6.5—Disease-resistant crops are safe for humans

Disease-resistant crops do not pose risks for humans.

See Genetic Roulette’s False Claims at Bottom of Page

Analysis of Peer-Reviewed Research:

Plant viruses are often spread by aphids or other insects and cause considerable losses of food crops such as squash, papaya, plum, sugar beet and maize, because of the damage that they cause to foods and crops. Fortunately no plant virus causes any harm to humans or causes any human disease, despite causing significant losses to food production. Smith’s book is about safety risks to humans. There are no human health risks posed by genetically engineered disease resistant plants.

Prevention of food losses from plant virus disease often requires the use of repeated insecticide spraying to eliminate insects that spread viruses from plant to plant.  The discovery some 20 years ago that plants could be genetically engineered to be resistant to virus disease was a big step forward in providing a new way of producing more food and undamaged vegetables. It also meant that less insecticide is needed to control the insects that spread plant viruses.

Successful interruption of plant virus diseases using genetic engineering relies on inserting a fragment of DNA corresponding to part of a virus gene into the chromosomes of plants. This DNA fragment insertion in the chromosome of the plant restricts the virus’s ability to multiply in that plant, and stops multiplication of the virus in the crop.

Twenty years of experience with such genetically engineered disease resistant squash, papaya, plums and grapes in continental USA, Hawaii, Spain, France and Romania shows that the worries Jeffrey Smith raises do not occur in practice. Smith worries about new viruses being generated in these crops. The practical experience is that virus epidemics stop and no new viruses are generated in the disease resistant crops growing in the field. Genetic Roulette gives theoretical predictions of ways in which new viruses could evolve  in these crops but avoids mentioning the practical experience that says the problems he fears are not found in farming experience that spans many years.

Genetic Roulette also avoids mentioning the extensive scientific investigations that say it is quite unlikely that the theoretical problems Genetic Roulette imagines will ever occur during farming a virus resistant crop.  It is also unlikely that the virus resistant crops will enable any changes that are different from what is already is possible with virus evolution in the field. Smith’s also avoids mentioning extensive deliberations by regulatory agencies about the safety of virus resistant crops (USDA-APHIS 2007).  The USDA-APHIS agency has recently approved disease resistant plum tree that resists the plum pox virus after careful and lengthy scientific finding that these crops are safe.

Smith’s worries that the new viruses — remember that these are not being noticed in the 20 years of use of virus resistant crops– will cause increased spraying for aphids to eliminate the insect that spread these viruses. In practical experience the absence of viruses causing disease in disease resistant crops means there is less need to spray insecticide to control insects such as aphids. Smith is worried about increased insecticide is at odds with the practical experience with virus resistant crops and is also illogical.

Genetic Roulette suggests that some plant viral proteins might be toxic to humans although this has not been demonstrated. It does not mention that virus-resistant plants reduce the amount of virus proteins in the plant and would reduce the amount of these hypothetically toxic proteins that could be consumed. None of the viruses that infect plants or their viral proteins have ever been found to cause any toxic effect or disease in humans. Plant viruses do not infect human cells even though we are frequently exposed to them from many different foods present in the diet.

Jeffrey Smith’s book indirectly speculates that the transgenic crops will give rise to novel viruses that are harmful to humans. Many different viruses are present in crops already, and mixed infections with two viruses, quite common in field situations, already has the potential to give rise by virus evolution to novel viruses, and occasionally does. Thus we are already exposed to low probability that a new virus could appear in our food, but there is no reason to believe such an event could be harmful to us, and there is no reason to believe that if such a virus evolved because of transgenic crop deployment, it would be harmful either. There is no reason to believe that transgenic crops will increase the chances of such a new virus occurring as speculated by Smith. Quite the reverse– the prevention of virus epidemics made possible by switching to disease resistant plants, decreases the chance the chances of novel virus appearing. No plant virus has been found that causes harm or disease in humans. Throughout this section Smith is living in an imaginary world of his own creation rather than the real world where virus and food production problems are being solved by disease resistant crops.

