The Case of the GMO Papaya

Sometimes a crops continued survival may depend on transgenesis.

By Michael Herbert , Contributing Editor

Until Californians go to the ballot box on Nov. 6, rhetoric from both supporters and opponents of the states Proposition 37 will only continue to intensify. Both viewpoints, however, share some common ground on at least one important point: The best way to persuade members of the voting public is by framing the debate based on what consumers want the mostis it an issue of price, of information, of safety, or of some other characteristic that consumers may weigh when making a purchase?

Perhaps another way to approach the GMO debate would be to step out of our own shoes for a moment and consider the viewpoints of some of the crops that we depend upon and enjoy.

No, seriously.

Very often, biotechnology firms attempt to market GMO products with claims that they offer some benefit to the consumer and grower, e.g., higher nutrient density, higher yields, pest resistance. In other cases, however, something as vital as a crops continued survival may depend on transgenesis.

The papaya and PRSV

Originally hailing from southern Mexico and Central America, Carica papaya was introduced to many Caribbean islands by the Spanish and other explorers from the Old World. The fruit was introduced to Hawaii in the 1800s. According to the Hawaii Papaya Industry Association, the Solo cultivar, the most prominent non-transgenic cultivar, was introduced to Hawaii in 1910 by Dr. Garritt Wilder. The island of Oahu, along with Florida, subsequently became the largest U.S. exporters of papaya.

But this all changed in the 1940s with the discovery of the papaya ringspot virus (PRSV). Primarily spread by wandering aphids, the virus causes chlorosis in the plants, a condition characterized by an inability to produce enough chlorophyll. If the virus infects young plants, they remain stunted; if it infects adult, fruit-bearing plants,  the papayas produced will have ringspots. According to Dennis Gonsalves, Ph.D., center director, United States Pacific Basin Agricultural Research Center, Hilo, HI, it is difficult to combat the spread of virus by going after the aphids because aphids do not have a high preference for papaya plants; they merely probe the plants and move on, not bothering to colonize in the papaya orchards.

In response to the rapidly-spreading virus, farmers fled to Hawaii's Big Island, which, according to Gonsalves, became responsible for 95% of papaya production by the 1970s. The island-hopping campaign, however, only managed to slow the spread of PRSV; the virus appeared in Puna, the last large papaya growing region in 1992, and, according to Sarah Nell Davidson of Cornell University, by 1995 the industry was in crisis, with trickle-down effects that threatened the economy of Hawaii as a whole." Florida fared worse: by the 1960s, virtually all but small-scale individual harvesting had ceased.

Building a better papaya

After the exodus to the Big Island, farmers knew it was only a matter of time before PRSV spread to their new sanctuary. Thus, in the 1980s, they enlisted the help of scientists and began to fight back. Their first course of action was attempting to breed resistance into the papaya plant. This met with only limited success; the lines produced by these efforts are merely PRSV-tolerant, not PRSV-resistant or immune. PRSV-tolerant plants can still produce papayas when infected, but the fruit, according to Gonsalves, is not high-quality. Worse, aphids that prey on infected PRSV-tolerant plants can still pick up the virus and spread it further, continuing the cycle.

A second technique, called cross-protection, works in a similar manner to Edward Jenner's classical smallpox vaccine: use a weakened virus to protect against a stronger one. Unfortunately, while the technique showed some promise, certain papaya cultivars were hurt more than they were helped, and cross-protection itself was not easy to implement from an economical perspective.

Beginning in 1985, Gonsalves began work on a genetically engineered version of papaya using the concept known as pathogen-derived resistance. According to Gonsalves, this concept basically states that plants that contain gene(s) ... of a pathogen are protected against detrimental effects of the same or closely related pathogens." Gonsalves and his team isolated and sequenced the gene that coded for the protein coat of PRSV and introduced it into callus cells from the Sunset papaya plant, which eventually resulted in a resistant plant line that was successfully field-tested in 1991. The result was SunUp, the first genetically engineered commercial variety of papaya. SunUp was later crossed with the non-GMO Kapoho Solo to create the Rainbow cultivar. Both SunUp and Rainbow quickly passed regulatory testing by the U.S. government and were officially released to farmers in 1998. Rainbow subsequently passed Japanese regulatory testing in 2011 and is currently being exported to Japan.

Resurgence and resistance

The speed at which SunUp and Rainbow were adopted rivaled the pace at which PRSV had infected the non-GMO varieties. Two years after the introduction of SunUp and Rainbow, more than half of fruit-bearing papayas were transgenic. After three years, papaya production rebounded to about 95% that of 1992 levels.

The current picture of GM papayas is mixed. As previously mentioned, the fruits are approved in the United States and Japan, and scientists from several countries in the developing world are working with Gonsalves and his team in order to develop their own resistant cultivars. However, the fruits are running into resistance from groups such as Greenpeace in other countries, such as Thailand, who question the safety and environmental impact of the GM crops. Similarly, Rainbow and SunUp may also never see distribution in Europe, explains Harry Cline of the Western Farm Press, because the legalization process is political."

The GM papayas also have their share of opponents in the United States. In July 2011, thousands of transgenic papaya trees were chopped down. Although the perpetrators were never found, local farmers speculated that the scale of the operation indicated eco-terrorists carried out the deed.

This story ends in the same way it beginswith conflict. The disagreements in California, fortunately, have not resulted in physical altercations or property damage. Ultimately, however, both those who support and oppose GMOs should take the time to discuss each side's viewpoint. As Davidson notes, scientists need to make themselves more available to answer questions the public undoubtedly has, and to illustrate that genetic modification is more than an issue of simple economics. Sometimes, as in the case of the papaya, the ability of future generations to partake in a highly nutrient-dense crop may be at stake.  An open, transparent forum of ideas will do much to alleviate the concerns of more moderate GMO activists and allow all of humanity to reap the benefits of this technology.

Michael Herbert is a freelance science writer based in the Chicago suburbs. He graduated from Knox College with a B.A. in Biology and History and is especially interested in new advancements in nutrition and the health sciences. In his spare time, he enjoys cooking, building personal computers, reading and a variety of games. Contact: [email protected] .

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