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Why engineer virus resistance?
How does GE virus resistance work?
Which plants have been genetically engineered to be virus resistant?

Many plants are susceptible to diseases caused by viruses, often transmitted by insects (like aphids) from plant to plant across a field. The spread of viral diseases can be very difficult to control, and damage to a crop can be severe. Insecticides are sometimes applied to control populations of aphids, but often have little impact on the spread of the disease. Often the most significant weapons against viral diseases are cultural controls (such as removing diseased plants) and plant varieties bred to be resistant (or tolerant) to the virus, but they may not always be practical or available. Scientists have discovered new genetic engineering methods that provide resistance to viral disease where options may have been limited before.
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Viruses are very primitive organisms composed of little more than a protein-based "coat" encasing a short piece of genetic material. This short strand of DNA or RNA contains instructions for the virus' coat and movement proteins, and a few other genes to assist reproduction. After a virus infects a cell, it removes its coating of proteins and then reproduces itself by tricking the cells they infect into manufacturing copies of their proteins and genetic material. New viruses assemble themselves from the newly made parts and then escape to infect other cells. Disease symptoms in the plant are the result of cellular damage caused by the viruses.

Scientists discovered that by genetically engineering plant cells to deliberately overproduce one of the genes important to the virus (usually the "coat protein" gene), the virus is not able to reproduce. This is thought to work by a process called cosuppression-- the plant cell senses that a gene is being overproduced, and responds by blocking both the engineered gene and the virus' copy of the gene. The disease-resistance strategy of engineering a virus gene into a plant is often called coat protein-mediated viral resistance.
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Yellow Squash and Zucchini
Asgrow seed markets several varieties of squash and zucchini resistant to three important viral diseases: Zucchini Yellow Mosaic Virus (ZYMV), Watermelon Mottle Virus 2 (WMV-2), and Cucumber Mosaic Virus (CMV). These viral diseases can cause devastating losses to squash-related crops (Cucurbits: squash, zucchini, pumpkins, cucumber, watermelon) due to leaf mottling and yellowing, stunted plant growth, and deformed fruit. GE resistance to the viral diseases is much better than conventional sources of resistance, but it is still somewhat susceptible to variations in pathogen strain and environment. The Asgrow varieties are marketed with names like Independence II, Liberator III, Freedom III, and Destiny III.

Papaya
Beginning in 1992, a devastating outbreak of Papaya Ring Spot Virus (PRSV) swept through the papaya plantations of Hawaii-- papaya production dropped 40% in the course of 5 years. Control of the disease was limited to careful observation to remove infected plants. Researchers in Hawaii and at Cornell University developed two varieties of GE papaya resistant to PRSV using coat protein-mediated resistance. Seed of the two varieties, named "SunUp" and "Rainbow," have been freely distributed to papaya growers since May of 1998.

Potato
The "NewLeaf" line of Bt-potatoes, marketed by Monsanto's NatureMark to provide resistance to the Colorado Potato Beetle, was expanded in the last few years to include two additional virus-resistant varieties, NewLeaf Plus and NewLeaf Y. These include the Bt gene plus genetically engineered resistance to potato viral diseases (caused by the Potato Leaf Roll Virus and Potato Virus Y, respectively). The "Lew Leaf" line, however, was discontinued by the developer in early 2001 and has not been marketed since.
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