Plant transformation technology offers an array of

opportunities for basic scientific research and for genetic modification of crops. Transgenic plants are typically produced from cultured cells or tissues in which foreign DNA is integrated in the nuclear or chloroplast genome by numerous methods. These methods can be biological (e.g. Agrobacterium tumefaciens), physical (e.g. electroporation, particle gun), or chemical (e.g. using polyethyleneglycol). Methods using A. tumefaciens are easy and inexpensive, and thus have been used extensively for transformation of several plant species (Lloyd et al. 1986; Binns and Thomashow 1988; Zambryski 1988). Generally, successful genetic transformation requires not only efficient gene delivery, but also an efficient selection system. During transformation of plant tissues