1. Plant viruses and their genes are already found in our food and do not cause any harm. A wide range of viruses affect foods that we eat. They do not cause human diseases but they can damage plants and reduce yields and they can cause vegetables and fruits to be malformed and unattractive to consumers. Additionally, fragments of DNA from viruses are widely found as inserts in plant genomes including many food plant genomes. This causes no problems, but all the hypothetical risks that Smith conjectures for genetically engineered plants, can also happen with these existing virus DNA- containing plants. Most people are not familiar with this fact about their food, and perhaps they would be less worried about all the talk that Smith has about viruses if he had carefully explained how widely spread viruses and their DNA are. None of the viruses causes any harm to humans except damaging their food.

Many different types of food are affected by one virus or another. Cereals like maize can be damaged by viruses that are spread by insects. This causes serious losses to food production in Africa for instance. Damage to staple foods by viruses is one reason why genetic engineering approaches that protect crops from damage by viruses are important as a tool for improving food output. Deregulated virus resistant papaya was introduced into the Hawaiian papaya industry in 1998 because the papaya crops were being decimated by viruses.  One of the reasons that regulatory agencies approved this commercialisation is that the pre-existing presence of substantial numbers of Papaya ringspot virus in fruit had caused no problems with allergy or toxic reactions to consumers of that fruit (Fuchs and Gonsalves 2007). Viruses are present in Brassica plants such as cabbages.  Plant viruses are present in potatoes, in grapes, in beans, peanuts, tomato, soybean, cucumbers, lettuce, and sweet potato. There is no evidence that any of these viruses cause any problems to people who eat these foods (Fuchs and Gonsalves 2007).

Plant chromosomes are riddled with parasitic DNA, including DNA related to viruses. Such insertions of fragments of DNA permanently carried in plant genomes can come from any one of a variety of different types of virus—retrovirus, pararetrovirus, geminivirus, and potyvirus are examples (Bejarano and others (1996), Harper and others 2002, Mette and others 2002, Tanne and Sela 2005). It is thus not unusual for chromosomes of a food plant to contain fragments of virus genes. These virus gene fragment insertions can even enable formation of virus proteins by plants, for example certain grapevines can produce some virus protein from the genes of a potyvirus that they have captured from an earlier viral infection (Tanne and Sela  2005). And there is no evidence that the presence of virus gene fragments in plant genomes causes any problems either (Harper and others 2002).

2. Virus resistant plants restrict multiplication of viruses and greatly limit epidemics of virus infection among plants. Introduction of a fragment of a virus gene into the plant chromosomes triggers silencing of viruses should they infect that plant. Normal multiplication of the virus inside the plant is shut down and silenced. The mechanism of this silencing of virus activity has been extensively investigated and is now well understood. Practical advantages of this silencing mechanism are that virus resistant plants created by genetic engineering need not contain any virus protein. In any case, whether or not they contain small amounts of virus protein or none whatsoever, they minimise the amount of virus that can multiply in the plant and so reduce the total amount of virus proteins are present in food. The overall total of virus multiplication that occurs in the crop that has been made resistant to virus disease is greatly reduced. This is almost self-evident, but it is a point that Jeffrey Smith does not seem to have understood in terms of its implications for safer and better food.

Reduction in the total amount of virus multiplication that can occur in crops because viruses are silenced counteracts evolution of new virus types. During virus replication in the field among large numbers of growing plants tremendous numbers of virus particles get formed and it is this large number of virus particles and their genetic material that provide opportunities for novel virus evolution. It provides opportunities for two different kinds of viruses to infect the same plant (Achon, Alonso-Duenas  2008, USDAS-APHIS 2007). When two different types of viruses occasionally infect plants they can exchange genes in one another. By preventing virus epidemics, disease resistant transgenic plants minimise this opportunity for viruses to exchange genes with one another (Fuchs and others 1998, Fuchs, Gonsalves 2007, Gonsalves 1998, , Hoekema and others 1989, Lawson  and others 1990, Ling and others 1991).

3. Years and years of practical experience with field cultivation of disease resistant crops shows that many theoretical problems are not real environmental problems. A large amount of scientific effort has been devoted to assessing environmental issues associated with virus resistant crops. Genetic Roulette provides a totally inadequate coverage of this scientific knowledge about plant viruses in food crops and their environmental safety.

