Stable transformation of tobacco by electroporation: evidence for plasmid concatenation

Electroporation (electric field-mediated DNA transfer) of tobacco protoplasts in the presence of the linearized plasmid pMON200 has led to the formation of transgenic plants. Defined electric shocks were delivered by capacitive discharges with readily available, low-cost electrical components. This transformation procedure is simple and efficient and may suggest a quick method for determining the appropriate electric fields for new cell systems. An optimal transformation frequency of 2.2 X 10(-4) (based on the number of cells subjected to the shock) was obtained with a single 2000-V/cm, 250-microseconds-duration capacitive discharge. Calli transformed to kanamycin resistance have been regenerated into whole plants. Southern blots of DNA from the transgenic plants demonstrate the integration of the selectable marker gene (neomycin phosphotransferase) at single or multiple genomic sites. In some cases, the plasmid appears to be integrated intact; in others, it is rearranged. The blots also provide evidence of plasmid recircularization and/or the formation of head-to-head and head-to-tail concatemers in most of the plants analyzed. Although some plants apparently have multiple integration sites, analysis of progeny obtained by self-fertilization of the transgenic plants indicates that the kanamycin-resistance marker is inherited as a single dominant gene.