STABLE TRANSFORMATION OF PLASTIDS IN HIGHER-PLANTS

Stable genetic transformation of the plastid genome is reported in a higher plant, Nicotiana tabacum. Plastid transformation was obtained after bombardment of leaves with tungsten particles coated with pZS148 plasmid DNA. Plasmid pZS148 (9.6 kilobases) contains a 3.7-kilobase plastid DNA fragment encoding the 16S rRNA. In the 16S rRNA-encoding DNA (rDNA) a spectinomycin resistance mutation is flanked on the 5' side by a streptomycin resistance mutation and on the 3' side by a Pst I site generated by ligating an oligonucleotide in the intergenic region. Transgenic lines were selected by spectinomycin resistance and distinguished from spontaneous mutants by the flanking, cotransformed streptomycin resistance and Pst I markers. Regenerated plants are homoplasmic for the spectinomycin resistance and the Pst I markers and heteroplasmic for the unselected streptomycin resistance trait. Transgenic plastid traits are transmitted to the seed progeny. The transgenic plastid genomes are products of a multistep process, involving DNA recombination, copy correction, and sorting out of plastid DNA copies.