Chloroplastic protein PORC undergoes heat-induced condensation and enhances thermotolerance in Arabidopsis

Chloroplast stress granules (cpSGs) are emerging as dynamic suborganellar condensates that play a crucial role in mediating stress response in plants. In this study, we demonstrate that the chlorophyll biosynthesis enzyme PROTOCHLOROPHYLLIDE OXIDOREDUCTASE C (PORC) localizes to cpSGs in response to acute and prolonged heat stress in Arabidopsis (Arabidopsis thaliana). While PORC promoter activity was developmentally regulated and remained unresponsive to heat, PORC protein re-localized from a diffuse chloroplast distribution into punctate structures under elevated temperatures. This condensation was reversible, translation-dependent, and absent under optimal growth conditions. Genetic disruption of PORC resulted in compromised thermotolerance, whereas overexpression enhanced photosynthetic recovery following both acute (42°C) and prolonged (35°C) heat stress. High-throughput phenotyping and chlorophyll fluorescence imaging confirmed enhanced Photosystem II (PSII) efficiency and increased post-stress growth rate. Proteomic profiling of heat-induced PORC-cpSGs revealed functional enrichment of photosystem I/II components, proteases (e.g., FtsH), and proteins involved in chlorophyll biosynthesis and photoprotection, suggesting a stress-protective role of cpSG under heat. These findings establish PORC as a key player in the chloroplast stress response, implicating cpSGs as protective hubs that facilitate the maintenance of photosynthetic integrity under elevated temperatures. Our study provides insight into chloroplast-specific biomolecular condensates, enhancing our understanding of plant stress resilience and paving the way for future studies on the regulation of their dynamics. Additionally, it highlights how components such as PORC could be utilized to develop heat-tolerant crops.