For instance Genetic Roulette doesn’t mention that Nieves Capote and colleagues studied aphids and viruses in Spanish plum tree orchards over an eight-year period. Their study compared how plant viruses behaved in genetically engineered virus resistant plums with non-engineered plums. Capote and colleagues concluded that transgenic and non-transgenic plants were very similar in terms of the risks of novel virus types or changes in virus and aphid populations over time (Capote and others 2008).

Another study not mentioned by Smith is that of American workers Klas and, Gonsalves and colleagues (Klas and others 2006) who spent two years looking at the spread of aphid-borne Zucchini yellow mosaic virus and Watermelon mosaic viruses in crops of commercial transgenic and non-transgenic squash. These plants were deliberately exposed to heavy virus infections from infected crops bordering the experimental trials that enabled virus movement from plant to plant in the field to be assessed. All the fruits of transgenic plants remained without symptoms and of marketable quality but the majority of the plants from non-transgenic squash were discoloured and malformed and of no marketable value. Klas and colleagues were able to show the transgenic virus resistant plants did not transmit viruses from plant to plant.

Many genetically engineered virus resistance plants produce a viral packaging protein. It is possible for this packaging protein to package a virus of a different type if happens to attack the genetically engineered crop, and there was repackaging may incur in create environmental problems. For instance, some viruses cannot be transmitted by aphids, but other viruses of a different type can be transmitted by aphids and this ability to be transmitted in is determined by which coat the virus is packaged within. If a virus that is unable to be aphid transmitted enters a genetically engineered crop is able to produce a coat protein that helps aphid transmission it may gain a new route for infecting plants.

Fortunately scientists working with genetically engineered virus resistant crops have long recognised this possible problem. It has been the focus of lots of investigations to work out if it is a real problem. Experiments carried out more than 10 years ago by Fuchs, Klas, and Gonsalves and their colleagues are not mentioned by Smith, showed that such assisted transmission of virus was not very efficient and did not give rise to epidemics of infection (Fuchs and others 1999, Fuchs M,  Klas FE, McFerson JR, Gonsalves D 1998). One reason this transmission is not very efficient and fails to give rise to epidemics is that the repackaging does not confer a prominent ability to be transmitted by aphids as the virus does not gain genetic inheritance for this ability only the coat protein itself (Tepfer 2002).

Smith has raised the possibility that virus resistant plants might help transmit new viruses by packaging them in a novel protein. But he makes no mention of all the scientific work has been carried out to solve this problem and to demonstrate that in practical plumbing with these crops the problems he mentions do not occur.

French workers led by Emmanuelle Vigne studied Grapevine fanleaf virus in a vineyard in France. This is a virus that is spread through vineyards by nematode worms. They studied transgenic grapevines expressing the coat protein of this virus. Emmanuelle Vigne made a comparison of virus infections in vineyards containing either virus resistance grapevine stock or virus sensitive non-transgenic grapevines. Their study, not mentioned by Smith,  indicates that transgenic grapevines do not assist the emergence of novel Grapevine fanleaf virus recombinant type virus to any detectable level, nor did they change the diversity of virus populations in these vineyards over the three-year trial period. This study specifically addresses the concerns that Smith raises and finds that the problems he imagines don’t occur (Vigne, Komar and others 2004, Vigne Bergdoll and others 2005

Californian scientists have carried out careful surveys of viruses from zucchini, straightneck and yellow crookneck squash. These are all called cucurbits and are infected by a virus called Cucumber mosaic virus that has a very wide range of crops that it can infect. Cucumber mosaic virus susceptible plants exist in about 365 different genera of plant and the virus cause a lot of economic damage to food production. These scientists carried out a field survey of the numerous Cucumber mosaic viruses that circulate in Californian is  virus susceptible cucurbits, genetically engineered cucurbits with tolerance to a several different viruses, and non-genetically engineered cucurbit varieties. They discovered that Cucumber mosaic virus variants able to insects genetically engineered virus resistant squash existed in California before genetically engineered disease resistant plants were cultivated. Thus the supposedly novel virus variants that Smith postulates that genetically engineered disease resistant plants will cause to emerge as epidemics in field populations already existed prior to the introduction of genetically engineered squash. Genetically engineered plants will not cause the creation of novel types of cucurbit disease postulated by Smith because they already exists (Lin and others 2002). Genetic Roulette does not mention this investigation even though it shows worries that Smith thinks are important are without foundation.

