Genetic Engineering and agronomic crops: The Science and the Potential Benefits
Part 3 of 3 |1 | 2 | 3|

J. Lynne Brown and Koushik Seetharaman
Department of Food Science

Making GE plants

Genetic Engineering, a technique based on work done by several Nobel prize winning scientists, is used widely today in the biotechnology industry and at university research laboratories. While the actual technique is pretty well defined, the claims made about the preciseness of the technology are often not accurate. Here is a description of how it is done and an example of how it has been applied in plants.

How is genetic engineering generally done in plants? Scientists usually use a ‘natural’ genetic engineer, a common soil bacterium called Agrobacterium tumefacien, to accomplish this. This bacterium contains a small circle of-free floating DNA called a plasmid.

The bacterium normally uses this plasmid to transform certain plants for its benefit resulting in the crown gall disease.

Scientists first identify the gene that controls the desired characteristic or trait they want to transfer to the target plant. They remove the gene or piece of DNA that has this trait from the donor organism using special enzymes that act like scissors. They also take the plasmid out of A. tumefaciens and snip out some unnecessary DNA from the plasmid making an open circle of DNA. They then mix the ‘open’ plasmid with the gene taken from the donor organism and using special enzymes that paste DNA together they can produce a plasmid that contains the desired gene. This plasmid is called recombinant DNA. The recombinant plasmid is inserted back into A. tumefaciens. These bacteria containing the altered plasmid are mixed with cells from the target plant. Some of the plant cells take up the desired gene from the plasmid and insert it into their own DNA. When these plant cells are grown in tissue culture to small plants, they can be tested to see if they have taken up the new gene. Those that have are called transformed plants and are tested further.

What is a specific example of the use of this technique? Summer squash like yellow and zucchini are attacked by a number of viruses. These viruses kill the plants and reduce the yield of squash from a farmer’s fields. Viruses are very small organisms that are composed of specific DNA enclosed in a capsule or coat of protein. Scientists have learned that if a plant contains the gene for the coat protein of the virus, it is resistant to the virus. Using this information, scientists have produced squash plants that are resistant to specific viruses.

The A. tumefaciens containing the rDNA plasmid are mixed with the target squash cells in tissue culture. Some of the squash cells take up the virus coat protein gene from A. tumefaciens and integrate it into their own chromosomal DNA. These cells grow into plants and they are tested to see which ones are resistant to the virus. Those that are resistant to the virus are further tested.

Are there other ways to do genetic engineering?

Yes. Scientists also use a ‘shotgun method.’ In this case, the scientist isolates the gene or genes of interest. These are used like very small shotgun pellets. The DNA-coated pellets are placed on a support that is in front of an array of target plant cells. A blast of helium gas is aimed at the pellets so that they are shot at the target plant cells.

Some of the pellets hit and enter the cells and do not go straight through. What happens in the cells that receive and retain the pellets is a mystery. But some of these cells take up the foreign DNA into their own DNA. These cells are grown into plants and tested. This method is more commonly used in cereal plants like wheat, rice and corn where Agrobacterium is not suitable to use.

What are marker genes? Scientists need a way to tell if the desired gene has been transferred into the host plant’s DNA. The actual act of transformation or incorporation of the new gene into the plant’s DNA does not happen very often. The process of transformation is not as efficient as might be desired. Scientists need a way to identify the few plants that actually were transformed from the far larger number that were not. So scientists place a ‘marker’ gene in front of the actual gene to be transferred. Think of the marker and the gene of interest as the head and the tail of a worm. If the head can be detected in the transformed plant, it is more likely the tail was also transferred.

Marker genes are more useful if they help the scientist ‘select’ for successful transformation events. Many times the successful transformation does not provide cells or plants with easily identifiable traits. So the marker provides a trait that can identify those plants that were transformed. Antibiotic resistance genes have been used successfully as marker genes. These genes allow plant cells to produce proteins that protect it from the effects of a specific antibiotic. Usually antibiotics inhibit the growth of normal plant cells that are not antibiotic resistant. If an antibiotic marker is used in a plant transformation, the scientist takes the cells exposed to the transformation and grows them in the presence of the antibiotic. Only those cells that received the antibiotic resistance gene will grow into plants. These plants were more likely to be transformed. These plants can then be tested to see if they received the gene linked to the marker gene. This greatly reduces the amount of testing necessary to identify transformed cells.

What is tissue culture? This is a process in which plant cells or small pieces of plants can be grown in flasks or petri dishes in the laboratory. The container has material in it called media that supplies the nutrients needed by the plant tissue to grow. This technique allows scientists to perform tests on plants more quickly than if working with the entire plant in a green house.

Real and Potential Benefits

Genetic engineering is the latest in the series of technological advances achieved in agriculture, since humans first started cultivating crops more than 10,000 years ago. Early on we depended on visual selection of beneficial traits and replanted only those seeds that showed some beneficial attribute such as surviving a disease outbreak or increased seed size. In doing so we carried over the inherited traits into the next generation. This is a practice that is used by farmers and plant breeders to this day. More than 2300 years ago, the Greeks developed and recognized the benefits of grafting plants resulting in the creation of orchards and groves. About 150 years ago, we understood the principles of heredity and realized that specific inherited traits could be transferred from one generation to the next. Less than 50 years ago, we recognized that genes located in the DNA are responsible for individual traits. Since then, the technology has rapidly advanced over the past two decades so that today we can specifically insert, remove or manipulate specific genes to alter one specific trait.

The advances in agricultural technology have yielded great benefits to societies and helped shape civilizations over generations. Crop yields, quality and agricultural practices have improved dramatically resulting in the abundant, safe and cheap food supply that we take for granted today. However, these same technological advances have also given us problems ranging from increased use of chemicals in agriculture to soil erosion and desertification.

What are the benefits from the first generation of GE crops? First generation GE crops are typically referred to those that were not only the first to be released, but also to traits that were targeted primarily to benefit farmers to improve agricultural practices and increase yields. These traits do not benefit consumers directly. Examples of first generation traits are Roundup Ready crops, Bt crops or virus resistant crops. The benefits derived from these modifications include:

What are the potential benefits from the second generation of GE crops?

Second generation GE crops are those that will directly benefit consumers by way of improved nutrition, quality or other attributes. There are several examples of these crops currently being developed and tested for commercial release.

The real and potential benefits of GE need to be evaluated in the context of the global scenario. World population is expected to increase from 6 billion now to about 9 billion by the year 2025. Meeting the food and feed demands of the increasing population has to occur within the context of diminishing natural resources such as arable land and water. Much of the increase in population and the reduction in natural resources will take place in the developing and underdeveloped regions of the world. The benefits offered by this technology will have greater impact in these regions of the world by improving crop productivity and utilizing marginal or diminishing land and water resources. However, inappropriate use of GE technology can also result in greater damage to the environment in this region as well as put small farmers out of business, and more dependent on a technology on which they have no control. Therefore, GE technology needs to be regulated and evaluated on a case-by-case basis based on the crop being developed and the region for which it is being developed.

For more information:

http://biotech.cas.psu.edu – this site provides some informational articles plus a variety of links to other organizations.

http://www.colostate.edu/programs/lifesciences/TransgenicCrops – this site explains in more detail (with pictures) how GE plants are created.

http://www.comm.cornell.edu/gmo – an excellent site to help you think about genetic engineering applications and their implications.