them. While the overall effectiveness of these measures remain controversial, let’s take a look at how it’s done.
The first step is to select a “gene of awesome” that you want your plant to express. Monsanto’s Genuity Brand Roundup Ready crops, for example, contain a gene that imparts resistance to the herbicide glyphosphate (trade name, Roundup). This has allowed farmers to use Roundup to control weed populations in their fields without killing the crop. This is desirable not only because Roundup is relatively cheap, it is also less likely to run off into drinking water supplies than other herbicides (which given the controversy regarding its toxicity is no bad thing).
Once you have your gene of awesome you need to put it into a plasmid. This circle of DNA will also contain at least one other gene, which can then be used to track the insertion of the gene of awesome. This is called a genetic marker, and plant biologists often use a gene called GUS. GUS is useful because in the presence of a particular chemical it will turn a plant blue.
Then you need to get your plasmid into a plant cell. This is done in a variety of ways, including my personal favorite, the “gene gun”. In this method plasmid DNA is applied to tiny particles of gold. These microscopic bullets are then fired at the plant, and the DNA is incorporated into the plants genome.
After growing the cells into seedlings, the GUS marker can then be used to select the seedlings that contain the gene of awesome. The new transgenic plants are then grown and propagated, et voila! You have a genetically modified plant.
The most recent Monsanto-related headlines have pertained to Bt-corn being approved for sale at Walmart with no indication to the consumer that that’s what they’re buying. If you have a problem with Monsanto, then I fully understand why you might not want to eat their corn. However, that it is poisonous to humans is, as far as I can tell, a spurious claim.
Bt stands for Bacillus thuringiensis, a bacteria that produces pesticidal toxins. These toxins, called Cry proteins, attack the larvae of particular insect species (including moths, butterflies, beetles, and wasps) and kill them. Cry proteins do this by recognizing proteins found on the cells of the larval gut wall. They then insert themselves into the membranes of these cells forming a channel through which water can flow. When enough water flows into the cell it bursts, and when this happens to enough cells the larva will die. Importantly, the proteins that Cry recognizes are specifically expressed in these insects, which means it represents a safe and specific pesticide.
The use of Bt in agriculture dates back to the 1920’s, when French farmers began using it for pest control, and continues to be used extensively in approved organic pesticides (Dipel and Thuricide).
The Specter of Resistance
Recently it has become apparent that insects are increasingly becoming resistant to Cry. Understandably this has infuriated organic farmers who rely on biological pesticides too. Monsanto has attempted to combat this through the use of “refuges”. Refuges are small amounts of non-Bt seeds that are mixed into Bt products (about 5% of the seed is non-Bt) that when planted form regions of the field where insects can flourish without relying on resistance. This reduces the selection pressure on the insects to become resistant to Cry.
So…Monsanto. Friend or Foe?
I will leave that for you to decide, but would love to hear your thoughts in the comments section. Equally, if you have any questions, I will do my best to wrangle the information out of the internet, although the polarizing nature of this topic makes finding unbiased information challenging to say the least!