Binary division of plastids involves endoplasmic reticulum mediation

Plastids divide through binary division, involving the cytosolic protein Accumulation and Replication of Chloroplast 5 (ARC5), suggested to constrict and sever the plastid envelope membrane. However, the mechanisms involved in ARC5 recruitment to the mid-plastid division site and the final separation of daughter plastids are not fully understood. Using time-lapse imaging of Arabidopsis thaliana stable transgenics expressing fluorescently tagged endoplasmic reticulum (ER) and ARC5 proteins, we investigated the role played by the ER in the late stages of plastid division. Our observations establish that prior to its mid-plastid localization at the division plane, ARC5 associates with ER membranes. ARC5-ER association generates an ER band around the plastid mid-plane that persists throughout division. Progressive tightening of the ER band narrows the middle of the plastid to form an isthmus. Concomitantly, tandem plastid-ER dynamics facilitated by membrane contact sites (MCSs) move and rotate the dividing plastid and ultimately lead to the separation of daughter plastids. Our findings strongly indicate a pivotal role for the ER in facilitating plastid division.