Chloroplasts are descendants of cyanobacteria and divide by binary fission. Several components of the division apparatus have been identified in the past several years and we are beginning to appreciate the plastid division process at a mechanistic level. In this review, we attempt to summarize the most recent developments in the field and assemble these observations into a working model of plastid division in plants. Plastids: Characteristics, Origins and DistributionWithout chloroplasts, plant and animal life would not exist. Plastids have been studied most extensively with regard to their role in photosynthesis, and are essential to the plant itself, being required for a select of amino acid and lipid metabolism, assimilation of nitrogen and sulfur into organic compounds, signaling in response to environmental cues and biosynthesis of plant hormones (1)(2)(3)(4)(5). Despite all we have learned over the past century regarding this organelle, we still know relatively little about its replication-which, superficially, seems a simple process of binary fission. Because plastid division and biogenesis are an integral part of plant growth and development, understanding of the division process is paramount to our understanding of plant and organelle biology.Plastids arose from an endosymbiotic relationship between a primitive biciliate protozoan and an ancient cyanobacterium that began 1.2-1.5 billion years ago (Figure 1) (6,7). Initially, each partner derived some mutual benefit: the cyanobacterium occupied and exploited an untapped protective niche and the host extracted reduced carbon and other nutrients from its new tenant. Time, selective pressure and mutation bring us to the present day where the chloroplast is a prisoner in its own home, having lost more than 90% of the gene content present in its free-living ancestor. Most of the plastid division genes identified to date are cyanobacterial in origin and encode division proteins localized inside the organelle. These are directed to the plastid post-translationally by N-terminal transit peptides that are cleaved upon import. A few components of the division complex were invented by the host after endosymbiosis and function on the surface of the organelle in contact with the cytosol; the mechanisms targeting these proteins to the chloroplast surface are unknown. In land plants, all known plastid division genes reside in the nucleus-none are found in the plastid genome-although in at least one unicellular green alga two plastid division genes remain associated with the plastid genome (8).The terms chloroplast and plastid are often used interchangeably, but in vascular plants chloroplasts are actually a subset of plastids, each of which is specialized for a given set of functions within a specific cell type. All plastids are surrounded by two envelope membranes and are derived by division from a population of proplastids within the meristematic (stem) cells of the plant. Beyond land plants, red algae, green algae and diatoms also contain plastids. Even apicomplex...

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.

In traditional mutant screening approaches, genetic variants are tested for one or a small number of phenotypes. Once bona fide variants are identified, they are typically subjected to a limited number of secondary phenotypic screens. Although this approach is excellent at finding genes involved in specific biological processes, the lack of wide and systematic interrogation of phenotype limits the ability to detect broader syndromes and connections between genes and phenotypes. It could also prevent detection of the primary phenotype of a mutant. As part of a systems biology approach to understand plastid function, large numbers of Arabidopsis thaliana homozygous T-DNA lines are being screened with parallel morphological, physiological, and chemical phenotypic assays (www.plastid.msu.edu). To refine our approaches and validate the use of this high-throughput screening approach for understanding gene function and functional networks, approximately 100 wild-type plants and 13 known mutants representing a variety of phenotypes were analyzed by a broad range of assays including metabolite profiling, morphological analysis, and chlorophyll fluorescence kinetics. Data analysis using a variety of statistical approaches showed that such industrial approaches can reliably identify plant mutant phenotypes. More significantly, the study uncovered previously unreported phenotypes for these well-characterized mutants and unexpected associations between different physiological processes, demonstrating that this approach has strong advantages over traditional mutant screening approaches. Analysis of wild-type plants revealed hundreds of statistically robust phenotypic correlations, including metabolites that are not known to share direct biosynthetic origins, raising the possibility that these metabolic pathways have closer relationships than is commonly suspected.

SUMMARYThe Arabidopsis arc1 (accumulation and replication of chloroplasts 1) mutant has pale seedlings and smaller, more numerous chloroplasts than the wild type. Previous work has suggested that arc1 affects the timing of chloroplast division but does not function directly in the division process. We isolated ARC1 by map-based cloning and discovered it encodes FtsHi1 (At4g23940), one of several FtsHi proteins in Arabidopsis. These poorly studied proteins resemble FtsH metalloproteases important for organelle biogenesis and protein quality control but are presumed to be proteolytically inactive. FtsHi1 bears a predicted chloroplast transit peptide and localizes to the chloroplast envelope membrane. Phenotypic studies showed that arc1 (hereafter ftsHi1-1), which bears a missense mutation, is a weak allele of FtsHi1 that disrupts thylakoid development and reduces de-etiolation efficiency in seedlings, suggesting that FtsHi1 is important for chloroplast biogenesis. Consistent with this finding, transgenic plants suppressed for accumulation of an FtsHi1 fusion protein were often variegated. A strong T-DNA insertion allele, ftsHi1-2, caused embryo-lethality, indicating that FtsHi1 is an essential gene product. A wild-type FtsHi1 transgene rescued both the chloroplast division and pale phenotypes of ftsHi1-1 and the embryo-lethal phenotype of ftsHi1-2. FtsHi1 overexpression produced a subtle increase in chloroplast size and decrease in chloroplast number in wild-type plants while suppression led to increased numbers of small chloroplasts, providing new evidence that FtsHi1 negatively influences chloroplast division. Taken together, our analyses reveal that FtsHi1 functions in an essential, envelope-associated process that may couple plastid development with division.Keywords: FtsHi, plastid division, embryogenesis, chloroplast biogenesis, plastid development, Arabidopsis thaliana. INTRODUCTIONThe Arabidopsis arc (accumulation and replication of chloroplasts) mutants exhibit various defects in chloroplast size and number in leaf mesophyll cells and define a suite of 12 nuclear genes, ARC1-ARC12, that influence chloroplast division and expansion (Pyke and Leech, 1992, 1994;Marrison et al., 1999;Pyke, 1999). Most of the arc mutants have fewer and larger chloroplasts per cell than the wild type, a hallmark indicator of impaired chloroplast division, and several ARC loci with mutations causing such phenotypes have been shown to encode components of the chloroplast division machinery (Gao et al., 2003;Vitha et al., 2003;Fujiwara et al., 2004;Shimada et al., 2004;Glynn et al., 2007;Yoder et al., 2007). The arc1 mutant is distinct in having smaller and more numerous chloroplasts per cell than the wild type, suggesting that plastid division is accelerated rather than inhibited in this mutant. arc1 seedlings are also pale, indicating a possible role for ARC1 in chloroplast development (Pyke and Leech, 1992, 1994) (Figure 1a). Analysis of double mutants has shown that ARC1 functions in a separate process from other ARC genes (...