Molecular analysis of mitochondria from a fertility restorer line of maize

Inbred line Ky21 carries nuclear genes which restore fertility to all three cytoplasmic male sterile (cms) types of maize: T, C and S. By substituting the Ky21 cytoplasm into a nuclear background lacking all of the nuclear restorer genes, we have demonstrated that Ky21 contains a fertile (normal) cytoplasm. Gel electrophoresis of mitochondrial DNA from Ky21 demonstrated an approximately 2.1 kb plasmid and no evidence for a 2.35 kb plasmid found in many normal cytoplasms of North American lines of maize. A 2.1 kb plasmid had been reported to be diagnostic for the T-type cms. However, the restriction endonuclease digestion pattern of Ky21 mtDNA more closely resembled that of normal lines than T-cms. Furthermore, mitochondria of Ky21 plants did not synthesize a 13 kilodalton polypeptide, which has only been found to be synthesized by T-type mitochondria. From these molecular criteria, as well as from the genetic analysis, we conclude that the mitochondria of the Ky21 universal restorer line are normal. In having a shorter form of a linear mtDNA plasmid, Ky21 resembles cytoplasms found in Mexican races of maize.