First Identification of MORF Family in Ferns: Molecular Regulation of Organellar RNA Editing in Osmunda japonica and Plenasium vachellii

RNA editing is a crucial mechanism regulating gene expression in plant organellar genomes, which optimizes protein structures through base substitution and plays a vital role in plant growth, development, and stress adaptation. This study revises the conventional understanding restricting MORF proteins to seed plants by reporting their first identification in ferns, an early vascular plant lineage. We sequenced chloroplast genomes of <i>O. japonica</i> and <i>P. vachellii</i>, revealing one MORF9 homolog in <i>O. japonica</i> and three homologs (MORF1/8/9) in <i>P. vachellii</i> through comparative transcriptomics and structural validation. All identified MORF proteins harbor conserved MORF-box domains, suggesting structural and potentially functional conservation with angiosperms. Crucially, MORF members differentially regulate organellar RNA editing: chloroplast editing frequencies are predicted to show dose-dependent enhancement (0.7-1.0 in conserved sites), potentially influenced by MORF presence or copy number. In <i>O. japonica</i>, chloroplast editing exhibits tissue-specific patterns (conserved sites 0.7-1.0; tissue-specific sites lower efficiency at 0.1-0.2), while this study's mitochondrial editing results show a balanced frequency distribution (0-1 range). Amino acid substitution analysis demonstrates MORF-mediated hydrophobic optimization (Ser→Leu > 30%, Pro→Leu > 18%), likely underpinning fern adaptability. This work provides crucial initial evidence for a conserved MORF-mediated RNA editing module shared between these early vascular plants (ferns) and angiosperms, offering fundamental insights into the evolutionary trajectory of plant organellar gene regulation.