Summary: Hornworts are crucial to understand the phylogeny of early land plants. The emergence of ‘reverse’ U‐to‐C RNA editing accompanying the widespread C‐to‐U RNA editing in plant chloroplasts and mitochondria may be a molecular synapomorphy of a hornwort–tracheophyte clade. C‐to‐U RNA editing is well understood after identification of many editing factors in models like Arabidopsis thaliana and Physcomitrella patens, but there is no plant model yet to investigate U‐to‐C RNA editing. The hornwort Anthoceros agrestis is now emerging as such a model system. We report on the assembly and analyses of the A. agrestis chloroplast and mitochondrial genomes, their transcriptomes and editomes, and a large nuclear gene family encoding pentatricopeptide repeat (PPR) proteins likely acting as RNA editing factors. Both organelles in A. agrestis feature high amounts of RNA editing, with altogether > 1100 sites of C‐to‐U and 1300 sites of U‐to‐C editing. The nuclear genome reveals > 1400 genes for PPR proteins with variable carboxyterminal DYW domains. We observe significant variants of the ‘classic’ DYW domain, in the meantime confirmed as the cytidine deaminase for C‐to‐U editing, and discuss the first attractive candidates for reverse editing factors given their excellent matches to U‐to‐C editing targets according to the PPR‐RNA binding code.

Programmable RNA editing offers significant therapeutic potential for a wide range of genetic diseases. Currently, several deaminase enzymes, including ADAR and APOBEC, can perform programmable adenosine-to-inosine or cytidine-to-uridine RNA correction. However, enzymes to perform guanosine-to-adenosine and uridine-to-cytidine (U-to-C) editing are still lacking to complete the set of transition reactions. It is believed that the DYW:KP proteins, specific to seedless plants, catalyze the U-to-C reactions in mitochondria and chloroplasts. In this study, we designed seven DYW:KP domains based on consensus sequences and fused them to a designer RNA-binding pentatricopeptide repeat (PPR) domain. We show that three of these PPR-DYW:KP proteins edit targeted uridine to cytidine in bacteria and human cells. In addition, we show that these proteins have a 5′ but not apparent 3′ preference for neighboring nucleotides. Our results establish the DYW:KP aminase domain as a potential candidate for the development of a U-to-C editing tool in human cells.

A subfamily of pentatricopeptide repeat (PPR) proteins, known as PPR-DYW:PG, catalyzes the cytidine to uridine (C-to-U) RNA editing in plant organelles. A related PPR subfamily, PPR-DYW:KP, catalyzes the uridine to cytidine (U-to-C) reaction, via a crosslinking mechanism involving a lysine residue. We demonstrate that Lys88 in the DYW:KP domain is essential for the U-to-C editing activity of PPR-DYW:KP proteins. Substituting Lys88 with other amino acids in designer proteins switches the protein activity to C-to-U and prevents crosslinking with the edited RNA. However, this mutation leads to C-to-U off-target editing downstream the targeted site. Finally, other modifications can modulate the catalytic activity and alter the type of reaction catalyzed by the DYW domain. Altogether, our results suggest that subtle modifications in the DYW domain can influence the position of the edited nucleotide and the type of RNA editing reaction.

Programmable RNA cytidine deaminase tools have been developed to convert cytidine-to-uridine (C-to-U) using CRISPR systems with guide RNAs. These tools, however, have limitations such as low editing efficiency, limited targetable sequence flexibility, and off-target RNA editing. Here, we present a novel guide-free C-to-U editing tool, named RECODE (RNA Editor for C-to-U with an Optimized DYW Enzyme), based on the RNA-binding pentatricopeptide repeat proteins, naturally fused to a C-terminal DYW cytidine deaminase domain. The RECODE specificity domain was engineered to enable retargeting, while its length and sequence were optimized to reduce off-target effects. Further optimization of the C-terminal catalytic region increased both the editing activity and the translation of the edited RNA. We showed that RECODE efficiently edits a wide range of targets in human cells, without affecting adjacent cytidines. It achieved over 50% editing efficiency for most sites, except those with an upstream guanine. Furthermore, we showed that RECODE is functional in mice, with high editing efficiency observed in specific tissues such as skeletal muscles using an AAV delivery system, suggesting its therapeutic potential for various diseases.

Plants possess a unique C-to-U RNA editing mechanism mediated by PPR-DYW proteins, wherein the PPR domain recognizes specific RNA sequences while the DYW deaminase domain precisely edits the target C base-a process essential for functional protein expression in plant chloroplasts and mitochondria. The coordination of these two domains is considered crucial for precise RNA editing. In nature, this site-specific and precise base editing by PPR-DYW proteins distinguishes them from other base-editing deaminases. However, the absence of structures containing both PPR and DYW domains has limited our understanding of the precise RNA-editing mechanism of PPR-DYW proteins. Here, we present crystal structures of the consensus PPR-DYW (consPPR-DYW) protein, a representative of the PPR-DYW proteins, in both RNA-free and target RNA-bound states. Comparison between these states demonstrates domain movements upon target RNA binding, whereby the PPR domain accommodates the upstream sequence of the target C base in the proper conformation for editing while the DYW domain is optimally positioned for precise C-to-U conversion. These results, combined with comprehensive biochemical analyses, provide the foundation for a mechanistic model that explains the coordinated action of the PPR and DYW domains in achieving precise C-to-U editing.

Plant mitochondrial biology is undergoing a rapid transformation driven by advances in genomics, structural biology, quantitative imaging, and genome engineering. Once focused primarily on respiration and bioenergetics, the field now encompasses diverse areas, including genome evolution, gene expression, organelle dynamics, stress signaling, metabolism, and biotechnology. The 14th International Conference for Plant Mitochondrial Biology (ICPMB), held in Kagoshima, Japan, from 18 to 22 May 2026, brought together researchers to discuss recent advances across these rapidly expanding research areas. This meeting report summarizes the major scientific advances presented at ICPMB 2026 and highlights emerging directions that are defining the future of plant mitochondrial biology.