Double ring structure around the constricting neck of dividing plastids ofAvena sativa

Summary

Ultrastructure of the constricting neck of dividing proplastids and young chloroplasts in the first leaves ofAvena sativa was examined by electron microscopy. An electron-dense, “double” ring structure (plastid-dividing ring doublet; PD ring doublet) with a width of 15–40 nm was revealed around the narrow neck of the constricted and dividing plastids by serial section technique. The inner and outer ring of the doublet coated the inside (stromal side) of the inner envelope membrane and the outside (cytoplasmic side) of the outer envelope membrane, respectively. However, electron-dense materials were not observed within the lumen between the outer and inner envelope membranes.

Although the PD ring doublet was commonly observed in the constricted plastids with a 70–140 nm wide neck, they could be scarcely observed in the constricted plastids with a 160 or more nm wide neck. The components of the PD ring were assumed not to be concentrated enough to identify by electron microscopy in the early stage of constriction and the PD ring may be formed and recognized at the final stage.

The significance of the formation of the PD ring and its role in plastokinesis (plastid kinesis) were discussed.