Transgenic Papaya

Image: USDA

Papaya is an important source of vitamin A in many parts of the developing world; in fact, the Center for Science in the Public Interest ranked it No. 1 on its list of most nutritious fruits (C. Gonsalves). After a seed is planted, fruit appears in just nine months, and within a year the tree can reach 12 feet in height. Fruit is produced throughout the year (D. Gonsalves).

Papaya is the second-largest fruit crop grown in Hawaii. Type P papaya ringspot virus (PRSV-p), transmitted by aphids (C. Gonsalves), destroyed the papaya industry on Oahu in the 1950s. At the time, farmers had no defenses against PRSV-p, so the industry relocated to the island of Hawaii, which was not afflicted by the virus’ presence. However, by 1970 PRSV-p was detected on the island, though it had not yet affected the primary growing area of papaya farmers. Anticipating the arrival of PRSV-p to papaya farms, scientists started to investigate methods for controlling the pathogen (Ronald, 58).

In 1979, Dennis Gonsalves and scientists at the University of Hawaii started to investigate a method called cross protection, which is a phenomenon in which infecting a plant with a mild form of a plant virus will confer protection against the effects of infection by a more virulent form. (This is similar to the first smallpox vaccine, which involved infecting humans with cowpox, which caused a mild infection but conferred immunity to smallpox, a very serious disease.) After conducting an investigation of different strains of the virus, the researchers settled on PRSV-HA, which mainly affected squash and was shown in laboratory experiments to have a mild effect on papaya. Field tests of cross protection using PRSV-HA were conducted in Taiwan and the method was shown to be successful in delaying the effects of PRSV-p infection, but not completely protecting against them either. Similar trials in Hawaii were more successful, but cross protection was not a magic bullet (D. Gonsalves).

In 1992, PRSV-p was detected in the papaya farms on Hawaii, and within three years the virus was epidemic; papayas in the United States were on the verge of extinction. By this time, however, a transgenic papaya with resistance to PRSV-p had been developed (Ronald, 58). In the 1980s, the concept of parasite-derived resistance was born; it was the idea that incorporation of segments of viral genetic information into the genome of a host plant could confer resistance to that virus (or related viruses). The strategy was first demonstrated to work in transgenic tobacco plants that had been engineered to resist tobacco mosaic viruses (D. Gonsalves).

Scientists from Cornell University and the University of Hawaii, largely funded by the USDA and led by Gonsalves, used the strategy of parasite-derived resistance to inoculate papaya trees against PRSV-p, incorporating the PRSV-HA gene coding for the virus’ surface protein into the genome of the papaya. The gene was inserted into the papaya by using a gene gun to bombard the papaya’s embryonic tissue with tiny pieces of tungsten coated with genetic information from PRSV-HA (D. Gonsalves). Like a vaccine, this gene rendered the transgenic papaya resistant to PRSV-p infection. It is thought to achieve this by post-transcriptional gene silencing (Kohli), which in its natural form is a defense evolved by many organisms to protect against viral infection. Its not-so-natural form is a method called RNA interference, in which geneticists can engineer into a plant pieces of siRNAs (“small interfering RNAs”) that bind to viral RNA during transcription, disrupting the effects of viral gene expression (Tortora, 259). In other words, we have primed the papayas to respond to infection by PRSV-p by nipping viral gene expression in the bud, before the virus can do any damage.

Two varieties of transgenic PRSV-p-resistant papaya were created: SunUp and UH Rainbow, which became commercially available in 1998 following APHIS, EPA, and FDA approval. The transgenic papaya trees were crossed with other varieties of papaya to propagate the gene and create GMO seed, which were given to Hawaiian papaya farmers at no charge, rescuing the U.S. papaya industry. Yields increased dramatically – from 71 million pounds in 1992 when the virus was first detected, to 26 million pounds in 1998 after the virus had taken hold, and back up to 40 million pounds in 2001 after transgenic plants were introduced (Ronald, 58; C. Gonsalves).

Just as a vaccine opponent might benefit from herd immunity, so too might an anti-GMO papaya fan benefit from the “herd immunity” afforded to organic papaya by transgenic papaya. Organically grown papaya trees were protected from PRSV-p because the presence of transgenic trees decreased the availability of host cells for the virus, slowing its spread. Organic papaya trees were also more literally protected when they were shielded inside of a circle of transgenic papaya trees (Ronald, 59). Despite the embrace of GMO seed by an overwhelming majority of U.S. papaya growers (C. Gonsalves), many anti-GMO activists do not see the presence of transgenic papaya as protective; rather, they see it as posing a threat of contamination. A Greenpeace flier describes a “tainted” organic papaya farm and makes vague reference to “environmental risks posed by genetically engineered food” (Greenpeace), but does not mention the greater threat posed by PRSV-p or explain how transgenic papaya might harm the ecosystem.

Ironically, while many fans of organic produce shun GMOs because they don’t want to eat food with genes from other organisms, organic papayas, if infected by PRSV-p, might contain huge amounts of viral proteins and genes – even more foreign matter than that expressed in transgenic papaya (D. Gonsalves) – making a PRSV-p-infected organic papaya a transgenic organism itself, albeit in a more “natural” sense.

