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Unintended Effects of Genetic Manipulation
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Potatoes with transgene for virus resistance were variably resistant, and some lines without the target gene nevertheless became highly resistant.

Manipulated Organism: Potato (Solanum tuberosum), varieties Russet Burbank and Ranger Russet.

Inserted Transgenes: Three different gene constructs: (1) coat protein (CP) gene derived from the potato leaf luteovirus and fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the gene would be expressed in all parts of the plant; (2) a modified form of the CP gene fused to the same promoter; and (3) as a control, a gene construct identical to the above-mentioned ones, but without the CP or modified CP gene. Using Agrobacterium as a vector, potato plants of both varieties (Russet Burbank and Ranger Russet) were transformed separately with the three constructs.

Goal: Make potato plants resistant to the potato leaf luteovirus, which is transmitted by aphids that feed on potato plants. Potato leaf luteovirus infections can cause severe losses in potato crops. Previous experiments had shown that transgenic potatoes with the CP gene had some resistance to the virus, although they did not make the coat protein. By modifying the CP gene, the researchers hoped that the plants would make the coat protein and show increased resistance to the virus. They created different lines of potatoes each containing one of the three constructs.

Intended Effect: All Russet Burbank transgenic lines containing the CP gene or the modified CP gene showed significantly lower virus infection than did unmanipulated potato plants.

Unintended Effects:
  • Although the Russet Burbank transgenic lines containing the CP gene or the modified CP gene showed significantly lower virus infection than did unmanipulated potato plants, they did not actually produce the coat protein itself. The cause of the increased resistance is unknown.
  • The Ranger Russet transgenic lines did contain the transgenes and produced transgenic RNA for the coat protein, but only 4 of 15 lines showed an increase in virus resistance, indicating some unknown variety-dependent effect.
  • Remarkably, the most resistant Ranger Russet line was one that had been transformed with the construct that did not contain the CP gene or the modified CP gene. This resistance was subsequently inherited as a dominant trait.
  • There were "statistically significant differences in virus resistance among independently derived transformants of the same variety-construct combination. . . . It thus appears that the variability associated with generating an individual clone [transgenic line] is larger than the effect of the construct" (p. 439).

Additional Comments: This study illustrates how a target effect in a transgenic plant - virus resistance in this case - can arise although the relation of that phenotypic change to the genetic manipulation remains a complete riddle.

Source: Presting, G. G., O. P. Smith, and C. R. Brown (1995). "Resistance to Potato Leafroll Virus Transformed with the Coat Protein Gene or With Vector Control Constructs," Phytopathology vol. 85, pp. 436-42.

Author Affiliations: USDA, Agricultural Research Service.

Funding: Not mentioned.

Product Status: Not on the market as of 2008.

Copyright 2008 The Nature Institute.

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