| Plant mitochondrial genomes have many interesting and unusual properties. For example, plant mitochondrial genes usually evolve extremely slowly, about 10-fold slower than plant and animal nuclear genes, and 100 times slower than animal mitochondrial genes. However, two lineages of plants show a remarkable increase in their mitochondrial substitution rate that is not found in plastid or nuclear genes. Using a novel approach to measure the magnitude and timing of rate change in these lineages, rates were shown to exceed those in animal mitochondria. Often, these rate increases were temporary, with rates returning to normally low levels in descendent lineages. Plant mitochondrial genomes are also unusual because, unlike most other multicellular eukaryotic genomes, their genes undergo horizontal gene transfer (HGT) with surprising frequency. Several additional cases of HGT were discovered, further supporting the idea that HGT of plant mitochondrial genes is a frequent occurrence. More importantly, the donors in each transfer event were identified as parasitic plants, indicating that the intracellular penetration of the host by these parasites facilitated gene transfer. In parallel, other laboratories discovered that genes can also be transferred in the opposite direction, from host to parasite plant. Taken together, these results have provided the first clear evidence for a mechanism for the horizontal transfer of plant mitochondrial genes. A third interesting property of plants is that their organellar transcripts undergo RNA editing, which converts specific cytidines to uridines and thereby modifies the information that was coded in the genome. By analyzing the RNA editing content of a complete mitochondrial genome, and by comparison to editing in other plant mitochondria, it is evident that RNA editing sites are not distributed randomly; rather, selection has played an important role in shaping the editing content in plant mitochondrial genomes. |
