Nicotiana tabacum protoplasts were transformed by PEG-mediated uptake and electroporation, with circular and linear DNA, and with or without X-ray irradiation. We have looked at the influence on the transient expression by the different parameters mentioned above as well as on the frequencies for stable transformation. Plants were regenerated and selfed and finally, the progenies of the transformed plants were analysed and used to compare the pattern of gene integration by these different variations in transformation methods. The results from the transient expression as judged by GUS-activity, showed electroporation to give higher and more reproducible results than PEG-mediated uptake. Using linear instead of circular DNA increased the rate of stable transformation about 3 times. Including a mild X-ray treatment gave an increase in the same range. When the inheritance of the transferred trait was investigated, it was found that protoplasts transformed using linear DNA resulted in the highest number of plants with single-copy insertions. Protoplasts transformed with circular DNA showed the highest incidence of losing the trait, while plants, in which the transformation included an X-ray treatment, had the highest frequency of multicopy insertion events.

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