Plant Mitochondrial Genomes

Abstract

Plant mitochondria share a functional role with their fungal and animal counterparts, and their genomes have a number of genes in common. In almost every other aspect, however, plant mitochondrial genomes differ. Whereas animal genomes are very small (14-18 kb) and fungal genomes somewhat larger (70-100 kb), known plant mitochondrial genomes range in size from 187 kb to over 2400 kb. Plant mitochondrial genomes have a variety of other distinctive or unique features. They often have multiple large repeated regions, some of which recombine frequently to yield a multipartite genome structure. Some subgenomes are present at extremely low (substoichiometric) levels, but can become amplified to become major constituents. Plant mitochondrial genes contain mainly group II introns, some of which are trans-spliced. The transcripts of many mitochondrial protein-coding genes undergo C-to-U RNA editing. Several mitochondrial tRNAs are known to be transcribed from nuclear genes and imported into the mitochondrion. While only a few mitochondrial genomes from plants have been completely sequenced to date, at least half of the DNA in each of these genomes is still of unknown origin. Comparisons between monocot and dicot mitochondrial genomes show that, in general, only genic exons are conserved. Even between rice and maize, or between Arabidopsis and rapeseed, most of the intergenic space is not conserved. On relatively short evolutionary time scales, plant mitochondrial genomes are in flux, taking up exogenous DNA, losing portions of their DNA and rearranging the order of their sequences.