None of the problems imagined by Smith have occurred in large scale virus resistant papaya farming over many years (Fuchs, Gonsalves 2007).  Squash, papaya, plum and grapevine are all examples of practical experience with successful farming of virus resistant crops that Smith does not mention. Years and years and years of laboratory and field investigations have been reported in the scientific literature say that the problems Smith worries about do not occur or are not practically important. Smith doesn’t cite any of this scientific literature. He is only interested in theoretical predictions of problems and not their solutions.

3. In the field during 20 years of experience with disease resistant crops, no breakdown of disease resistance requiring the use of insecticide to control virus vectors has occurred. Smith speculates that cultivation of virus resistant crops will give rise to new viruses leading to increased use of pesticides to control the imaginary new virus epidemics triggered by reliance on virus resistant plants. The problem with this speculation is that no new virus epidemics are observed in practice. Remember the disease resistant plants are introduced because virus epidemics are a real problem that causes dramatic losses to food production. The introduction of virus resistant papaya and virus resistant squash prevented new outbreaks of disease. They also minimises the amount of insecticide needed to manage outbreaks of insects by viruses. Smith does not talk about this saving of insecticide spraying, but instead talks about hypothetical disease outbreaks predicted to occur in the future that have never happened in years of experience with disease resistant crops. It seems as if he is trying to distract attention away from the very practical and environmentally beneficial outcome of using disease resistant crops. They prevent virus outbreaks and eliminate the need for insecticide to control the carriers of virus disease (Fuchs and Gonsalves 2007, Gonsalves 1998).

4. Genetically engineered disease resistant plants do not introduce a novel risks. Genetic Roulette speculates that genetically engineered plants containing virus DNA may enable new viruses to infect plants and allow evolution of novel viruses. It does not mention evolution of new viruses can easily occur in non-engineered plants. It can happen in at least two ways: (i) Plants can capture genes from viruses they are infected with and carry those capture genes within plant chromosomes. These captured genes are then able to exchange with other viruses that infect the plant just like Smith imagines a genetically engineered virus gene might do. (ii) Plants can get infected with two different viruses and when there is a mixed infection two different viruses can exchange genes with one another.

Many different types of virus DNA are carried in plant chromosomes and provide opportunities to new viruses to evolve were the first mechanism (Harper and others 2002, Tanne and Sela  2005). One example was given earlier when it was explained that certain grapevines produce a viral protein from the virus gene they have captured (Tanne and Sela  2005). Another example is in tobacco plants which have the numerous copies of virus that is thought to provide disease protection against infection of that plant. In other words it behaves in exactly the same way as the virus fragments that are genetically engineered only this time it’s happened during natural evolution and has not been assessed for safety by regulatory agencies (Mette and others 2002). Nevertheless these natural DNA inserts of virus gene fragments in plant chromosomes don’t appear to cause problems.

The second way that new viruses can be generated is during mixed infections of a plant with two different viruses. Mixed virus infection of plants occurs frequently in crops that are cultivated in the field (Achon, Alonso-Duenas 2008, Tepfer 2002). So virus evolution by exchange of genes between different types of virus already occurs in field crops. Transgenic crops introduced into the new kinds of risk issue as far as far as evolution is concerned.

5. Regulatory agencies and scientists have extensively evaluated the safety of virus resistant crops and have worked out strategies for ensuring that they are safe. The genetic engineering of disease resistant plants has been an exciting era for biologists and plant breeders. Scientists have extensively discussed the implications of the discovery that virus do diseases could be prevented by insertion of a DNA version of virus genes into plant chromosomes. The mechanism by which this DNA is able to silence virus activity has been of enormous interest to all biologists, and has led to remarkable discoveries about how plants and animals and humans protect himself from parasitic DNAs and virus attack. The work has been part of the recognition of a special system for silencing gene activity called RNA silencing or RNA interference (Tepfer 2002). There has been vigorous debate about what the implications are for insertion of virus genes into plant chromosomes and the possible implications of genetically engineered plants containing virus genes have been thoroughly explored. Government agencies have devoted careful attention to this topic and have developed policies and regulations which ensure that it is safe (USDA-APHIS 2007). The scientific community now recognises that many of the initial worries about whether this was environmentally manageable approach to crop breeding were overstated (Fuchs , Gonsalves 2007). Jeffrey Smith repeats these overstated early worries without providing the reader with access to the current state of scientific knowledge about this topic.

