ORDERLY FORMATION OF THE DOUBLE-RING STRUCTURES FOR PLASTID AND MITOCHONDRIAL DIVISION IN THE UNICELLULAR RED ALGA CYANIDIOSCHYZON MEROLAE

The formation of the plastid-dividing ring (PD ring) and mitochondrion -dividing ring (MD ring) was studied in a highly synchronous culture o f the unicellular red alga Cyanidioschyzon merolae. The timing and the order of formation of the MD and PD rings were determined by observin g organelles around the onset of their division, using transmission el ectron microscopy. In C. merolae, there is one chlloroplast and one mi tochondrion per cell, and the shape of the chloroplast changes sequent ially from acorn-like, to round, to trapezoidal, to peanut-shaped, in that order, during the early stage of chloroplast division. None of th e cells with acorn-shaped or round chloroplasts contained organelles w ith PD rings or MD rings, while all of the cells with peanut-shaped ch loroplasts contained organelles with both PD rings and MD rings. In ce lls with peanut-shaped chloroplasts, the PD and MD rings were double r ing structures, with an outer ring located on the cytoplasmic face of the outer membrane of the organelle, and an inner ring located in the matrix beneath the inner membrane. These results suggested that the do uble ring structures of the PD ring and the MD ring form when chloropl asts are trapezoidal in shape. Detailed three-dimensional observation of cells with trapezoidal chloroplasts revealed the following steps in the formation of the double ring structures of the PD and MD rings: ( i) the inner ring of the PD ring forms first, followed by the outer ri ng; (ii) then the MD ring forms and becomes visible; (iii) when the do uble ring structures of the two rings have formed, the microbody then moves from its remote location to the plane of division of the mitocho ndrion and contraction of the PD and MD rings commences. These steps w ere also confirmed by computer-aided three-dimensional reconstruction of the images from serial thin sections. This study reveals the order of formation of the double ring structures of the PD and MD rings, and the behavior of the microbody around the onset of division of plastid s and mitochondria. The results also provide the first evidence that t he inner PD ring is not a tension element formed by the contractile pr essure but a definite structure, independent of the outer ring.