Amplification of substoichiometric recombinant mitochondrial DNA sequences in a nuclear, male sterile mutant regenerated from protoplast culture in Nicotiana sylvestris.

ANicotiana sylvestris plant regenerated from protoplast culture was found to be mutated in both the mitochondrial (mt) and nuclear genomes. The novel mt DNA organization, called U, is due to the amplification of recombinant substoichiometric DNA sequences that preexist in the parent line. The recombination event involves two 404 by repeats, which hybridize to a 2.1 kb transcript. Although the sequence of both repeats was not altered by the recombination, an additional transcript of 2.5 kb was detected in U mitochondria. In addition to this mitochondrial reorganization, the protoclone carried a recessive nuclear mutation conferring male sterility (ms4). A possible role ofms4 in the appearance of the U mt DNA organization was investigated by introducing this gene into normalN. sylvestris cytoplasm. No mt DNA change could be found in homozygousms4/ms4 plants of the F2 generation.