Solid-like condensation of MORF8 inhibits RNA editing under heat stress in Arabidopsis

Heat stress inhibits photosynthesis efficiency, thereby suppressing plant growth and crop yield. However, the mechanism underlying this inhibition is not fully understood. Here, we report that the multiple organellar RNA-editing factor 8 (MORF8) forms condensates with solid-like properties in chloroplasts upon heat stress. In vitro data show that the MORF8 condensation is intrinsically heat-dependent and primarily determined by its IDR (intrinsically disordered region). Purification and characterization of MORF8 condensates show that numerous editing factors including PPR proteins and MORFs are partitioned. We provide both genetic and biochemical evidence that MORF8 condensation inhibits chloroplast RNA editing. In agreement, we find that both heat stress and MORF8 condensation lead to reduced editing of RNAs encoding NADH dehydrogenase-like (NDH) complex and impaired NDH activity and photosynthesis efficiency. These findings uncover MORF8 as a putative chloroplastic thermosensor that mediates photosynthesis inhibition by heat and highlight the functional significance of solid material properties of biomolecular condensates.