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

## TL;DR

This study discovers MORF proteins in ferns for the first time, showing they regulate RNA editing in chloroplasts and mitochondria, challenging previous assumptions about their presence only in seed plants.

## Contribution

The first identification of MORF family proteins in ferns, revealing conserved RNA editing regulation in early vascular plants.

## Key findings

- MORF9 homolog identified in Osmunda japonica and MORF1/8/9 in Plenasium vachellii with conserved MORF-box domains.
- Chloroplast RNA editing frequencies are dose-dependent and tissue-specific, influenced by MORF presence.
- Mitochondrial RNA editing shows uniform frequency distribution, contrasting with chloroplast patterns.

## Abstract

RNA editing critically regulates gene expression in plant organelles through post-transcriptional base substitution. We report the first identification of the Multiple Organellar RNA Editing Factor (MORF) family in ferns (Osmunda japonica and Plenasium vachellii). Through comparative transcriptomics and structural validation, we identified one MORF9 homolog in O. japonica and three homologs (MORF1/8/9) in P. vachellii, all containing conserved MORF-box domains. Our analysis demonstrates tissue- and organelle-specific RNA editing regulation: chloroplast editing frequencies are predicted to show dose-dependent enhancement (conserved sites 0.7–1; tissue-specific sites 0.1–0.2) potentially influenced by MORF presence, while mitochondrial editing exhibits uniform distribution. These findings challenge the traditional paradigm of MORF restriction to seed plants, revealing evolutionary conservation of RNA editing mechanisms in land plants.

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 O. japonica and P. vachellii, revealing one MORF9 homolog in O. japonica and three homologs (MORF1/8/9) in P. vachellii 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 O. japonica, 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.