Glyphosate-resistant crops are prime examples of genetic engineering. These crops have been genetically modified to withstand the effects of herbicides that use glyphosate as the active ingredient, most notably the product Roundup.
How Glyphosate-Resistant Crops Resist Herbicides
Like all other living things, plants need fatty acids to survive. Unlike animals, however, plants do not need to consume these fatty acids in their diets. They can manufacture them with the help of an enzyme called EPSPS.
A glyphosate herbicide works by binding to EPSPS. In a bound state, EPSPS cannot begin the metabolic process by which fatty acids are synthesized. Therefore, a plant treated with glyphosate will gradually die of malnutrition.
Extensive experimentation led to the discovery of a variety of the Agrobacterium genus that could survive glyphosate treatment. This bacteria has a different form of the enzyme EPSPS. Glyphosate can still bind to the the enzyme, but it must take a slightly more compacted shape which cannot interfere with the work of synthesizing fatty acids. Although this form of EPSPS is perhaps not as efficient as the normal form, it is still effective enough to keep plants alive and growing.
Through genetic engineering, this enzyme was introduced into soybeans and corn first, but later alfalfa, canola, cotton, and sorghum.
How Glyphosate-Resistant Crops Are Engineered
Creating glyphosate-resistant crops is not a simple process. The gene from Agrobacterium species that produces the altered form of EPSPS is just one piece of the puzzle.
Another component that has to go into glyphosate-resistant crops is a promoter. A promoter is a region of DNA that cues transcription of a gene, putting RNA molecules to work replicating and executing the code, so to speak. To ensure that the gene for altered EPSPS production is properly replicated throughout the plant, a promoter is borrowed from the cauliflower mosaic virus. This promoter was chosen because it ensures that the desired EPSPS gene is expressed in all cells.
EPSPS does its work in the chloroplasts of the plant, the food factories where photosynthesis takes place. Therefore, part of creating a glyphosate-resistant crop is making sure that the new type of EPSPS will be able to enter the chloroplasts. Besides the EPSPS gene, a chloroplast transit peptide was added to the coding sequence. A chloroplast transit peptide is a short chain of amino acids that plays a role in regulating secretions. In this case, the transit peptide helps the EPSPS enzyme into the chloroplasts to do its work. A transit peptide coding sequence from a hybrid petunia is fused to the EPSPS coding sequence to make glyphosate-resistant crops.
Once all of the necessary components have been combined, the resulting package of DNA is cloned. Now the genetic material is ready for insertion into the desired crop. For this purpose, a device called a gene gun is used to bombard a plant tissue culture. The gun fires microscopic pellets made of a heavy metal, such as gold. The pellets are coated in DNA.
After the plant culture has been bombarded, it is usually in a state of disarray, with some of the cells being damaged beyond use. All intact cells are carefully gathered up, treated with hormones, and kept in a culture to allow entire plants to develop from them. These plants are then tested to ensure that they are truly glyphosate-resistant. The genetically modified plants serve as sources of glyphosate-resistant seeds.
Helpful Resources
Roundup Ready
An easy-to-follow explanation, part of a University of Nebraska lesson on genetic engineering.
“Molecular Basis for the Herbicide Resistance of Roundup Ready Crops”
A technical explanation of how glyphosate resistance works.
“Soybean Genetic Transformation”
Information on how crops are engineered from the Genetics and Molecular Biology journal.
