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256-271
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A major unresolved problem in the evolution of green plants is which bryophytes are the earliest land plants, liverworts, hornworts, or mosses. Therefore, we tried to resolve this problem using a newly determined structure of the hornwort Anthoceros chloroplast genome and RNA editing in the chloroplast. A phylogenetic tree based on amino acid sequences derived from the complete nucleotide sequences of chloroplast genomes partially supports liverworts to be the earliest land plants. The occurrence of abundant RNA editing in the hornwort chloroplasts supports liverworts as the earliest, because RNA editing is detected in major land plant lineages but not detected any algae or in the complex thalloid liverworts (Marchatiidae). However, another explanation cannot be ruled out, that RNA editing was required for the hornworts to survive on land and then liverworts lost the ability as they diverged. Charales and Coleochaetales are the closest relatives to land plants among other charophytes. The land plants consist of two groups: bryophytes and tracheophytes (vascular plants). Extant vascular plants comprise pteri-dophytes and seed plants. Pteridophytes consist of four groups: lycophytes, Equisetum, Psilotum, and ferns. The orientation of a 30-kb gene cluster in the chloroplast genome shows that lycophytes are the earliest vascular plants and ferns, including Psilotum, are the basis of seed plants.
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