The horsetail Equisetum arvense mitochondria share two group I introns with the liverwort Marchantia, acquired a novel group II intron but lost intron-encoded ORFs.

We studied the genomic structure and RNA editing of mitochondrial cox1, cox2, cob and atp9 from the horsetail Equisetum arvense, a representative of an old fern lineage. Editing of cox1, cob and atp9 mRNAs occur only by C-to-U transitions. No changes were found in cox2 transcripts constituting one of the rare examples of unedited mitochondrial mRNA in land plants. From three intervening sequences in cox1, cox1i395 and cox1i624 are group IB introns homologous to the Marchantia polymorpha cox1 introns, and cox1i747 is a group IIA intron different to other introns found in plant mtDNA. The group II intron cox2i373 is very similar to other introns found in cox2 from vascular plants. While cob and atp9 have no introns and display the gene structure found in seed plants, various nucleotide substitutions abolish the only potential ORF, a LAGLIDADG endonuclease present in cox1i395. Thus, E. arvense mitochondria conserve two group I introns from non-vascular plants, probably inherited from a common ancestor with liverworts. Analogous to seed plants, E. arvense has no potential mitochondrial splicing factors encoded in these introns. This is the first report concerning the presence of vertically inherited group I introns in vascular plant mitochondria.