See also

2 .4 The genetic switch region that turns on the activity of a transgene is no more likely to turn on the expression of other genes than are other highly unlikely genetic events that change chromosomes.

3.9 Disease resistant crops don’t cause human diseases

5.9 Transfer of viral genes in the gut



Achon MA and Alonso-Duenas N (2008). Impact of 9 years of Bt-maize cultivation and distribution of maize viruses. Transgenic Research DOI 10.1007/s11248-008-9231-2.

Bejarano ER, Khashoggi A, Witty M, and Lichtenstein C (1996). Integration of multiple repeats of geminiviral DNA into the nuclear genome of tobacco during evolution. Proc. Natl. Acad. Sci. U.S.A. 93:759– 764. www.pnas.org/content/93/2/759 this event mimics artificial transgenic incorporation into plant genome. There are multiple copies of virus DNA in tobacco. Their similarity to the viral DNA is compelling. Tobacco plants are easily regenerated from tissue fragments and this may explain the incorporation of the virus DNA into the germline. Tobacco also has a bacterial DNA inserted into its genome from Agrobacterium.

Capote N, Pérez-Panadés J, Monzó C, Carbonell E, Urbaneja A, Scorza R, Ravelonandro M, Cambra M. (2008) Assessment of the diversity and dynamics of Plum pox virus and aphid populations in transgenic European plums under Mediterranean conditions. Transgenic Res. 2008 Jun;17(3):367-77. Epub 2007 Jun 29.

Fuchs M, Tricoli DM, Carney KJ, Schesser M, McFerson

JR, and Gonsalves D (1998). Comparative virus resistance and fruit yield of transgenic squash with single and multiple coat protein genes. Plant Dis. 82:1350-1356.

Fuchs M,  Klas FE, McFerson JR, Gonsalves D (1998). Transgenic melon and squash expressing coat protein genes of aphid-borne viruses do not assist the spread of an aphid non-transmissible strain of cucumber mosaic virus in the field Transgenic Research 7, 449-462  “Field experiments conducted over two consecutive years showed that aphid-vectored spread of CMV strain C did not occur from any of the CMV strain C-challenge inoculated transgenic plants to any of the uninoculated CMV-susceptible non-transgenic plants”

Fuchs M, Gal-On A, Raccah B, and Gonsalves D (1999) Epidemiology of an aphid nontransmissible potyvirus in fields of nontransgenic and coat protein transgenic squash. Transgenic Research 8: 429–439 “ Despite the availability of numerous test plants and conditions of high disease pressure but low selection pressure, an epidemic of ZYMV strain MV did not develop in fields of transgenic plants. In contrast, the aphid transmissible ZYMV strain NY was aphid transmitted to 99% (446/450) of transgenic plants under similar conditions.”

Fuchs M and Gonsalves D. (2007). Safety of virus-resistant transgenic plants two decades after their introduction: Lessons from realistic field risk assessment studies. Annu. Rev. Phytopathol. 2007. 45:173–202

Gonsalves D  (1998). Control of Papaya Ringspot Virus in papaya: a case study. Annu. Rev. Phytopathol. 1998. 36:415–37. “measured amounts of coat protein in transgenic plants were much lower than those of infected plants”.

Harper G, Hull R, Lockhart B and Olszewski N (2002). Review. Viral sequences integrated into plant genomes. Annual Review of Phytopathology 40:119–36. Numerous bits of viruses are found inside the chromosomes of plants that we eat.

Hass M Bureau M, Geldreich A, Yot P and Keller M (2002) Review: Cauliflower mosaic virus: still in the news. Molecular Plant Pathology. 3(6): 419–429. Description of the virus from which the S35 promoter used in the first generation of GM plants was obtained.

