|Budget Amount *help
¥3,300,000 (Direct Cost : ¥3,300,000)
Fiscal Year 1999 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1998 : ¥2,800,000 (Direct Cost : ¥2,800,000)
Ultrastructures of plastid-dividing rings were studied using a liverwort Marchantia polymorpha and green algae of Klebsormidium flaccidum and Closterium ehrenberghii. The samples were fixed by quick freezing at the temperature of liquid helium followed by procedure of freeze-substitution or freeze-fracture. For germinating sporelings of M. polymorpha and, in particular, cells of K. flaccidum, cellular structures were preserved very well. A doublet of inner and outer plastid-dividing rings was clearly detected around the isthmus of the dividing chloroplasts in both plants. In K. flaccidum 'unclosed' plastid dividing-ring doublet was revealed in chloroplasts of an early stage of division. The side of the isthmus us where the unclosed ring is girdled is heavily constricted but the side where the ring has not reached is scarcely constricted. During the early stage of the chloroplast division the microbody changes in shape to have several protuberances, one of which enwinds the isthmus of the dividing chloroplast and associates the outer plastid-dividing ring. These findings suggest, that the unclosed plastid-dividing ring may exert mechanical force to constrict the chloroplast before its forming a closed ring, and that the microbody may be involved in formation of the plastid dividing-ring doublet.
Methods of freeze-fracture and replica were conducted to observe plastid-dividing rings. Although it was possible to prepare freeze replicas with good resolution of cells of K. flaccidum, however, unfortunately the replicas of the plastid-dividing rings were not obtained so far. Seemingly, required are cell samples where highly synchronous divisions of the chloroplasts take place. Examination as to whether the plastid-dividing rings contain actin using immunoelectron microscopy following quick freezing and freeze-substitution obtained no evidence for localization of actin at the plastid-dividing rings.