Genetically Altered Papayas Save the Harvest

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February, 2000: Hawaii

 

Overview map of Hawaii. (Map courtesy of Alternative-Hawaii.)

Agriculture is an important way of life on the Hawaiian Islands, an archipelago in the Pacific Ocean about 2,300 miles off the coast of California. According to the United States Department of Agriculture's 1997 census, 1.4 million acres of Hawaii's 4-million-acre total land area is farmland. The vast majority of these farms are between 1 and 99 acres in size and are individually or family owned. Any condition adversely affecting the crops directly interferes with the livelihood of the 56% of the population practicing farming as a principal occupation.

In addition to pineapple, sugarcane, and macadamia nuts, another economically important crop in Hawaii is papaya. The papaya plant, Carica papaya, is commonly known as papaw or melon tree, although it is technically an herb, not a tree. The papaya plant is a dicot in the Caricaceae family, with lightly scented, moth-pollinated flowers. Carica has many economic uses. Its palmate leaves can be wrapped around meat as a tenderizer or used to reduce the cloudiness of beer during processing. The fleshy fruit can be consumed fresh or used for ice cream, jam, jellies, beverages, and chewing-gum flavoring. The plant has also been grown for the extraction of an enzyme called papain that is produced within its unripe fruits and is now being investigated for antibacterial properties.

A papaya tree. (Photo © 1999 Erica Kipp.) Palmate leaves. (Photo © 1999 Erica Kipp.)

  Papaya fruit.

However, the papaya industry is currently in jeopardy because the plants are being attacked by the papaya ring spot virus (PRSV), also known as potyvirus, papaya leaf distortion virus, and papaya distortion mosaic virus. PRSV first appeared in the 1940s on the island of Oahu, where most of the papaya was then being produced. The virus has since spread slowly across the archipelago, destroying papaya plantations and farmers' hopes along the way. About seven years ago, it reached the island of Hawaii, where almost all of the state's papayas now grow.

PRSV is a microparasite, meaning that it uses a vector to transmit infection from host to host. While some microparasites spread through abiotic (nonliving) vectors such as the air when people cough or sneeze, PRSV relies on biotic, or living, vectors--one of two insects scientifically known as Myzus persicae and Aphis gossypii. These insects transfer the virus from an infected plant to a healthy plant. PRSV causes premature molting and malformation of existing leaves. It may also result in ring spots and other markings on plant organs such as the fruit, stems, and petioles. Papaya plants infected with PRSV are generally smaller, less productive, and less likely to survive than uninfected plants.

 

Steps in making transgenic plants

Attempts were made to rid the island of the virus by removing diseased plants, developing a vaccination for healthy plants, and applying traditional plant breeding systems, including hybridization and polyploidy. However, those methods proved futile. Then the Papaya Administrative Committee, a research and marketing group supported by local papaya farmers, worked with the University of Hawaii, the Agriculture Department, and Cornell University's Department of Plant Pathology to produce a genetically modified papaya called Rainbow. Rainbow is resistant to PRSV because the virus's DNA has been incorporated into the DNA of the plant. Such genetically modified plants are called transgenic, meaning that one or more heterologous genes have been incorporated into their chromosomes, either artificially or naturally.

Transgenic papaya plants are causing quite a stir among local growers, organic farmers, and ecologists. Some growers are tremendously thankful for the chance to save a literally dying industry, while organic farmers are up in arms because their organic papaya may be fertilized with pollen from Rainbow papaya, thereby rendering the next generation of fruits no longer "organic" by definition. Meanwhile, a comparison of the financial benefits to Hawaiian farmers indicates that nonorganic papaya sells at sixty times the dollar amount of organic versions.

Because humans have been consuming the infected fruits without ill effect for quite some time, researchers believe the protein produced by the virus gene introduced into Rainbow papaya will also have no adverse effects. Rainbow seeds are available to the farmers at no charge from the Papaya Administrative Committee. At this point, more than 1,000 acres of Rainbow papaya have been planted, and the first fruits are already for sale in Hawaii and on the mainland.

References, Websites, and Further Reading

"Stalked by Deadly Virus, Papaya Lives to Breed Again," New York Times, 20 July 1999, Science Times issue.

Map of Hawaiian Islands; general information (including tourist spots)

Plant Viruses Online

Hawaii Agriculture Research Center (HARC)

Stern, Introductory Plant Biology, 8th Edition

Chapter 6: Stems
Diagrams of external stem anatomy, pp. 85, 87

Chapter 7: Leaves
Leaf arrangements and leaf types, pp. 107-10

Chapter 8: Flowers, Fruits, and Seeds
Monocots and dicots compared, pp. 129-30
Structure of flowers, pp. 130-32
Fruit types, pp. 132-39

Chapter 14: Plant Propagation and Biotechnology
Plant biotechnology and propagation, pp. 238-54

Chapter 16: Plant Names and Classification
Scientific vs. common names, pp. 268-72

Chapter 17: Kingdom Monera and Viruses
Viruses, pp. 296-301

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