转基因食品有利还是有害GM foods harmful or helpful

Discovery Guides © 2000 CSA Released April 2000 1 Genetically Modified Foods: Harmful or Helpful? Deborah B. Whitman Genetically-modified foods (GM foods) have made a big splash in the news lately. Euro- pean environmental organizations and public interest groups have been actively protest- ing against GM foods for months, and recent controversial studies about the effects of genetically-modified corn pollen on monarch butterfly caterpillars1,2 have brought the issue of genetic engineering to the forefront of the public consciousness in the U.S. In response to the up swelling of public concern, the U.S. Food and Drug Administration (FDA) held three open meetings in Chicago, Washington, D.C., and Oakland, California to solicit public opinions and begin the process of establishing a new regulatory proce- dure for government approval of GM foods.3 I attended the FDA meeting held in Novem- ber 1999 in Washington, D.C., and here I will attempt to summarize the issues involved and explain the U.S. government's present role in regulating GM food. What are genetically-modified foods? The term GM foods or GMOs (genetically-modified organisms) is most commonly used to refer to crop plants created for human or animal consumption using the latest molecu- lar biology techniques. These plants have been modified in the laboratory to enhance de- sired traits such as increased resistance to herbicides or improved nutritional content. The enhancement of desired traits has traditionally been undertaken through breeding, but conventional plant breeding methods can be very time consuming and are often not very accurate. Genetic engineering, on the other hand, can create plants with the exact desired trait very rapidly and with great accuracy. For example, plant geneticists can isolate a gene responsible for drought tolerance and insert that gene into a different plant. The new genetically-modified plant will gain drought tolerance as well. Not only can genes be transferred from one plant to another, but genes from non-plant organisms also can be used. The best known example of this is the use of B.t. genes in corn and other crops. B.t., or Bacillus thuringiensis, is a naturally occurring bacterium that produces crystal proteins that are lethal to insect larvae. B.t. crystal protein genes have been transferred into corn, enabling the corn to produce its own pesticides against insects such as the European corn borer. For two informative overviews of some of the techniques involved in creating GM foods, visit Biotech Basics (sponsored by Monsanto) http://www.biotechknowledge.monsanto.com/biotech/bbasics.nsf/index or Techniques of Plant Biotechnology from the National Center for Biotechnology Education http://www.ncbe.reading.ac.uk/NCBE/GMFOOD/techniques. 1 Transgenic pollen harms monarch larvae (Nature, Vol 399, No 6733, p 214, May 20, 1999) 2 Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterfly larvae in field studies (Pro- ceedings of the National Academy of Sciences, Vol 98, No 21, p11931-11936, Oct 2001) 3 Bioengineered Foods transcripts from the public meetings are available to download (http://www.fda.gov/oc/biotech/default.htm) Deborah Whitman: Genetically Modified Foods CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released April 2000 2 What are some of the advantages of GM foods? The world population has topped 6 billion people and is predicted to double in the next 50 years. Ensuring an adequate food supply for this booming population is going to be a major challenge in the years to come. GM foods promise to meet this need in a number of ways: • Pest resistance Crop losses from insect pests can be staggering, resulting in devastat- ing financial loss for farmers and starvation in developing countries. Farmers typi- cally use many tons of chemical pesticides annually. Consumers do not wish to eat food that has been treated with pesticides because of potential health hazards, and run-off of agricultural wastes from excessive use of pesticides and fertilizers can poi- son the water supply and cause harm to the environment. Growing GM foods such as B.t. corn can help eliminate the application of chemical pesticides and reduce the cost of bringing a crop to market.4,5 • Herbicide tolerance For some crops, it is not cost-effective to remove weeds by physi- cal means such as tilling, so farmers will often spray large quantities of different her- bicides (weed-killer) to destroy weeds, a time-consuming and expensive process, that requires care so that the herbicide doesn't harm the crop plant or the environment. Crop plants genetically-engineered to be resistant to one very powerful herbicide could help prevent environmental damage by reducing the amount of herbicides needed. For example, Monsanto