CP29A RNA-binding protein forms cold-dependent droplets adjacent to chloroplast nucleoids and facilitates chloroplast RNA splicing and plant cold resistance

Arabidopsis thaliana is capable of producing photosynthetic tissue with active chloroplasts at temperatures as low as 4 degrees celsius, and this process depends on the presence of the nuclear-encoded, chloroplast-localized RNA-binding protein CP29A. In this study, we demonstrate that CP29A undergoes phase separation in vitro and in vivo in a temperature-dependent manner, which is mediated by a prion-like domain (PLD) located between the two RNA recognition motif (RRM) domains of CP29A. The resulting droplets display liquid-like properties and are found in close proximity to chloroplast nucleoids. The PLD is required to support chloroplast RNA splicing and translation in cold-treated tissue. Together, our findings suggest that plant chloroplast gene expression is compartmentalized by inducible condensation of CP29A at low temperatures, a mechanism that we demonstrate to be crucial for plant cold resistance.