分裂リングから見た色素体の起原と進化

抄録

Summary: Plastid-dividingrings have been observed at the isthmus region of the dividing plastids in the wide range of land plants and algae, and have been envisaged as a division apparatus of plastids. As proteins composing the ring are suggested to be nulear-encoded, the mechanism of plastid divislon may have been aquired during the course of the evolutlon from cyanobacterium-like. endosymbionts to true organelles, i. e. plastids. ln most of the land plants examined so far and some green algae, the plastid-dividing rings are detected as double ring structures girdling the double envelope; the outer cytoplasmic ring and the inner stromal ring. In the other green algae and red algae, a single cytoplasmic ring has been recorded. The evidence now encompasses those of chromophytic algae of which chloroplasts are surrounded by four membranes. Their plastid dividing ring is observed as a single ring girdling the periplastidal face of the outer membrane of the inner pair of the four surrounding membranes. This finding demonstrates that the inner pair represents the true chloroplast envelope, providing a new evidence supporting the secondary endosymbiosis theory for the origin of the chloroplasts of chromophyta. Discussions are made on themorphology and localizations of plastid-dividing rings in relevance to the origins and evolution of plastids.