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Environmental Effects of Transgenic Plants: The Scope and Adequacy of Regulation
with substantially reduced linolenic acid (Green 1986). Oil from this mutant is virtually identical to high-quality oils from safflower or sunflower. The mutant linseed varieties are now in commercial production in several countries.
Plant breeders were not as successful developing altered oil-quality soybean genotypes using mutagenesis, although mutant soybean oil varieties do exist (Brossman and Wilcox 1984, Kinney 1994). The exact mechanisms or genotypes of most mutations are unknown. In mutation breeding, plant material is exposed to a mutagenic agent, such as ionizing radiation, in the hope of obtaining a mutation with a desirable phenotype. Mutagenic agents alter the DNA of the subject organism, causing destruction or duplication of a gene or parts of genes. Plant breeders select genetically based phenotypes and rarely characterize mutants at a molecular genetic level. It is almost certain that any mutation resulting in a desired phenotype will also contain mutations at other less obvious genes. These conceivably could affect ecological fitness, nutritional or antinutritional composition, or other important characteristics. In practice, however, such accessory or pleiotropic effects are identified during the breeding evaluation process prior to commercial release.
To avoid some of the problems and uncertainties with mutation breeding, DuPont used transgenic technology to improve the soy oil quality by increasing the proportion of oleic acid from approximately one-quarter to over one-half of the fatty acids. This change was accomplished by inserting another copy of a soybean gene to interfere with the enzyme responsible for converting oleic acid to polyunsaturated fatty acids. Sometimes, introducing an extra copy of a gene results in a phenomenon called cosuppression, the simultaneous silencing of the activity of both the endogenous and the inserted copies of the gene. This enzyme is encoded by a gene called Gm fad 2-1, which catalyses the biochemical reaction converting oleic acid to linoleic acid. DuPont’s strategy was to develop transgenic soybean lines in which the inserted soybean gene interferes with the normal activity of that enzyme through cosuppression, resulting in a buildup of oleic acid and a reduction in linoleic and other polyunsaturated fatty acids. DuPont inserted DNA, including the Gm fad 2-1 gene, into soybean (cultivar Asgrow A2396) using the particle gun delivery method, regenerated transgenic soybean plants, and analyzed them for activity of the Gm fad 2-1 gene.
In the DNA plasmid containing Gm fad 2-1, the gene was linked to the soybean seed-specific promoter of the beta-conglycinin gene and the transcription terminator region from the phaseolin gene of Phaseolus vulgaris (common bean). This plasmid included two additional genes from E. coli: the ampicillin resistance beta-lactamase gene (bla) with a bacterial promoter, which was used in selecting the initial DNA constructs in bacteria