Hoekema A Huisman MJ, Molendrijk L, van den Elzen PJ and Cornelissen BJC (1989). The genetic engineering of two commercial potato cultivars for resistance to potato virus X. Bio/technology 7:273-278. Transgenic potatoes has less viral protein than do infected potatoes.

Hull R, Covey S & Dale P (2000). Genetically modified plants and the 35S promoter: assessing the risks and enhancing the debate. Microbial Ecology in Health and Disease 12, 1–5

Klas FE and Fuchs M, Gonsalves D. (2006) Comparative spatial spread overtime of Zucchini Yellow Mosaic Virus (ZYMV) and Watermelon Mosaic Virus (WMV) in fields of transgenic squash expressing the coat protein genes of ZYMV and WMV, and in fields of nontransgenic squash. Transgenic Res. 2006 Oct;15(5):527-41. Epub 2006 Jul 13.

Klas FE Fuchs M, and Gonsalves D (2007). Environmental safety of transgenic squash: a geostatistical analysis. ISB News Report. www.isb.vt.edu/news/2007/May07.pdf accessed Jan 21 2009

Lawson C, Kaniewski W, Haley L, Rozman R, Newell C, Sanders P and Tumer NE (1990) Engineering resistance to mixed virus infection in a commercial potato cultivar: resistance to potato virus X and potato virus Y in transgenic Russet Burbank.Biotechnology (N Y). 1990 Feb;8(2):127-34.Infected potato has more virus coat protein than do transgenic potatoes.

Lemaux P (2008). Section 3.12. Do viral sequences used in plant genetic engineering create a human health risk? In Review: Genetically engineered plants and foods: a scientist’s analysis of the issues (Part I). Annual Review Plant Biology 59:771–812.

Lin HX, Rubio L, Smythe A, Jiminez M, Falk BW. (2003) Genetic diversity and biological variation among California isolates of Cucumber mosaic virus. J Gen Virol. 2003 Jan;84(Pt 1):249-58.

Ling KS, Namba S, Gonsalves C, Slightom JL, and Gonsalves D (1991) Protection against detrimental effects of potyvirus infection in transgenic tobacco plants expressing the papaya ringspot virus coat protein gene. Biotechnology. 1991 Aug;9(8):752-8.


Mette MF, Kanno T, Aufsatz W, Jakowitsch J, van der Winden J, Matzke MA and Matzke AJM (2002). Endogenous viral sequences and their potential contribution to heritable virus resistance in plants. The EMBO Journal 21(3):461-469.

Staginnus C and Richert-Pöggeler KR (2006) Endogenous pararetroviruses: two-faced travelers in the plant genome. Trends Plant Sci. 11(10):485-91. Describes the virus sequences that are found in plant genomes. We eat these almost every day.

Tanne E and Sela I (2005). Occurrence of a DNA sequence of a non-retro RNA virus in a host plant genome and its expression: evidence for recombination between viral and host RNAs.Virology.332(2):614-22.

Tepfer M (2002). Risk assessment of virus-resistant transgenic plants. Annu Rev Phytopathol. 2002;40:467-91. Epub 2002 Feb 20

USDA-APHIS (2007). Approval of USDA-ARS Request (04-264-01P) Seeking a Determination of Non-regulated Status for C5 Plum Resistant to Plum pox Virus. Finding of No Significant Impact and Decision Notice.

Vigne E, Bergdoll M, Guyader S, Fuchs M. (2004) Population structure and genetic variability within isolates of Grapevine fanleaf virus from a naturally infected vineyard in France: evidence for mixed infection and recombination J Gen Virol. 2004 Aug;85(Pt 8):2435-45.

Vigne E, Komar V, Fuchs M. (2004) Field safety assessment of recombination in transgenic grapevines expressing the coat protein gene of Grapevine fanleaf virus. Transgenic Res. 2004 Apr;13(2):165-79.

Genetic Roulette Falsely Claims:

Disease-resistant crops may promote new plant viruses, which carry risks to humans.

1. Virus-resistant transgenes protect crops from one type of virus, but may increase susceptibility to other plant viruses.

2. Infected plants to which humans at risk due to increased pesticide use.

3. They may also lead to increased consumption of potentially harmful viral proteins.

Genetic Relates speculates that disease resistant crops may promote new viruses and that this could be harmful to humans.

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