ABSTRACT
Genetic transformation involves transfer of a DNA sequence into a plant cell and its subsequent integration into the host genome. Transformation serves as an important tool in plant biology for studying gene function and expression. It also allows for the addition of a single or few desirable traits in the existing elite varieties of plants without altering their genetic constitution. The essential prerequisites for a gene transfer system include the availability of a target tissue, a method to introduce foreign DNA, and a procedure to select transformed cells and regenerate transgenic plants (Birch, 1997). Although several methods have been used for delivery of transgenes into plant cells, Agrobacterium-mediated transformation has been the preferred method for a number of species. In nature, Agrobacterium tumefaciens is a pathogenic bacterium of many dicotyledonous and a few monocotyledenous plants. During the process of infection, Agrobacterium transfers a part of its DNA (T-DNA) present on a tumor-inducing (Ti) plasmid to the nuclear genome of the host plant. The components needed for transfer of T-DNA, known as the vir (virulence) genes, are present on the Ti plasmid in Agrobacterium. Transfer of the T-DNA region is achieved by the activation and action of the vir genes. Proliferating plant cells at the site of infection result in formation of tumors or crown galls. Infected cells produce amino acids known as opines that are utilized by the bacterium. The genes for opine synthesis are present in the T-DNA region, which is transferred to the host genome (Gelvin, 2003). The T-DNA transfer mechanism is utilized for the genetic transformation of crop plants, with the T-DNA region responsible for tumor formation being replaced by a T-DNA containing the desired genes of interest to be studied. Such Agrobacterium strains can no longer cause tumors upon infection and are termed disarmed strains. Genes of interest can either be inserted into the T-DNA region of a Ti plasmid or the T-DNA region of a smaller plasmid that is then introduced into disarmed Agrobacterium. The latter, known as a binary vector system, is more commonly adopted for transformation (Hellens et al., 2000).
