MORF proteins: A small family regulating organellar RNA editing and beyond

In the chloroplasts/plastids and mitochondria of flowering plants, RNA editing alters hundreds of cytidines to uridines at specific sites mediated by the editosome. Over the past decade, Multiple Organellar RNA Editing Factor (MORF) proteins have emerged as essential regulators that affect the editing efficiency of most editing sites in plastids and mitochondria. In Arabidopsis, the MORF family consists of nine members, each possessing a single conserved MORF-box that is distributed among flowering plants. Accumulating studies have demonstrated that MORF proteins interact with many other factors, including the PPR proteins and enzymes in different biosynthetic pathways, indicating that the MORF proteins play a more extensive role in regulating organellar development than RNA editing. Recent studies reveal that MORF2 and MORF9 possess holdase activity and may act as chaperones and that MORF8 undergoes heat-dependent phase separation to inhibit RNA editing in chloroplasts. In this review, we provide an overview of our current knowledge of the MORF family proteins and discuss the biological and molecular functions of this family in plants.