The down low on GMOs
Examining some of the science behind genetic engineeringDecember 2016
By Trista Crossley
Can you pick out which of these plants are genetically modified according to the definition used by U.S. regulatory agencies?
1) Grapefruit mutagenized with thermal neutrons;
2) The fusion of cells from two different plant species to generate a hybrid with both sets of chromosomes;
3) Doubling the number of chromosomes in a cell/species; or
4) Reduced expression of a specific gene in a species through silencing.
Despite all four sounding like something that would happen through genetic modification (GM or GMO for genetically modified organism), only the last one would actually fall under the definition of a GM plant according to most U.S. regulatory agencies (and to much of the public as well). The other three options are classified as conventional breeding techniques despite the fact that in breeding, conventional or otherwise, DNA is being modified. That idea was one of the main points of the presentation given by Joseph Kuhl, a plant molecular biologist from the University of Idaho, at the 2016 Tri-State Grain Growers Convention last month.
Kuhl’s presentation, “The Dilemma: The Science behind GMOs,” focused on what GM crops are and how they fit with conventionally bred crops. Among other topics, he talked about different types of genetic engineering, common ways researchers insert DNA into cells and his belief that GM products are misunderstood.
“One point I really want to make is that GMO plants and animals are not defined by what they are or what traits they carry, they are defined by the process through which they are generated. It is the method of breeding that is used to generate them that has given rise to this term GMO,” he said.
Without getting too deeply mired into the complex science of genetic engineering, Kuhl walked attendees through the most common methods of conventional breeding and contrasted them with genetic engineering. He also touched briefly on some of the regulatory hurdles GM crops have to jump through before being approved for commercialization. Here are some of the highlights from his presentation.
All of today’s fruits and vegetables were domesticated from wild ancestors. Kuhl used maize and cucumbers as examples of wild plants that were modified by humans over thousands of years of selective breeding to produce bigger ears of maize and less bitter, less spiny cucumbers. “As far as I’m concerned, this would fit my definition of genetic modification, but it doesn’t fit the definition of genetic modification as used by regulatory agencies,” he said. “All crops, all fruits, all vegetables have been modified by humans over time to a more domestic form.”
Different methods of conventional plant breeding. “Before GMOs came along, this was the only game out there,” Kuhl said. In conventional plant breeding, the genes from two parents are crossed to produce progeny with various traits. Hybridization, or cross pollination, is the most widely known method of conventional breeding. Kuhl explained that boundaries between plant species are much more flexible than those between animal species.
“Generally, you can reach out from the crop species you are working with into some close relatives, and you can generate progeny. In many cases, those progeny will be fertile and you can use them in further crosses,” he said.
On average, plants have about 30,000 genes, so there is widespread genomic exchange between the parent lines to the progeny. With no way to direct the exchange of genes, researchers have to search through the resulting progeny in hopes of finding one with the right combination of genes that express the trait they are looking for.
Other types of conventional plant breeding methods include:
• Wide crosses are crosses between species that do not normally reproduce with each other. In animals, the resulting progeny are usually infertile, such as a mule. In plants the resulting progeny can be fertile.
• Embryo rescue. This method takes a seed from a cross that might germinate but would never survive to grow into a seedling. As Kuhl said, this allows researchers to recover plants from crosses that would otherwise fail.
• Mutagenesis happens when seeds or plants are exposed to chemicals or radiation, causing mutations to appear in their DNA. “There are thousands of cultivars in the grocery store today that have been given rise through mutagenesis,” Kuhl said. “It is a bit of a crap shoot when it comes to breeding. It takes a lot of screening, and you don’t always know what you are going to get, but it has given rise to traits that have been very valuable and not present in the parental lines.” Rio Red and Star Ruby grapefruit are two popular example of mutagenesis. In Europe, mutagenesis is considered a form of genetic modification, but not in the U.S.
The definition of a GM plant. A GM plant contains a gene or gene fragment via recombinant DNA that has been artificially inserted instead of the plant acquiring it through pollination. Kuhl focused his discussion less on the recombinant DNA part of the definition and more on how that DNA is inserted. Researchers can insert DNA into plant tissue in many ways, but the two most common are via a “gene” gun, which physically blasts genes or gene fragments into a plant’s cell, or by using a bacteria to introduce a gene or gene fragment into a plant’s cell.
Agrobacterium tumefaciens, sometimes called agrobacterium, is a naturally occurring soil bacterium that has evolved over millions of years the ability to introduce fragments of DNA into plant cells. Crown galls, large growths usually found on woody plants, are the outcome of an agrobacterium infection.
“So the inserted DNA sequence, in the case of genetic modification, can come from the same species, such as wheat genes into wheat, potato genes into potatoes. But you have flexibility here,” Kuhl said. “It’s wherever you can recover a gene from, and with today’s science, we can recover genes from just about every biological organism out there.”
Types of genetic engineering. Any plant using one of the following three breeding processes would be considered genetically modified:
• Transgenic, which is using DNA from a distant relative or a nonrelative of the species being modified.
• Cisgenic, in which the gene or DNA fragment being introduced is coming from a closely related, sexually compatible species. Silencing a gene that causes an apple to go brown after being cut or silencing a gene that causes bruises in potatoes to go brown are examples of this type of genetic engineering.
• Intragenic means you are doing a target modification of a gene that is in the cultivar naturally. In most cases, researchers are not introducing new genes or new DNA or DNA fragments, they are modifying the existing DNA.
“So in conventional crops, only genes that are in closely related species are involved in the development of new cultivars,” Kuhl summarized. “In the case of genetic modification, those genes can come from a wide range of organisms. Traditional methods are going to focus on exchanging the entire genome of two parental lines, so you are going to be mixing up tens of thousands of genes. In the case of genetic modification, you are focused on one, two, three or four genes. It’s very targeted. If it’s one gene one trait, that may be the only alteration you observe in the genetically modified organism. Those are two of the primary differences between conventional breeding methodologies and modern breeding methods of genetic modification.”
Observations on the regulatory process. GM crops are tested to make sure the modified cultivar is substantially the same as the unmodified cultivar with the exception of the targeted trait the breeder was aiming for. The testing, also done by the breeder, needs to show that an introduced protein is neither toxic or allergenic.
Kuhl used a graph from a 2004 report on the safety of genetically engineered foods from the Institute of Medicine and the National Research Council of the National Academies that illustrated the unintended changes associated with different breeding methods. He pointed out that the risks associated with using agrobacterium are fairly low compared to several methods of conventional breeding, especially mutation breeding, or mutagenesis.
“This is one reason why, when mutation is used in breeding, it is extensively tested and screened over many, many years to ensure that the recovered line is similar to the originating, unmutated line,” he said. “There’s a tremendous amount of testing, but intriguingly, this doesn’t fall under the general public’s definition of GMO.”
GM crops are heavily regulated in the U.S., and there are usually many patents associated with a GM variety. Genes can be patented. The process that isolates a gene can be patented, not to mention the process leading to genetic modification can be patented. Before a variety can be commercialized, every patent associated with that variety must be resolved.
“That gives you a little insight as to why GMOs are primarily being released by some of the largest agriculture companies out there. They have the money and the legal background to push through the regulatory and the patent protection side of things,” Kuhl said.