Hawaii is not the only place where papaya trees have fallen victim to PRSV; the virus is a threat in other parts of the world, including Malaysia, Australia, Thailand (C. Gonsalves), and most notably Brazil (where it was also similarly driven nearly to extinction [D. Gonsalves]). In such countries, it is not only industry that is threatened by the virus; papaya is a common tree in household gardens. After the development of transgenic papaya seed by university-affiliated scientists, licensing agreements were negotiated with Monsanto, Asgrow Seed Company, Cambia Biosystems, and MIT; the agreement only extended to the seeds’ use in Hawaii, possibly placing it in the public domain and thus leaving open the opportunity for similar techniques to be employed for developing countries where gardeners and farmers can’t afford to buy seed from biotech corporations (C. Gonsalves). Researchers reached out to papaya-growing developing nations to share the biotechnological techniques with scientists in those countries, to employ as an appropriate technology in the battle against papaya ringspot virus worldwide (D. Gonsalves).

In 2008, Nature published SunUp’s full genetic sequence, making it the third multicellular plant genome and the first transgenic genome to be completely sequenced. Sequencing of the transgenic papaya seemed to provide preliminary evidence that transgenes lack a propensity to rearrange themselves within the genome – good news in terms of biosafety. Functionally, transgenic papaya is similar to non-GMO papaya in all ways other than resistance to PRSV-p; however, more exact comparisons cannot be made without a sequencing of the region on the non-GMO papaya genome that corresponds to the insertion site in the transgenic genome (Kohli).

Some scientists fear that genetically modified crops can have adverse effects on soil microbiota by way of horizontal gene transfer from transgenic plants to underground microorganisms. A Chinese team found evidence that transgenic papaya developed specifically for China might change the chemical properties, enzymatic activities, and microbial makeup of the soil. Their experiment showed that the presence of Chinese transgenic papaya correlated with a reduction in nitrogen and sulfur content compared with the control groups, as well as a change in the activities of some enzymes normally found in soil (Wei).

More interestingly, there seemed to be an increase in kanamycin (Km) resistance among microorganisms in the soil planted with transgenic papaya, raising the possibility that the gene for Km resistance had been acquired through horizontal gene transfer[1] – though it is more likely that the Km-resistant microbes were introduced into the soil by the transgenic papayas when they were planted. Because horizontal gene transfer is so rare, the principle of parsimony would dictate that the microbes were introduced rather than created by the presence of a transgenic plant. In any case, the inclusion of antibiotic-resistant genes in transgenic plants could be risky in that it could change the microbial profile of soil (Wei).

Though microbial life might be affected by antibiotic-resistant marker genes, how are humans likely to be affected? The National Academy of Science declared human risk from antibiotic genes used as markers in GMO foods to be low. The DNA from GMO soybeans degrades too quickly for there to be much of a chance for it to be acquired by the gut flora via horizontal gene transfer. Still, after the lessons we learned from the overuse of antibiotics over the last 60 years, should we be wary about introducing more opportunities for antibiotic resistance to develop in the world? Luckily, new genetic markers are being developed, such as a sugar-enablement marker that allows resistance to high levels of sugar. Antibiotic-resistance genes are being used less frequently, and some transgenic organisms, such as Xa21 rice, are being engineered without the use of any markers at all. Thus far, it seems that the overuse of antibiotics in other areas of life – in animal feed, in hospitals – has led to more tangibly negative outcomes (Ronald, 97). Because of this, many feel that transgenic organisms should be made without marker genes conferring antibiotic resistance — and the technology to do so is emerging.

To this day, the only effective defense papaya farmers have against the papaya ringspot virus is in the form of transgenic papaya (Ronald, 59). Other defenses are usually unsuccessful, and include certain pruning methods, netting, pesticides, planting different varieties of papaya, and quarantining and incinerating affected papaya trees (C. Gonsalves). It seems that in the case of papaya at risk for PRSV-p, genetic engineering is the most appropriate solution – especially in areas where papaya is an important source of nutrition.

References

Gonsalves, C. C., Lee, D. R., & Gonsalves, D. D. (2007). The Adoption of genetically modified papaya in Hawaii and its implications for developing countries. Journal of Development Studies, 43(1), 177-191. doi:10.1080/00220380601055650

Gonsalves, D. (1998). CONTROL OF PAPAYA RINGSPOT VIRUS IN PAPAYA: A Case Study. Annual Review of Phytopathology, 36(1), 415.

Greenpeace (2006). Tainted Fruit in Paradise. Greenpeace Update, 8.

Kohli, A., & Christou, P. (2008). Stable transgenes bear fruit. Nature Biotechnology, 26(6), 653-654. doi:10.1038/nbt0608-653

Ronald, P.G., Adamchak, R.W. (2008). Tomorrow’s Table: Organic Farming, Genetics, and the Future of Food. Oxford University Press.

Tortora, G.J., Funke, B.R., Case, C.L. (2010). Microbiology: An Introduction. San Francisco: Pearson Benjamin Cummings.

Wei, X. D., Zou, H. L., Chu, L. M., Liao, B. B., Ye, C. M., & Lan, C. Y. (2006). Field Released Transgenic Papaya Affects Microbial Communities and Enzyme Activities in Soil. Plant & Soil, 285(1/2), 347-358. doi:10.1007/s11104-006-9020-8


[1] A gene for Km resistance had been used as a marker gene in the engineering of transgenic papaya. A marker gene is a piece of DNA that confers a certain phenotype onto the transformed organism. This makes it readily apparent to scientists that the insertion of the other bits of foreign DNA, such as those conferring virus resistance, was successful. Genes conferring resistance to antibiotics are often used as genetic markers.

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