For decades, the concept of foreign DNA transfer into plant cells and its integration into the genome of the recipient cell was an intriguing curiosity (Lurquin, 1977). It was not until the advent of Southern hybridization (Southern, 1975) and the availability of restriction enzymes that the presence of `non-plant’ DNA (in this case, the T-DNA of Agrobacterium tumefaciens) in plant cells was unequivocally demonstrated (Chilton et al., 1977; Roberts, 1982). Once it became known that Agrobacterium could transfer a defined segment of its plasmid DNA into plant cells and that this DNA could become covalently integrated into the genomic DNA of the host cell, where it was expressed, plans for the transfer of known foreign genes into plant cells could be laid out (Roberts, 1982). An understanding of the roles of auxin and cytokinin biosynthetic genes that were present on the T-DNA and the availability of tools to selectively recombine genes with the Ti plasmid led to the possibility of genetic engineering for improvement of plants. Since these discoveries in the late 1970s, both the tools and the concepts of genetic manipulation of plants using specific genes have improved tremendously. During this period, we have revisited the earlier approaches of direct passive uptake of DNA, improved the efficiency of gene transfer by direct and vector-mediated techniques, designed genes for optimal expression in the transgenic cells/plants, and cloned a plethora of genes for potential use in genetic improvement of plants. As a result, during the past two years (1998–1999), a large proportion of the field-planted material of some of our major crops has been genetically modified using the tools of genetic engineering (Birch, 1997; Moffat, 1998). In addition, these techniques have led to a quantum leap in our understanding of some very basic aspects of plant development, physiology, and biochemistry (Birch, 1997). In this regard, transformation technology has helped us to tag and clone genes that were previously known only from the phenotypes they produce, to test critical hypotheses regarding the developmental roles of several of these genes, and to explore the regulatory aspects of key metabolic pathways (Koncz et al., 1989; Klein et al., 1990; Feldmann, 1991; Lindsey, 1998; Maes et al., 1999).
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