The Science Behind GM Crops

UPDATE: This is part of a series on GMOs. Links for all of the posts for this series are indexed on my highlights page. Check out all of them.

If you’ve been wondering about the lower amount of posts this week- sorry! I’m back now and this one’s a doozie.

Let’s start by breaking down the basics on genetics and how scientists can manipulate genes to create GMOs. This will be somewhat vocabulary intensive. However, some of these words have made their way into common usage in modern English, but they may not always be used properly.

DNA double helix Credit: mstroeck via Wikimedia Commons http://commons.wikimedia.org/wiki/File:DNA_Overview2.png

For all life* forms on Earth, DNA is the molecule that dictates how life happens. It is present in every cell of every creature orchestrating the biochemical reactions that occur undera given set of environmental conditions. DNA is comprised of units called nucleotides, which are joined together in strings of sequences to code for the information necessary for life. This is remarkable because there are only four different kinds of nucleotides. Thus, all of the different combinations of these building blocks put together along sequences of varying length generate the diversity of life on Earth. The sum total of the DNA within an organism is called its genome. Certain lengths of DNA sequences within the genome make up units called genes. For simplicity, these genes encode information that ultimately makes proteins, which are the workhorses of biochemistry in cells. The repertoire of biochemistry performed by a cell or group of cells in response to the environment creates an organism with certain characteristics called traits. Individual organisms have some variations (sometimes called mutations) in their traits, which are traced back to variations in the DNA sequences of their genes. These variations occur naturally a couple of different ways. Variations can be the result of errors when DNA must be copied every time cells divide. Variations also result from sexual reproduction with the DNA of two different individuals combines to form a new unique offspring.

DNA nucleotides
Credit: Madeleine Price Ball via Wikimedia Commons
http://commons.wikimedia.org/wiki/File:DNA_chemical_structure.svg

If this sounds too general for you, here’s an example. For cultivated organisms, like our crop plants, humans have identified individuals with useful traits like bigger, sweeter ears of corn or hotter chili peppers. These new individuals with more useful traits probably came from the selective breeding of individuals with promising characteristics. While we are talking about plants, I will briefly mention hybrids as well. Hybrids are the offspring of two different parent individuals that yield a set of desirable traits, but these traits cannot be faithfully passed on to the offspring of the hybrid. Many tomato varieties are hybrids. Often seed companies do not tell you what the parents were if you were even inclined to do your own tomato breeding. Nevertheless, someone has manually forced the reproduction of two significantly different parent tomato plants to provide the seeds in the packet you bought.** These seeds will grow into a plant that will make the tomato with the desirable hybrid traits you want (Better Boy or Early Girl are examples). After you have enjoyed your harvest of tomatoes, you are more than welcome to try to save some of the seeds from those hybrid tomatoes, but the plants they produce will not necessarily have all of the characteristics as their parents. This is why you must buy them new every year. On the other hand true-breeding varieties will always produce plants with their particular characteristics (Cherokee Purple or Yellow Pear tomatoes).

OK, on to other methods of making new varieties. Humans can also induce variations or mutations in organisms with treatments that alter DNA- things like certain chemicals or radiation treatment. Obviously, these treatments can result in less useful or often inviable organisms because it is easier to mess things up than make something new. However, on a large enough scale, these treatments can result in individuals with new desirable traits that can be propagated or passed on to offspring. If you are starting to pucker a little at this kind of genetic manipulation when it comes to your food, hold on to your taste buds while we consider the history of the grapefruit. The grapefruit is actually a hybrid of two other citrus species (pummelo and sweet orange). Not long after it began to be grown in Texas, farmers started noticing some trees with darker red buds that yielded darker fruit. Some of these natural variants were subsequently propagated and given proper variety names like Ruby Red grapefruit. After this, researchers at the A&M Citrus Center screened the offspring of parent grapefruit plants treated with with ionizing radiation. They were able to identify grapefruit with redder, sweeter flesh and greater cold tolerance. This fruit is known as the Rio Star and accounts for up to 65% of the grapefruit crop in Texas. For other examples check out this link on atomic gardening (also touches on GM at the end).

