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BOX 2-3 Genetic Engineering Methods Traditional breeding is restricted to mobilization of genes within related plant species. In contrast, genetic engineering, through the process called transformation, allows scientists to transfer genes between not only unrelated species but also the kingdoms of living organisms. Transformation involves the introduction of DNA into plant cells and tissues. It changes the hereditary material in each cell of the altered plant, as well as the plant's biochemical reactions. Newly introduced traits might affect plant growth, development, nutrition requirements, nutrient content, or composition of harvested plant parts. Plant transformation is one of the fundamental tools by which genetic engineers modify plants. However, the techniques have only been developed over the past two decades. In the late 1970s, scientists discovered that the common bacterium Agrobacterium tumefaciens causes plant tumors when oncogenes are transferred from the bacterial Ti plasmid into plant chromosomes. Scientists at Monsanto Company and Washington State University, St. Louis, developed methods to delete the oncogenes, and replace them with different genes, of interest, thereby using the Ti plasmid as a vehicle to transfer desired genes into plant chromosomes. To ensure that all plant cells in an experimental mixture were transformed, they added a selectable marker gene (e.g., kanamycin resistance) to the transferred DNA (T-DNA). Exposure to selective growth conditions (e.g., a medium containing kanamycin) would then kill all of the nontransformed cells. Many plant cells are totipotent—an individual cell can grow into a whole plant. Thus, researchers could grow whole plants from individual transformed plant cells and select the plants that passed T-DNA to their progeny in a Mendeilan-deminant manner. Various dicotyledon plants have been transformed using the Agrobacterium technology, including tomato, hybrid poplar, potato, soybean, cotton, rape, and sunflower. Agrobacterium-mediated transformation initially did not work for most monocotyledon plants, including the majority of grain crops (the exceptions are rice and banana). Various academic and industrial laboratories developed new technologies for monocotyledon transformation based on particle acceleration. "Biolistic" guns shoot DNA into plant cells; the cells incorporate the DNA into their chromosomes and recover. Electroporation involves putting cells into an electric field.

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animal, or microbe). As our understanding of plant metabolism continues to improve, scientists will be able to manage more sophisticated manipulations of these systems to produce the desired biochemicals in the desired quantities. Separations of plant components for industrial uses can also be improved by genetic engineering.

Biochemical pathways and genes can be mobilized within plants to create new products based on molecules that originate from nonplant sources such as microorganisms. Further, biomolecules often can be modified to facilitate purification. Such capabilities have no parallel in