Another major area of genetic engineering activity is manipulation of the genes of key crop plants. In plants the primary experimental difficulty has been identifying a suitable vector for introducing recombinant DNA. Plant cells do not possess the many plasmids that bacteria do, so the choice of potential vectors is limited. The most successful results thus far have been obtained with the Ti (tumor-inducing) plasmid of the plant bacterium Agrobacterium tumefaciens, which infects broadleaf plants such as tomato, tobacco, and soybean. Part of the Ti plasmid integrates into the plant DNA, and researchers have succeeded in attaching other genes to this portion of the plasmid (figure 19). The characteristics of a number of plants have been altered using this technique, which should be valuable in improving crops and forests. Among the features scientists would like to affect are resistance to disease, frost, and other forms of stress; nutritional balance and protein content; and herbicide resistance. Unfortunately, Agrobacterium generally does not infect cereals such as corn, rice, and wheat, but alternative methods can be used to introduce new genes into them.
A recent advance in genetically manipulated fruit is Calgene’s “Flavr Savr” tomato, which has been approved for sale by the USDA. The tomato has been engineered to inhibit genes that cause cells to produce ethylene. In tomatoes and other plants, ethylene acts as a hormone to speed fruit ripening. In Flavr Savr tomatoes, inhibition of ethylene production delays ripening. The result is a tomato that can stay on the vine longer and that resists overripening and rotting during transport to market.
Recently, broadleaf plants have been genetically engineered to be resistant to glyphosate, the active ingredient in Roundup, a powerful, biodegradable herbicide that kills most actively growing plants (figure 20). Glyphosate works by inhibiting an enzyme called EPSP synthetase, which plants require to produce aromatic amino acids. Humans do not make aromatic amino acids; they get them from their diet, so they are unaffected by glyphosate. To make glyphosate-resistant plants, agricultural scientists used a Ti plasmid to insert extra copies of the EPSP synthetase genes into plants. These engineered plants produce 20 times the normal level of EPSP synthetase, enabling them to synthesize proteins and grow despite glyphosate’s suppression of the enzyme. In later experiments, a bacterial form of the EPSP synthetase gene that differs from the Figure 19
The Ti plasmid. This Agrobacterium tumefaciens plasmid is used in plant genetic engineering.