Recent origination of circular RNAs in plants.

Circular RNA (circRNA) is a kind of new regulatory RNA with diverse biological functions. Numerous circRNAs have been identified in many plant species; however, evolution of plant circRNAs remains largely unknown. In this study, we assembled full-length sequences of 6,519 rice (Oryza sativa) circRNAs and analyzed their conservation in other 46 plant species based on comparison of sequences and expression patterns. We found that, at the genomic level, 8.7% of the 6,519 circRNAs were conserved in dicotyledonous plants and 49.1% in Oryza genus. Meanwhile, 57.8% of parental protein coding genes of the rice circRNAs originated recently after divergence of monocotyledonous plants, implying recent origin of the majority of rice circRNAs, a conclusion further supported by the results based on analysis of 4,663 full-length circRNAs in Arabidopsis thaliana. Accordingly, we proposed three models to address the origination of different types of circRNAs. Taken together, the results obtained in this study provide new insights for the evolutionary dynamics of plant circRNAs and candidate circRNAs for further functional exploration.