Coevolution of Organelle RNA Editing and Nuclear Specificity Factors in Early Land Plants

RNA editing alters genomic information by site-specific conversion of cytidines into uridines in land plant chloroplast and mitochondrial transcripts. Key to this process are nuclear-encoded RNA-binding pentatricopeptide repeat (PPR) proteins imported into the two endosymbiotic organelles. The model moss Physcomitrella patens has been proven useful in identifying those specificity factors and has now become the first organism with a full mutual assignment of specific PPR proteins and organelle RNA editing sites. With only 11 mitochondrial and two chloroplast RNA editing events, however, Physcomitrella is at the lower end of the tremendously variable editing frequencies among land plants. At the other end of the spectrum, thousands of editing sites change genetic information in both organelles of club moss (Selaginella) species. Moreover, reverse U-to-C editing operates in parallel to C-to-U in other lycophyte genera, in hornworts and in ferns. In contrast to dozens of editing factors meantime characterized for C-to-U editing in model taxa like Arabidopsis, Oryza or Physcomitrella, no factors are yet identified for U-to-C editing. Gains and losses of RNA editing sites and adaptations of their cognate nuclear specificity factors are an outstanding opportunity to study 500 million years of coevolution between the three genetic systems in plant cells. A particular interest in this subject also results from necessary improvements of a recently proposed PPR–RNA binding code. We here summarize the state of research, highlighting the particular value of nonseed plants in studies of RNA editing and give an outlook on the likely future developments of the field.