Coordination of Cell and Chloroplast Division Cycles

S. quadricauda has also been extensively used to study the relationship between cell and chloroplast division. The division of a cell and its organelles is coordinated; in plants there are two types of organelles, the division of which has to be coordinated: chloroplasts and mitochondria. Algae with a single chloroplast provide a nice model to uncover the basis of this coordination. Very early in the study of green algae it was observed that there is a very tight correlation between the division of chloroplasts and of cells; the chloroplast divides first, and is followed by cell division. In S. quadricauda the numbers of chloroplast and nuclear replications/divisions are also usually equal. However, a detailed analysis has shown that chloro-plast DNA replication and the number of nucleoids (chloroplast nuclei) depends on growth rate. Depending on the growth rate there could be both increase or decrease in the number of nucleoids per daughter cell compared to the mother cell (Zachleder and Cepak 1987a,b; Zachleder et al. 1995). More importantly, the processes of chloroplast DNA replication and nucleoid division can be uncoupled from nuclear DNA replication and division by application of 5-fluorodeoxyuridine (Zachleder et al. 1996), an inhibitor of thymidylate synthase that in S. quadricauda specifically inhibits nuclear DNA replication and does not affect the chloroplast DNA replication (Zachleder 1994). In the presence of 5-fluorodeoxyuridine replication of nuclear DNA is stopped immediately, while chloroplast DNA replicates, the nucleoids divide, and ultimately the chloroplasts are also able to divide in a giant cell with uninuclear genomic content of DNA. This implies that there is no checkpoint control that interconnects chloroplast and nuclear DNA replication/division. The same phenomenon was also observed in C. rein-hardtii (Harper and John 1986) and C. merolae (Itoh et al. 1996). Therefore, it seems that despite a strikingly tight correlation between chloroplast and nuclear division cycles there is no regulatory checkpoint interconnecting both processes.

I propose the following hypothetical model to explain the correlation between chloroplast and nuclear cycles. In phototrophically growing green algae photosynthesis leads primarily to the growth of chloroplast which may trigger chloroplast DNA replication/division. Since chloroplast occupies the majority of the cell volume, its growth directly affects the cell size which in turn leads to attaining of the commitment point and to nuclear DNA replication and division.

S. quadricauda has proven to be a useful model for unraveling the relationship between different events of the multiple fission cell cycle and also between cell and chloroplast division cycles. Unfortunately, no efforts have been made so far to establish genetic tools and/or stable DNA transformation techniques or to sequence its nuclear genome. Since C. reinhardtii and S. quadricauda are close relatives, one could assume that methods used in C. reinhardtii should work for S. quadricauda. S. quadricauda is definitely a potent biochemical model for cell cycle studies. Since its cell cycle organization is slightly different from that of C. reinhardtii, it would be interesting to see how the cell cycle is regulated genetically.

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