ABSTRACT
The ability of plant cells cultured in vitro, under suitable conditions, to form completely normal plantlets efficiently and reproducibly is the cornerstone for the production of transgenic plants. The retention of totipotency in differentiated plant cells provides the theoretical basis for this unique phenomenon in plants. Thus, plant tissues cultured in vitro can differentiate to form de novo organs such as shoots, roots, flowers, and embryos. Although the concept of plant cell totipotency dates back to the late 1800s, it was first demonstrated in 1939 when Gautheret (1), Nobécourt (2), and White (3) independently reported that continuously growing callus cultures were derived from meristematic tissues. However, Skoog (4) and Skoog and Tsui (5) are credited with the induction of callus and adventitious shoots from isolated mature and differentiated cells. They also implicated plant growth substances in the induction of shoot and/or root initiation from
NRCC No. 43799
tobacco callus, thereby providing the principle on which all micropropagation depends. Since these pioneering discoveries related to de novo organ formation from plant tissue cultures, this research area has developed into a scientific discipline that has become critical to the production of transgenic plants. Although the major impediments encountered in in vitro culture of most plant species have been overcome, the process still requires careful optimization in order to develop an efficient transgenic technology for plant improvement. In addition to development of high-fre-quency transformation efficiencies, the transgene must be stably integrated into the genome, inherited in a predictable manner, and expressed with fidelity. The foundation for fulfilling these requirements lies in the efficient regeneration of plants from a single cell that has received the gene of interest.
