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Background  Plastids arose from a free-living cyanobacterial endosymbiont and multiply by binary division as do cyanobacteria. Plastid division involves nucleus-encoded homologs of cyanobacterial division proteins such as FtsZ, MinD, MinE, and ARC6. However, homologs of many other cyanobacterial division genes are missing in plant genomes and proteins of host eukaryotic origin, such as a dynamin-related protein, PDV1 and PDV2 are involved in the division process. Recent identification of plastid division proteins has started to elucidate the similarities and differences between plastid division and cyanobacterial cell division. To further identify new proteins that are required for plastid division, we characterized previously and newly isolated plastid division mutants of  Arabidopsis thaliana .    Results  Leaf cells of two mutants,  br04  and  arc2 , contain fewer, larger chloroplasts than those of wild type. We found that  ARC2  and  BR04  are identical to nuclear genes encoding the plastid chaperonin 60α (ptCpn60α) and chaperonin 60β (ptCpn60β) proteins, respectively. In both mutants, plastid division FtsZ ring formation was partially perturbed though the level of FtsZ2-1 protein in plastids of  ptcpn60β  mutants was similar to that in wild type. Phylogenetic analyses showed that both ptCpn60 proteins are derived from ancestral cyanobacterial proteins. The  A. thaliana  genome encodes two members of ptCpn60α family and four members of ptCpn60β family respectively. We found that a null mutation in  ptCpn60α  abolished greening of plastids and resulted in an albino phenotype while a weaker mutation impairs plastid division and reduced chlorophyll levels. The functions of at least two ptCpn60β proteins are redundant and the appearance of chloroplast division defects is dependent on the number of mutant alleles.    Conclusion  Our results suggest that both ptCpn60α and ptCpn60β are required for the formation of a normal plastid division apparatus, as the prokaryotic counterparts are required for assembly of the cell division apparatus. Since moderate reduction of ptCpn60 levels impaired normal FtsZ ring formation but not import of FtsZ into plastids, it is suggested that the proper levels of ptCpn60 are required for folding of stromal plastid division proteins and/or regulation of FtsZ polymer dynamics.

Plastid chaperonin proteins Cpn60α and Cpn60β are required for plastid division in Arabidopsis thaliana