I should also mention another commonly used genetic manipulation technique used in agriculture for the cultivation of plants before we move too far on from fruit trees because they provide a good example. Growers can clone certain plant varieties to ensure that an exact copy of the desirable material is reproduced. This is possible because plants have the ability to re-grow starting from only parts of themselves. For example, entire blackberry brambles can efficiently grow back from just a small root segment. Anyone that has grown mint knows that part of that plant can be removed to another container and it will continue to grow into another larger plant (and take over most of your yard if you don’t give it any boundaries). Cloning is also particularly useful when it comes to growing fruit trees. In this case, part of a desirable variety can be grafted onto a standard rootstock. As this grafted tree grows, only the top portion will grow and make the desirable fruit, while the bottom portion will grow and form the root system. This is efficient because in the cases of many modern fruit varieties, the seeds to do not yield true copies of their parents. When it comes to the scale (time and space) involved in growing orchards, it is important for growers to produce consistently desirable fruit. Cloning via grafting makes this possible. Even on the scale of home fruit tree planting where there is space for only a handful of trees, grafted trees are the way to go because you are not likely to get a desirable variety from seed from just a few trees. Established varieties like Golden Delicious or Gala apples are propagated in this way. However, when breeders have identified a new variety from some other genetic manipulation, pieces of that new variety can be cloned onto waiting rootstocks for production within a reasonable time frame.

From the examples given above, it is clear that humans have been manipulating the genes of plants for food and aesthetics for centuries. The only differences have been with the tools and scale in terms of time and numbers of individuals. To this point, I haven’t even started talking about the genetic modification technology that is such a polarizing topic in our modern food system. The often vilified GM crops are modified in such a way that they are transgenic. This means they have been engineered by humans to contain genes (pieces of DNA sequence) from other organisms with which they would otherwise never be able to exchange DNA. These new genes confer new useful traits to the organism. Let’s start with how this happens.

From decades of studying how DNA determines the biochemistry of organisms and how that biochemistry translates into traits, scientists have learned quite a bit and borrowed tools from nature along the way. So through a lot of research in a number of different organisms, scientists have identified a gene (or genes) that confer(s) a particular desirable trait that might be useful in a plant. In the simplest case, that trait ultimately corresponds to a single piece of DNA (gene). The DNA molecule itself can be removed from or, more accurately, it can be reconstructed from whatever source organism in which it was found. This is possible because the properties of DNA are universal among all organisms. There are some enzymes (a general term for proteins that do useful biochemical work) that act as molecular scissors to specifically cut DNA and there are other enzymes that act like molecular glue to paste DNA piece together. There are other techniques that allow scientists to specifically change a single (or multiple) nucleotide(s) within a DNA molecule. All of these manipulations fall under the umbrella term molecular biology. Without going into all of the details, modern scientists are well-versed in techniques that allow them to construct any possible DNA sequence originating from any organism. There are other techniques that allow scientists to introduce this engineered DNA molecule into the organism of their choice so that it becomes part of the genome of that organism. Once the engineered DNA is integrated within the organism’s genome, it will confer the desired trait to that organism. Also, the engineered DNA will be present in every cell of the organism and passed on to offspring along with the rest of the organism’s genome.

Check out this awesome animation of the process presented by the University of Nebraska Department of Agronomy.

