840.119:Developing Glyphosate Resistance in Tobacco Plants (Jaime and Hanna)

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840.119: Introduction to Biotechnology

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Contents

Project Description

The herbicide glyphosate has been around for over twenty years and since the technological advances in genetically modified organisms, the development of a resistance in crops has become a desired and beneficial product. A gene that has been found to be unresponsive to glyphosate is inserted in crops to develop a transgenic plant. This plant can then be grown in abundance, and the field it's growing in can be sprayed with an herbicide containing glyphosate. Glyphosate has been favored by growers due to its low costs and effectiveness. This helps in destroying weeds that could harm the crop, without harming the actual crop itself, and it allows growers to develop an easily manageable program.

Objectives

The main objective of this website is to discuss the impacts and risks associated with glyphosate resistance. We will also discuss a bit about how glyphosate works and the technology behind developing glyphosate resistance.

Current Usage

Tobacco plants that have been modified for glyphosate resistance are currently being used by farmers and the results of the resistance to the herbicide have been successful. There has been widespread use of glyphosate on crops such as soybeans, cotton, and tobacco since the late eighties or early nineties. But with new technology, using the combination of glyphosate and glphosate resistant crops has become hugely popular. About eighty percent of the U.S. market in farm crops is now in plants that tolerate glyphosate. Glyphosate is an inexpensive herbicide, that is potent yet mostly enviornmentally friendly, so it has great appeal. Environmentally friendly refers to the idea that it is biodegradable under most circumstances, so it doesn't harm the soil or organisms that would be indirectly effected by the spray of a crop. It has been trademarked as "Roundup" by the coorporation Monsanto. Because of the qualities that glyphosate possesses, resistance to the herbicide in crops is very important.

Herbicide resistance is the most widely planted transgenic crop trait. About 75% of all genetically modified crops are engineered for herbicide resistance.

Scientific approach

Methods Used

Image:Fig_6_10.jpg

Glyphosate is a chemical herbicide that inhibits 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS). EPSPS is an enzyme that is important in aromatic amino acid production. Basically, when this enzyme is inhibited, the plant dies. However, scientist have found a form of EPSPS in bacteria that is naturally resistant to glyphosate. They have used this resistant form to engineer resistance to the herbicide in many plants.

1. Take the resistant EPSPS from bacteria and insert it into a vector.

2. Put the vector into the plant. Scientists now put the vector into the cholorplast of the tobacco plant. Cholorplasts are maternally inherited, so this process prevents the transmission of the foreign gene by pollen to nearby plants.

3. With the bacterial form of EPSPS in the tobacco plants, you can then spray all plants with glyphosate.

4. The EPSPS form naturally found in plants is inhibited, and only the plants with the bacterial form can survive the glyphosate application.

5. All plants that were not modified with the vector containing the bacterial EPSPS (i.e. weeds) die.

Alternative Methods, Why aren't they used?

Researchers have been looking for herbicides that will spare the crop, but kill the pest. Often crops and pests have similar biochemical properties, therefore finding an herbicide that won't affect both has been difficult. Today, with new advances in biotechnology, researchers have found it to be more beneficial to use genetic engineering to create a crop that is less susceptible to glyphosate-based herbicides.

(Potential) Impact

The development of herbicide resistant crops has helped spare the environment from harsh chemicals and reduce the cost to farmers on chemicals for pest control. Chemicals in herbicides designed to be used on herbicide tolerant crops tend to be much more environmentally friendly and much milder than those used on non-tolerant crops. Also, the price of these milder chemicals is much less than traditional herbicides.


Top soil loss is a major problem in modern agriculture. Traditional weed control policies usually involve tillage during the growing season and after the harvest. However, using glyphosate resistant crops allows no till or low till policies to be implemented. Glyphosate resistant crops have increased the use of low/no till by 35% in the United States. This may not sound like much, but it actually saves about one billion tons of soil from erosion each year. This is an incredible amount, and the potential to save tons more exists if more farmers adopt no/low till policies.

Economic Impact

Monsanto was the first company to create a glyphosate-based herbicide. It was also the first company to develop crops that are glyphosate resistant. In 1996 RoundUp ready soybeans were introduced by Monsanto. They were the first genetically engineered crops be glyphosate resistant. Monsanto promptly obtained a patenet on the technology. Since then, many other crops including corn, canola, tobacco, and cotton have been engineered to be resistant to glyphosate.


However, Monsanto's patent on the glyphosate-resistant crop technology expired in September of 2000, but it is still the most widely used herbicide in the world. It still maintains about 80% of the market share of glyphosate-based herbicides, despite the fact that there are about 30 different companies that offer their own versions of a glyphosate-based herbicide. RoundUp is, in general, much more expensive than these alternatives, but six years after the expiration of their patent, they still dominate the market. Monsanto is actually being sued in a class action law suit due to their market domination.