has created a strain of soybeans genetically modified to be not affected by their herbicide product Roundup ®.6 A farmer grows these soy- beans which then only require one application of weed-killer instead of multiple ap- plications, reducing production cost and limiting the dangers of agricultural waste run-off.7 • Disease resistance There are many viruses, fungi and bacteria that cause plant dis- eases. Plant biologists are working to create plants with genetically-engineered resis- tance to these diseases.8,9 • Cold tolerance Unexpected frost can destroy sensitive seedlings. An antifreeze gene from cold water fish has been introduced into plants such as tobacco and potato. With this antifreeze gene, these plants are able to tolerate cold temperatures that normally 4 Insecticidal proteins from Bacillus thuringiensis protect corn from corn rootworms (Nature Biotechnol- ogy, Vol 19, No 7, pp 668-672, Jul 2001) 5 Lepidopteran-resistant transgenic plants (US Patent 6313378, Nov 2001, Monsanto) 6 Roundup Ready Soybeans http://www.biotechknowledge.monsanto.com/biotech/knowcenter.nsf 7 The use of cytochrome P450 genes to introduce herbicide tolerance in crops: a review (Pesticide Science, Vol 55, No 9, pp 867-874, Sep 1999) 8 Transgenic Approaches to Combat Fusarium Head Blight in Wheat and Barley (Crop Science, Vol 41, No 3, pp 628-627, Jun 2001) 9 Post-transcriptional gene silencing in plum pox virus resistant transgenic European plum containing the plum pox potyvirus coat protein gene (Transgenic Research, Vol 10, No 3, pp 201-209, Jun 2001) Deborah Whitman: Genetically Modified Foods CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released April 2000 3 would kill unmodified seedlings.10 (Note: I have not been able to find any journal ar- ticles or patents that involve fish antifreeze proteins in strawberries, although I have seen such reports in newspapers. I can only conclude that nothing on this application has yet been published or patented.) • Drought tolerance/salinity tolerance As the world population grows and more land is utilized for housing instead of food production, farmers will need to grow crops in lo- cations previously unsuited for plant cultivation. Creating plants that can withstand long periods of drought or high salt content in soil and groundwater will help people to grow crops in formerly inhospitable places.11,12 • Nutrition Malnutrition is common in third world countries where impoverished peo- ples rely on a single crop such as rice for the main staple of their diet. However, rice does not contain adequate amounts of all necessary nutrients to prevent malnutrition. If rice could be genetically engineered to contain additional vitamins and minerals, nutrient deficiencies could be alleviated. For example, blindness due to vitamin A de- ficiency is a common problem in third world countries. Researchers at the Swiss Fed- eral Institute of Technology Institute for Plant Sciences have created a strain of "golden" rice containing an unusually high content of beta-carotene (vitamin A).13 Since this rice was funded by the Rockefeller Foundation,14 a non-profit organization, the Institute hopes to offer the golden rice seed free to any third world country that requests it. Plans were underway to develop a golden rice that also has increased iron content. However, the grant that funded the creation of these two rice strains was not renewed, perhaps because of the vigorous anti-GM food protesting in Europe, and so this nutritionally-enhanced rice may not come to market at all.15 • Pharmaceuticals Medicines and vaccines often are costly to produce and sometimes require special storage conditions not readily available in third world countries. Re- searchers are working to develop edible vaccines in tomatoes and potatoes.16,17 These vaccines will be much easier to ship, store and administer than traditional injectable vaccines. 10 Type II fish antifreeze protein accumulation in transgenic tobacco does not confer frost resistance (Transgenic Research, Vol 8, No 2, pp 105-117, Apr 1999) 11 Transgenic salt-tolerant tomato plants accumulate salt in foliage but not in fruit (Nature Biotechnology, Vol 19, No 8, pp 765-768, Aug 2001) 12 Peroxidase activity of desiccation-tolerant loblolly pine somatic embryos (In Vitro Cellular & Develop- mental Biology Plant, Vol 36, No 6, pp. 488-491, Dec 2000) 13 Genetic engineering towards carotene biosynthesis in endosperm (Swiss Federal Institute of Technology Institute for Plant Sciences) 14 New rices may help address vitamin A- and iron deficiency, major causes of death in the developing world (Rockefeller Foundation) 15 RICE BIOTECHNOLOGY: Rockefeller to End Network After 15 Years of Success (Science, Vol 286, No 5444, pp 1468-1469, Nov 1999) 16 Medical molecular farming: