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.
In my last post, I described the generalities of genetically modifying organisms by a variety of methods. Today, let’s talk specifics about currently cultivated GM crops. The overwhelming majority of GM crops have been engineered for herbicide- or pest-resistance or both. These traits were relatively easy to engineer because the modes of action were well understood because of years of previous studies and they each require only a single gene to confer. When I posted about the GMO numbers, these are the engineered traits that have been introduced into commodity crops like corn, soybeans, cotton, and sugar beets.
The herbicide glyphosate, marketed under the trademark RoundUp, specifically targets an enzyme plants use in their biochemical pathway for making a class of amino acids (essential building blocks of proteins) along with other important plant hormones and electron carriers. The enzyme is called 5-enolpyruvylshikimate-3-phosphate synthase, but we’ll just call it EPSPS for short. EPSPS is found only in plants and microbes and not in animals (we have to eat plants to acquire the class of amino acids produced in the EPSPS pathway). Glyphosate is easily absorbed through leaves and most effective on actively growing plants. It effectively inhibits EPSPS and plants have a difficult time metabolizing glyphosate into anything less toxic. Some crop plants have been genetically engineered with a bacterial version of the EPSPS, which is not susceptible to glyphosate. In this case the EPSPS transgene encodes an enzyme to replace one that the plant already makes. No new biochemistry is gained, only the ability to bypass this otherwise lethal inhibitor. This means that fields of genetically engineered ‘RoundUp Ready’ plants can be sprayed with glyphosate to stop the growth of all surrounding weeds leaving the GM plants impervious to the treatment.
The pest resistance is conferred by a toxic protein produced by the bacterium Bacillus thuringiensis (Bt). This is a naturally occurring soil bacterium that has been used in pest control for decades. The bacteria make this pesticidal protein that crystallizes within their cells. When the crystals are ingested by insects (like cotton bollworms or corn borers) the toxic proteins are activated within their acidic digestive tracts where they destroy the insects from the inside out. Bt in various forms (sprays, wettable powders etc) has been used for agricultural pest control for decades and is approved for use in organic farming. Many safety studies have been conducted on Bt toxins, and they have reported few if any negative effects of Bt on humans or the environment. Given its effectiveness and long history of safety, crop plants have been engineered to produce the pesticidal Bt protein in their cells to eliminate the need to spray the crops with Bt or other pesticide formulations. In this case, the plants have acquired a truly novel protein. It doesn’t add any biochemistry to the plants, but offers them protection from insects. While there is a long history of safety with use of Bt, these Bt-plants produce the Bt pesticidal protein in every cell and this cannot be washed away as it can be when used as a spray.
Plants can be susceptible to diseases (viral or bacterial) in the same way that humans and other animals can be. Plant diseases can be particularly devastating to agriculturally important crops. When it comes to battling plant diseases, genetic engineering represents an efficient way to get an edge over the pathogens. Sometimes traditional breeding just isn’t fast enough or doesn’t provide the tools necessary to confer resistance in time to save an industry. This is especially true in the context of fruit production where breeding and testing fruit trees takes years. One success story for genetic engineering disease resistance comes from the papaya. The papaya ringspot virus (PRSV) was devastating the Hawaiian papaya industry. Researchers at Cornell and the University of Hawaii were able to develop a genetically engineered papaya that contained a small amount of PRSV DNA which provides the plants with immunity to the virus. The original transgenic papaya variety was called SunUp, but it could be crossed with non-transgenic plants to yield better hybrid papayas designated the UH Rainbow variety.
These traits do wonders for the industrial scale farming of these crops, without adding or detracting nutritional value. It’s why (in the U.S.) our processed foods and meat products are so cheap.* It’s also very lucrative business for the companies that sell the seeds to farmers. It’s the acme of capitalism when sometimes the company that sells you the engineered seeds can also sell you the complementary agrochemical.** Clearly, these examples were the first on the market and have gained an enormous market share within our major crop species. Without discounting too much of the science that went into creating these varieties, these traits represent the low-hanging fruit of the transgenic world (single gene transfers conferring profitable traits).
The future of genetic engineering gets much more complicated. Scientists will continue to work with farmers to combat the issues they face in production (disease and pest resistance), but this will always be (as it has been throughout history) an escalating arms race between plants and their diseases and pests. The other problems scientists would like to address include things like getting plants to have higher yields, use nutrients more efficiently (aka use less fertilizer), and have higher drought tolerance. These traits are much more complicated involving multiple genes, which are likely to vary more among different plant species (meaning there won’t likely be a one-size-fits-all solution universal for all plants). Beyond optimizing these production-related issues with our crop plants, scientists would also like to engineer plants with increased nutrition or flavor. Again, these are more complicated traits that would require more sophisticated manipulations. When it comes to engineering these types of ‘next generation’ traits, scientists will likely be taking advantage of the wealth of genetic diversity within the plant kingdom.
In my next post, I’d like to cover some examples of ‘next generation’ GM food plants on the horizon.
* Whether our food system should be optimized for cost efficiency to the exclusion of all other virtues is a completely separate discussion. We’ll get into more details on these larger issues in our modern agricultural system, but suffice it to say that eliminating the use of genetic engineering will result in a higher cost for our food (either through money or increased labor energy). My intent is simply to describe the facts of our current system with respect to these plants. Obviously, our food system has many less-than-appetizing features, but they go unnoticed in our bountiful modern grocery stores filled with cheap food on every aisle.
** It’s the real-life equivalent of owning Park Place or Broadway with three hotels. Don’t hate the player, hate the game, right?