Overview of Genetic Engineering

Here’s another great resource page on Transgenic Plants from Colorado State: http://cls.casa.colostate.edu/transgeniccrops/index.html

Sometimes life is simple and there is a one to one relationship between the gene and the trait no matter the organism context. However, life is not always so simple. Sometimes moving a gene into a new context does not give the desired trait because of other biochemical pathways in the new host organism. Sometimes life is even more complicated and multiple genes determine a desired trait. In this case, a desired trait is present only when all of these genes work appropriately in the new host. The bottom line is that a great deal of effort is put into identifying candidate genes for desirable traits followed by tinkering with them to actually yield the desired trait in the organism you want. Then, scientists must make sure that no other important biological function of the host has been compromised in the modification- otherwise the work is all for nothing.***

It is important to compare the different genetic manipulations mentioned above with one another. When it comes to inducing mutations to yield new varieties, the technique is completely random and there is no way to know what you are going to get out of it. Only a few offspring will have desirable traits. Another issue with this technique is that the underlying mechanism for the new desirable trait may remain unknown for some time. In other words, radiation may give you a purple sweet potato, but you don’t know specifically what molecular changes have been made to the DNA to give you the variant proteins that resulted in this remarkable biochemical feat. In the case of selective breeding, there is a reasonable expectation that the offspring will have desirable traits, but there are no guarantees. Making new hybrids is probably on par with selective breeding for producing offspring with the traits you want. Cloning by grafting or other methods that completely bypass sexual reproduction ensure that the offspring will be exactly what you intend them to be. Genetic engineering by introducing novel genes from other organisms is done in a very precise manner to yield an expected outcome (sweetness, color, resistance). Genes from other species are not just randomly inserted in hopes of a desirable trait; many years are spent teasing apart the mechanisms for how a gene (or genes) confer(s) a certain trait before moving it into a separate species and expecting it to work in a similar way. Of course, there are no guarantees for success in the initial stages of making transgenic plants. There could always be unforeseen problems upon introduction of new genes that make the plants weaker or otherwise less desirable, in which case they would never make it to production scale.

One overarching issue with respect to the plants grown in our modern agricultural system is that we are relying on a smaller genetic pool to meet our needs. We are trying to pack in all of the human-desirable genes of a species into a single super variety. Once these new varieties are identified for characteristics like great flavor or convenience in growing, harvesting and processing, they are mass produced in large monocultures. This issue is completely separate from the manner in which their genetics came into existence. If biology has taught us nothing else, it is that genetic diversity is key to long term survival. When there is a change to the system, populations are better off with a larger gene pool vs. a smaller one. If a new disease is introduced, a larger, genetically diverse population is more likely to have resistant individuals that will survive than a smaller, genetically similar population. The same premise holds true for other challenges like changing environmental conditions and new invasive species. Check out this link from FAO about the current state of our global agrobiodiversity. You may think that we live in a world of seemingly limitless variety, but these statistics will surprise you. Here’s one:

“Today, 75 percent of the world’s food is generated from only 12 plants and five animal species.” FAO, WHAT IS HAPPENING TO AGROBIODIVERSITY?

Clearly, this is a topic for a broader discussion, but definitely something to keep in mind when thinking about the subject of GMOs and our modern food system.

Now that you have some background and context, I hope you understand genetic engineering a little bit better and how it compares to the kinds of genetic manipulations humans have been performing on plants for millennia. I believe that genetic engineering and transgenic plants in and of themselves are not inherently evil. It represents a new tool in our toolbox for getting the most out of the organisms we rely on for food, fiber and fuel. As consumers, it is important to make informed decisions about how this new tool should be used.

Next time I will discuss the specifics on the science behind some of the major GM crops currently in production. Maybe those specific examples will be more understandable than the general description given today. I will also address the role of transgenic technology for generating plants to meet our future demands.

Johnna

* I’m not counting viruses as life forms because things get more complicated.

** Try not to think about tomato or any other vegetable/fruit rape here.

*** There will be a future post delving into more details on the testing that goes into putting transgenic plants into production along with other safety considerations, so we’ll leave it here for now.

Advertisements

One thought on “The Science Behind GM Crops

  1. Pingback: A Plant Scientist’s Perspective on Genetic Engineering | New Under The Sun Blog

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s