2018 年 30 巻 1 号 p. 83-89
Chloroplasts arose from a cyanobacterial endosymbiont. Most algae with a single or a few chloroplasts per cell synchronize chloroplast division with the host cell cycle. This synchronization is regulated via interactions between the cell and chloroplast. The onset of chloroplast division is regulated by S-phase-specific expression of the nuclear-encoded chloroplast division genes; however, in the unicellular alga Nannochloris bacillaris, one of the plastid division genes, FtsZ2, is expressed throughout the cell cycle. Plastid-dividing (PD) machinery usually forms a ring complex only during the division phase, although the number of the FtsZ rings, and not of PD rings, is increased in N. bacillaris cells under phosphate-enriched mixotrophic condition where the chloroplast DNA is excessively replicated. Therefore, among components of the PD machinery, only FtsZ ring formation is promoted by chloroplast DNA synthesis under certain conditions. During chloroplast division, the onset of chloroplast constriction allows the progression of cell cycle to metaphase. The blockage of PD machinery formation before its assembly arrests the cell cycle in prophase in the unicellular alga, Cyanidioschyzon merolae; however, once DRP5B is recruited to the chloroplast division site, the cell cycle progresses despite the failure of chloroplast fission. A similar phenomenon has been observed in the glaucophyte Cyanophora paradoxa suggesting that the mechanism of the chloroplast division checkpoint is established early in chloroplast acquisition.
