Abstract
BackgroundPlastids 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.ResultsLeaf 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.ConclusionOur 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.
Highlights
Plastids arose from a free-living cyanobacterial endosymbiont and multiply by binary division as do cyanobacteria
Consistent with the endosymbiotic origin of plastids, molecular genetic studies in A. thaliana have defined several nucleus-encoded homologs of cyanobacterial cell division proteins that function in plastid division in photosynthetic eukaryotes [7,8,9,10,11,12,13]
By characterizing plastid division mutants, we found that the cyanobacteria-derived chaperonin proteins ptCpn60α and ptCpn60β are required for proper plastid division in A. thaliana
Summary
Plastids arose from a free-living cyanobacterial endosymbiont and multiply by binary division as do cyanobacteria. Most of the genes once present in the endosymbiont have been lost or transferred to the host nuclear genome; those nuclear-encoded proteins used by the plastid are translated by the host and targeted back into the organelle to express their functions [1,2]. Consistent with this scenario, plastids are never synthesized de novo and they cannot multiply independently. Mutations in several other cyanobacteria-derived genes, such as Giant Chloroplast 1 [17,18] and Crumpled Leaf [19], cause defects in plastid division, their roles in the division process are still not known
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