Heteroplasmy and expression of mitochondrial genes in alloplasmic and euplasmic wheat.

The plant chondriome confers a complex nature. The atp4 gene (formerly called orf25) of Aegilops crassa (CR) harbors the promoter sequence of the rps7 gene from common wheat (Triticum aestivum cv. Chinese Spring, CS). The rps7 gene of CR has the promoter sequence of CS atp6. The atp6 gene of CR contains an unknown sequence inside of its coding region. Since repeat sequences have been found around the breaking points, these structural alterations are most likely generated through homologous recombination. In this study, PCR analysis was performed to detect structural alterations in each of three lines: euplasmic lines of Ae. crassa, Chinese Spring, and alloplasmic Chinese Spring wheat with the cytoplasm of Ae. crassa ((cr)-CS). We found that each of these lines contained both genotypes, although mitochondrial genotypes of CR in Chinese Spring wheat and CS genotypes in Ae. crassa were still retained as minor fractions (less than 10%). On the other hand, CS mitochondrial gene frequencies in ((cr)-CS) were shown to be ca. 30%. SNP analysis after DNA sequencing of these genes indicated that minor types of all three mitochondrial genes in alloplasmic wheat contained the mitochondrial gene types from pollens. Since the frequencies of paternal mitochondrial gene types in F(1) were about 20%, successive backcrossing increased the frequencies of paternal mitochondrial gene types to around 30% in alloplasmic wheat. Expression profiles of these mitochondrial genes were quantitatively analyzed by RT-PCR. Transcripts of paternal mitochondrial gene types were scarcely found. This suggests that minor fractions including paternal mitochondrial gene types are maintained and silenced in the descendants.