Glyphosate resistant soybeans were first introduced in 1996, but currently they comprise about 80% of soybean crops in the US. And about 70% of the cotton crops in the US are engineered to be resistant to glyphosate. This is a huge percentage considering the technology was only introduced abou 10 years ago. It is also important to note that crops engineered to be glyphosate resistant have also been a huge success around the world, particularly in South America.


US farmers have saved about $1.2 billion associated with the costs of herbicide purchases, application and tillage. Actually, US farmers can eliminate about $385 million in tillage costs every year. These savings are due to the decrease in the costs of buying and maintaining equipment, as well as man hours and fuel costs.


Glyphosate is also a much more efficient chemical herbicide than many of the others available. To acheive the same effeciveness, much less glyphosate is necesssary compared to the other types. US farmers have reduced the amount of herbicide needed by 37.5 million pounds due to the use of glyphosate.


Glyphosate and glyphosate resistant crops have had a huge environmental and economic impact in the US and around the world. The changes are still developing as more and more countries adopt the use of glyphosate resistant crops. It will be interesting to see how this technology affects the world economy in the years to come.

Associated Risks

Weed Resistance

One of the potential risks associated with the development of herbicide resistance in tobacco plants is that with only one primary method of controlling weeds, some are worried that the weeds will become resistant themselves and this method will no longer be useful. With glyphosate as the primary herbicide, or in any case where one herbicide is the primarily used, there is potential for this. With such a high percentage of crop plants being glyphosate resistant it has cut down the use of other herbicides to virtually none. Some have suggested that with other herbicides on the market it is important to incorporate them all into use. It could make it necessary to produce crop plants resistant to the other herbicides, but it could also save the method of using herbicides, and herbicide resistance. If weed species are subject to only one type of herbicide, and they become resistant evetually, it could eliminate the entire tool and the time and money saved by the process of the herbicide and herbicide resistant crops, as well as possibly decreasing the farmland value. This is a problem that can have negative influences from the idea of herbicide resistance in general, not just glyphosate resistance, however there are some specific examples of how glyphosate resistance has impacted the idea of weed resistance because it is such a commonly used herbicide.


Horseweed was the first example of a glyphosate-resistant weed to a Roundup Ready formula. This weed become resistant to Monsanto's Roundup Ready soybean herbicide. In this case the horseweed was found in a field where the Roundup Ready formula was used for two years as the only herbicide, and prior to this it had been used very infrequently. After two years of the field being consistently sprayed with a single herbicide the horseweed developed a resistance to glyphosate.


Sometimes the weeds that have become herbicide resistant are referred to as "super weeds." With more weeds developing this trait the management of weeds and the cost of the management will increase, and the big question has been whether or not the increase will be great enough that serious changes should be made. The amount of impact that the glyphosate-resistant weeds have on the farm industry will all depend on what kinds of characteristics the weeds pick up and the rapidity of spreading of the weed. Basically, if alternative methods for controlling this issue are successful in defeating the glyphosate-resistant weeds, then there isn't much of a problem, however if the weeds do develop some kind of characteristic that hinders the effects of the alternative methods there could be a big problem


The most common alternative method used in helping with the weed control is adding an amount of another herbicide to the glyphosate containing Roundup Ready, and continuing with their program as they have in the past. This does add additional cost to the grower but the cost is very low because they don't need a great amount of the alternative herbicide being mixed in. Some growers already use this approach to help with the problem and prevent future problems.


If the resistant weed develops a characteristic that can no longer be managed by the alternative method then the costs can become much higher. The weeds may demand a more expensive herbicide for control, but mainly the big problem would be timing. The weeds may end up calling for a completely different program than what is used to manage the majority of the weeds that aren't resistant to the glyphosate. Some weeds require a post-emergence application of herbicide but it must be done before they reach a 4 inch height. This leaves a very small window for the grower to attack the weeds, and with the sizes of farms growing it can end up leading to less successful control of the weeds.


Since the first reports of glyphosate resistant weeds in 1996, there is now a total of 5 resistant weed species that have been documented. With this rate of new species developing resistance it is sure that new species with this trait will be continually seen. Scientists studying this problem at Iowa State University do not see this as a detrimental issue currently. Their best suggestions to growers is that if they create and implement a long term plan to help decrease the amount of resistant weeds, the issue will continue to be a lesser one with small costs to the farmer. This long term plan would be best if growers avoided growing Roundup Ready crops predominantly and rotating herbicides used on their fields annually. It is hoped that this plan can help keep the inexpensive use of glyphosate available to farmers. However, farmers must be willing to add some minor costs of rotating herbicides in order the help ensure the likelihood of avoiding more weeds with resistance.

Health Risks

Overall glyphosate is thought of as very environmentally and health friendly, however there are some circumstances where risks may be a possibility. When thinking about what could be affected by glyphosate which could cause health effects in humans, the possibility of it getting into the soil and eventually drinking water seems to be the main concern. In general, glyphosate becomes inactivated when it comes in contact with the soil. When glyphosate hits the soil its either bound or unbound glyphosate. The unbound glyphosate is degraded quickly into carbon dioxide by microbial activity. The bound glyphosate degrades more slowly and sometimes even remains undegraded but is completely inactive in the soil.


