Dual Cytoplasmic and Chloroplastic Mechanisms Fine-Tune Chloroplast Division through ARC3 Protein Stability.

Chloroplast division is essential for plant growth and photosynthesis, and the assembly of the FtsZ-ring is tightly regulated by ARC3 (ACCUMULATION AND REPLICATION OF CHLOROPLASTS 3). However, the mechanisms controlling its protein turnover remain poorly understood. This study reveals that ARC3 stability is co-regulated by both the ubiquitin-proteasome system (UPS) and chloroplast protease pathways. We demonstrate that the E3 ubiquitin ligase PUB52 mediates cytosolic degradation of ARC3 precursors via the 26S proteasome, whereas the chloroplast chaperone-protease CLPC1 (CLPC HOMOLOGUE 1) promotes ARC3 degradation within chloroplasts. Conversely, ARC2 stabilizes ARC3 by protecting it from proteolytic degradation, ensuring proper FtsZ-ring assembly. Either disruption or overexpression of PUB52, CLPC1, or ARC2 leads to aberrant chloroplast division phenotypes that resemble those of ARC3 dysregulation mutants. Genetic analyses place these regulators upstream of ARC3 in the chloroplast division pathway. These findings establish a post-translational regulatory network where PUB52, CLPC1, and ARC2 dynamically control ARC3 levels to fine-tune chloroplast division. This work provides important insights into the coordination of cytoplasmic and chloroplast protein homeostasis systems in maintaining organellar homeostasis, with broader implications for organelle biogenesis and plant stress adaptation.