production of antibodies, biopharmaceuticals and edible vaccines in plants (Trends in Plant Science, Vol 6, No 5, pp 219-226, May 2001) 17 Oral immunization with hepatitis B surface antigen expressed in transgenic plants (Proceedings of the National Academy of Sciences, USA, Vol 98, No 20, pp. 11539-11544, Sep 2001) Deborah Whitman: Genetically Modified Foods CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released April 2000 4 • Phytoremediation Not all GM plants are grown as crops. Soil and groundwater pollu- tion continues to be a problem in all parts of the world. Plants such as poplar trees have been genetically engineered to clean up heavy metal pollution from contami- nated soil.18 How prevalent are GM crops? What plants are involved? According to the FDA and the United States Department of Agriculture (USDA), there are over 40 plant varieties that have completed all of the federal requirements for com- mercialization (http://vm.cfsan.fda.gov/%7Elrd/biocon). Some examples of these plants include tomatoes and cantaloupes that have modified ripening characteristics, soybeans and sugarbeets that are resistant to herbicides, and corn and cotton plants with increased resistance to insect pests. Not all these products are available in supermarkets yet; how- ever, the prevalence of GM foods in U.S. grocery stores is more widespread than is commonly thought. While there are very, very few genetically-modified whole fruits and vegetables available on produce stands, highly processed foods, such as vegetable oils or breakfast cereals, most likely contain some tiny percentage of genetically-modified in- gredients because the raw ingredients have been pooled into one processing stream from many different sources. Also, the ubiquity of soybean derivatives as food additives in the modern American diet virtually ensures that all U.S. consumers have been exposed to GM food products. The U.S. statistics that follow are derived from data presented on the USDA web site at http://www.ers.usda.gov/briefing/biotechnology/. The global statistics are derived from a brief published by the International Service for the Acquisition of Agri-biotech Applica- tions (ISAAA) at http://www.isaaa.org/publications/briefs/Brief_21.htm and from the Biotechnology Industry Organization at http://www.bio.org/food&ag/1999Acreage. Thirteen countries grew genetically-engineered crops commercially in 2000, and of these, the U.S. produced the majority. In 2000, 68% of all GM crops were grown by U.S. farm- ers. In comparison, Argentina, Canada and China produced only 23%, 7% and 1%, re- spectively. Other countries that grew commercial GM crops in 2000 are Australia, Bul- garia, France, Germany, Mexico, Romania, South Africa, Spain, and Uruguay. Soybeans and corn are the top two most widely grown crops (82% of all GM crops har- vested in 2000), with cotton, rapeseed (or canola) and potatoes trailing behind. 74% of these GM crops were modified for herbicide tolerance, 19% were modified for insect pest resistance, and 7% were modified for both herbicide tolerance and pest tolerance. Glob- ally, acreage of GM crops has increased 25-fold in just 5 years, from approximately 4.3 million acres in 1996 to 109 million acres in 2000 - almost twice the area of the United 18 Phytodetoxification of hazardous organomercurials by genetically engineered plants (Nature Biotechnol- ogy, Vol 18, No 2, pp. 213-217, Feb 2000) Deborah Whitman: Genetically Modified Foods CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released April 2000 5 Kingdom. Approximately 99 million acres were devoted to GM crops in the U.S. and Ar- gentina alone. In the U.S., approximately 54% of all soybeans cultivated in 2000 were genetically-modi- fied, up from 42% in 1998 and only 7% in 1996. In 2000, genetically-modified cotton varieties accounted for 61% of the total cotton crop, up from 42% in 1998, and 15% in 1996. GM corn and also experienced a similar but less dramatic increase. Corn produc- tion increased to 25% of all corn grown in 2000, about the same as 1998 (26%), but up from 1.5% in 1996. As anticipated, pesticide and herbicide use on these GM varieties was slashed and, for the most part, yields were increased (for details, see the UDSA publica- tion at http://www.ers.usda.gov/publications/aer786/). What are some of the criticisms against GM foods? Environmental activists, religious organizations, public interest groups, professional as- sociations and other scientists and government officials have all raised concerns about GM foods, and criticized agribusiness for pursuing profit without concern for potential hazards, and the government for failing to exercise adequate regulatory oversight. It seems that everyone has a strong opinion about GM foods. Even the Vatican19 and the Prince