Do Plant Mitochondria Even Need Base Excision Repair

Substitution rates in plant mitochondrial genes are extremely low, indicating strong selective pressure as well as efficient repair. Plant mitochondria possess base excision repair pathways, however, many repair pathways such as nucleotide excision repair and mismatch repair appear to be absent. In the absence of these pathways, many DNA lesions must be repaired by a different mechanism. To test the hypothesis that double-strand break repair (DSBR) is that mechanism, we maintained independent lines of plants deficient in uracil-N-glycosylase (UNG) for 10 generations to determine the repair outcomes when that pathway is missing. In the absence of UNG, there is an increase in in double-strand breaks as assayed by recombination at repeated sequences. Surprisingly, given the single-seed descent bottleneck and the bottleneck of mitochondrial genomes in gametogenesis, no single nucleotide polymorphisms were fixed in any line in generation 10. The pattern of heteroplasmy was also unaltered through 10 generations. These results indicate that double strand break repair is a general system of repair in plant mitochondria, and appears to be so efficient that base excision repair is nearly dispensable. The existence of this general system may explain the seemingly anomalous differences between genes and non-genes in plant mitochondria.