Because of its inactivity in the soil it would be thought to be safe, but glyphosate is also used in the control of aquatic weeds. When the glyphosate is used on aquatic weeds, it can be carried by eroded soil into surface waters. The breakdown processes are much slower in water and glyphosate has occasionally been found in drinking water.


The EPA (Environmental Protection Agency) has set the acceptable limit of glyphosate in drinking water at 0.7ppm. It is believed by the EPA to be a safe level that could not cause any health problems. This is referred to as a maximum contaminant level. The EPA set these standards believing that if the level is above the 0.7ppm, then the possibilty of health effects becomes an issue. The short-term health effects are congestion of the lungs and increased breathing rate. The possibly long-term effects are possible kidney damage and reproductive effects, which have been seen in studies on rabbits that showed harmful effects on the quality of semen and spem count.


In general glyphosate is a safe herbicide, and as long as the presence of glyphosate in drinking water is kept under the EPA recommended amount, there should be no concerns.

Environmental Risks

As already mentioned, herbicides containing glyphosate overall appear to be rather environmentally friendly, and in general this is true, especially when compared to herbicides used in the past. However, there are some environmental risks that need to be discussed and kept in mind due to the great use of glyphosate containing herbicides.


One of the problems that the Forestry Commission acknowledges is that glyphosate has been known to cause die-back in hedgegrow trees. Also, in the US glyphosate-containing herbicides have been shown to reduce some trees' ability to stand up to winters, and it reduces their resistance to fungal diseases. These problems are thought to be caused because sometimes the glyphosate particles get bound to the soil particles while the glyphosate is still active. If the glyphosate stays active long enough, the herbicide may be released from the soil and taken up by the trees or other plants. It's actually been said that several plants may be at risk from glyphosate of becoming endangered.


Another issue with the glyphosate herbicides is its affects on certain animals. The toxicity of glyphosate to humans, other mammals, and birds is rather low in general. However habitats and food sources for some of these animal populations may be at risk. If an animals' habitat is disturbed by glyphosate, or their food sources are reduced or lost because of affects of glyphosate, then reductions in populations of certain species could occur. wThis could lead to possibility of new endangered species. Aquatic invertebrates and fish aren't as lucky as mammals and birds, and may be much more sensitive to glyphosate. The toxicity of glyphosate can increase with higher temperatures and pH levels of water. If glyphosate does get into the water and the temperature and pH levels are right, several aquatic species can be harmed by the glyphosate, killing several if the level is high enough.


Another major concern of glyphosate's affects on the environment is that it can be harmful to organisms that live within the soil and help keep the soil healthy and in good condition for growing the species found in that area. Certain bacteria, mites, and isopods can be affected by glyphosate, and this can end up making the soil worse for the species that depend on its conditions that the organisms help create. In one study where nine herbicides were tested, glyphosate-containing herbicides were found to be the number two most toxic to some bacteria and fungi found in soils.


Most of these risks are in cases where there has been severe usage of glyphosate and they are not standard results to be found everywhere that glyphosate is used, however they are important to remember. It is necessary to be aware of these risks and do everything possible to avoid them and to try to ensure that water and soils are not harmed, as well as certain plants and habitats. Glyphosate is successful at its job to get rid of the weeds easily and quickly, however like most things there is a cost, and it is important to be aware of its affects on the environment and other risks in order to avoid as mcuh harm to the world as possible.

References

Benbrook, Charles M. 1986. "Engineering Crops to Resist Herbicides." Technology Review. 89:54-59.

Dill, Gerald M. 2005. "Glyphosate-resistant crops: history, status and future." Pest Management Sciene. 61:219-224.

Dyer, William E. 1988. "Glyphosate tolerance in tobacco." Plant Physiology. 88:661-666.

Gianessi, Leonard P. 2005. "Economic and herbicide used impacts of glyphosate-resistant crops." Pest Management Science. 61:241-245.

Gray, Alan J. 1998. "Reducing transgene escape routes." Nature. 392:653-654.

Hartzler, Bob and Mike Owen. 2003. "Status and Concerns for glyphosate Resistance." www.ent.iastate.edu/Ipm/Icm/2003/3-17-2003/glyphosate.html.

Miller, Julie Ann. 1985. "Gene Splicing for Herbicide Resistance." Science News. 127:140.

Stokstad, Eric. 2004. "A New Task on Herbicide Resistance." Science. 304:574-1089.

Thieman, William J. and Michael A. Palladino. 2004. Introduction to Biotechnology. 136-149.

Ye, Guan-Ning and Peter T.J. Hajdukiewicz. 2001. "Plastid-expressed 5-enolypyruvylshikimate-3-phosphate synthase genes provide high level glyphosate tolerance in tobacco." The Plant Journal 25:261.

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