of Wales20 have expressed their opinions. Most concerns about GM foods fall into three categories: environmental hazards, human health risks, and economic concerns. Environmental hazards • Unintended harm to other organisms Last year a laboratory study was published in Nature21 showing that pollen from B.t. corn caused high mortality rates in monarch butterfly caterpillars. Monarch caterpillars consume milkweed plants, not corn, but the fear is that if pollen from B.t. corn is blown by the wind onto milkweed plants in neighboring fields, the caterpillars could eat the pollen and perish. Although the Na- ture study was not conducted under natural field conditions, the results seemed to support this viewpoint. Unfortunately, B.t. toxins kill many species of insect larvae indiscriminately; it is not possible to design a B.t. toxin that would only kill crop- damaging pests and remain harmless to all other insects. This study is being reexam- ined by the USDA, the U.S. Environmental Protection Agency (EPA) and other non- government research groups, and preliminary data from new studies suggests that the original study may have been flawed.22,23 This topic is the subject of acrimonious de- bate, and both sides of the argument are defending their data vigorously. Currently, 19 GMO Roundup (Nature Biotechnology, Vol 18, p 7, Jan 2000) 20 Questions about Genetically Modified Organisms: An article by The Prince of Wales (http://www.princeofwales.gov.uk/speeches/agriculture_01061999.html) and Seeds of Disaster: An article by The Prince of Wales (http://www.princeofwales.gov.uk/speeches/agriculture_08061998.html) 21 Transgenic pollen harms monarch larvae (Nature, Vol 399, No 6733, p 214, May 1999) 22 GM corn poses little threat to monarch (Nature Biotechnology, Vol 17, p 1154, Dec 1999) 23 Bt and the Monarch Butterfly: Update by Dr. Douglas Powell (AGCare Update Magazine http://www.agcare.org/AGCareUpdate.htm#Monarch) Deborah Whitman: Genetically Modified Foods CSA Discovery Guides http://www.csa.com/discoveryguides/discoveryguides-main.php Released April 2000 6 there is no agreement about the results of these studies, and the potential risk of harm to non-target organisms will need to be evaluated further. • Reduced effectiveness of pesticides Just as some populations of mosquitoes devel- oped resistance to the now-banned pesticide DDT, many people are concerned that insects will become resistant to B.t. or other crops that have been genetically- modified to produce their own pesticides. • Gene transfer to non-target species Another concern is that crop plants engineered for herbicide tolerance and weeds will cross-breed, resulting in the transfer of the herbi- cide resistance genes from the crops into the weeds. These "superweeds" would then be herbicide tolerant as well. Other introduced genes may cross over into non- modified crops planted next to GM crops. The possibility of interbreeding is shown by the defense of farmers against lawsuits filed by Monsanto. The company has filed patent infringement lawsuits against farmers who may have harvested GM crops. Monsanto claims that the farmers obtained Monsanto-licensed GM seeds from an un- known source and did not pay royalties to Monsanto. The farmers claim that their unmodified crops were cross-pollinated from someone else's GM crops planted a field or two away. More investigation is needed to resolve this issue. There are several possible solutions to the three problems mentioned above. Genes are exchanged between plants via pollen. Two ways to ensure that non-target species will not receive introduced genes from GM plants are to create GM plants that are male sterile (do not produce pollen) or to modify the GM plant so that the pollen does not contain the in- troduced gene.24,25,26 Cross-pollination would not occur, and if harmless insects such as monarch caterpillars were to eat pollen from GM plants, the caterpillars would survive. Another possible solution is to create buffer zones around fields of GM crops.27,28,29 For example, non-GM corn would be planted to surround a field of B.t. GM corn, and the non-GM corn would not be harvested. Beneficial or harmless insects would have a refuge in the non-GM corn, and insect pests could be allowed to destroy the non-GM corn and would not develop resistance to B.t. pesticides. Gene transfer to weeds and other crops would not occur because the wind-blown pollen would not travel beyond the buffer zone. Estimates of the necessary width of buffer zones range from 6 meters to 30 meters or 24 New tools for chloroplast genetic engineering (Nature Biotechnology, Vol 17, No 9, pp 855-856, Sep 1999) 25 Tandem constructs: preventing the rise of superweeds (Trends in Biotechnology, Vol 17, No 9, pp 361- 366, Sep 1999) 26 Containment of herbicide re