Plastid division is driven by a complex mechanism that involves differential transition of the bacterial and eukaryotic division rings

During plastid division, two structures have been detected at the division site in separate analyses. The plastid-dividing ring can be detected by tra nsmission electron microscopy as two (or three) electron-dense rings: an ou ter ring on the cytosolic face of the outer envelope, occasionally a middle ring in the intermembrane space, and an inner ring on the stromal face of the inner envelope. The FtsZ ring, which plays a central role in bacterial division, also is involved in plastid division and is believed to have desc ended to plastids. from cyanobacterial endosymbiosis. The relationship betw een the two structures is not known, although there is discussion regarding whether they are identical. Biochemical and immunocytochemical investigati ons, using synchronized chloroplasts of the red alga Cyanidioschyzon merola e, showed that the plastid FtsZ ring is distinct and separable from the pla stid-dividing ring. The FtsZ ring localizes in stroma and faces the inner p lastid-dividing ring at the far side from the inner envelope. The FtsZ ring and the inner and outer plastid-dividing rings form in that order before p lastid division. The FtsZ ring disappears at the late stage of constriction before dissociation of the plastid-dividing ring, when the constriction is still in progress. Our results suggest that the FtsZ ring-based system, wh ich originated from a plastid ancestor, cyanobacteria, and the plastid-divi ding ring-based system, which probably originated from host eukaryotic cell s, form a complex and are involved in plastid division by distinct modes.