Developmentally regulated association of plastid division protein FtsZ1 with thylakoid membranes in Arabidopsis thaliana

Characterization of thylakoid division using chloroplast dividing mutants in Arabidopsis

Chloroplasts are double membrane bound organelles that are found in plants and algae. Their division requires a number of proteins to assemble into rings along the center of the organelle and to constrict in synchrony. Chloroplasts possess a third membrane system, the thylakoids, which house the majority of proteins responsible for the light-dependent reactions. The mechanism that allows chloroplasts to sort out and separate the intricate thylakoid membrane structures during organelle division remain unknown. By characterizing the sizes of thylakoids found in a number of different chloroplast division mutants in Arabidopsis, we show that thylakoids do not divide independently of the chloroplast division cycle. More specifically, we show that thylakoid division requires the formation of both the inner and the outer contractile rings of the chloroplast.

Differential impacts of FtsZ proteins on plastid division in the shoot apex of Arabidopsis

FtsZ proteins of the FtsZ1 and FtsZ2 families play important roles in the initiation and progression of plastid division in plants and green algae. Arabidopsis possesses a single FTSZ1 member and two FTSZ2 members, FTSZ2-1 and FTSZ2-2. The contribution of these to chloroplast division and partitioning has been mostly investigated in leaf mesophyll tissues. Here, we assessed the involvement of the three FtsZs in plastid division at earlier stages of chloroplast differentiation. To this end, we studied the effect of the absence of specific FtsZ proteins on plastids in the vegetative shoot apex, where the proplastid-to-chloroplast transition takes place. We found that the relative contribution of the two major leaf FtsZ isoforms, FtsZ1 and FtsZ2-1, to the division process varies with cell lineage and position within the shoot apex. While FtsZ2-1 dominates division in the L1 and L3 layers of the shoot apical meristem (SAM), in the L2 layer, FtsZ1 and FtsZ2-1 contribute equally toward the process. Depletion of the third isoform, FtsZ2-2, generally resulted in stronger effects in the shoot apex than those observed in mature leaves. The implications of these findings, along with additional observations made in this work, to our understanding of the mechanisms and regulation of plastid proliferation in the shoot apex are discussed.

Adjustments of embryonic photosynthetic activity modulate seed fitness in Arabidopsis thaliana

In this work, we dissect the physiological role of the transient photosynthetic stage observed in developing seeds of Arabidopsis thaliana. By combining biochemical and biophysical approaches, we demonstrate that despite similar features of the photosynthetic apparatus, light absorption, chloroplast morphology and electron transport are modified in green developing seeds, as a possible response to the peculiar light environment experienced by them as a result of sunlight filtration by the pericarp. In particular, enhanced exposure to far-red light, which mainly excites photosystem I, largely enhances cyclic electron flow around this complex at the expenses of oxygen evolution. Using pharmacological, genetic and metabolic analyses, we show that both linear and cyclic electron flows are important during seed formation for proper germination timing. Linear flow provides specific metabolites related to oxygen and water stress responses. Cyclic electron flow possibly adjusts the ATP to NADPH ratio to cope with the specific energy demand of developing seeds. By providing a comprehensive scenario of the characteristics, function and consequences of embryonic photosynthesis on seed vigour, our data provide a rationale for the transient building up of a photosynthetic machinery in seeds.

The puzzle of chloroplast vesicle transport - involvement of GTPases

In the cytosol of plant cells vesicle transport occurs via secretory pathways among the endoplasmic reticulum network, Golgi bodies, secretory granules, endosome, and plasma membrane. Three systems transfer lipids, proteins and other important molecules through aqueous spaces to membrane-enclosed compartments, via vesicles that bud from donor membranes, being coated and uncoated before tethered and fused with acceptor membranes. In addition, molecular, biochemical and ultrastructural evidence indicates presence of a vesicle transport system in chloroplasts. Little is known about the protein components of this system. However, as chloroplasts harbor the photosynthetic apparatus that ultimately supports most organisms on the planet, close attention to their pathways is warranted. This may also reveal novel diversification and/or distinct solutions to the problems posed by the targeted intra-cellular trafficking of important molecules. To date two homologs to well-known yeast cytosolic vesicle transport proteins, CPSAR1 and CPRabA5e (CP, chloroplast localized), have been shown to have roles in chloroplast vesicle transport, both being GTPases. Bioinformatic data indicate that several homologs of cytosolic vesicle transport system components are putatively chloroplast-localized and in addition other proteins have been implicated to participate in chloroplast vesicle transport, including vesicle-inducing protein in plastids 1, thylakoid formation 1, snowy cotyledon 2/cotyledon chloroplast biogenesis factor, curvature thylakoid 1 proteins, and a dynamin like GTPase FZO-like protein. Several putative potential cargo proteins have also been identified, including building blocks of the photosynthetic apparatus. Here we discuss details of the largely unknown putative chloroplast vesicle transport system, focusing on GTPase-related components.

The pleiotropic effects of ftn2 and ftn6 mutations in cyanobacterium Synechococcus sp. PCC 7942: an ultrastructural study

Two cell division mutants (Ftn2 and Ftn6) of the cyanobacterium Synechococcus sp. PCC 7942 were studied using scanning electron microscopy and transmission electron microscopy methods. This included negative staining and ultrathin section analysis. Different morphological and ultrastructural features of mutant cells were identified. Ftn2 and Ftn6 mutants exhibited particularly elongated cells characterized by significantly changed shape in comparison with the wild type. There was irregular bending, curving, spiralization, and bulges as well as cell branching. Elongated mutant cells were able to initiate cytokinesis simultaneously in several division sites which were localized irregularly along the cell. Damaged rigidity of the cell wall was typical of many cells for both mutants. Thylakoids of mutants showed modified arrangement and ultrastructural organization. Carboxysome-like structures without a shell and/or without accurate polyhedral packing protein particles were often detected in the mutants. However, in the case of Ftn2 and Ftn6, the average number of carboxysomes per section was less than in the wild type by a factor of 4 and 2, respectively. These multiple morphological and ultrastructural changes in mutant cells evinced pleiotropic responses which were induced by mutations in cell division genes ftn2 and ftn6. Ultrastructural abnormalities of Ftn2 and Ftn6 mutants were consistent with differences in their proteomes. These results could support the significance of FTN2 and FTN6 proteins for both cyanobacterial cell division and cellular physiology.

Emerging facets of plastid division regulation

Plastids are complex organelles that are integrated into the plant host cell where they differentiate and divide in tune with plant differentiation and development. In line with their prokaryotic origin, plastid division involves both evolutionary conserved proteins and proteins of eukaryotic origin where the host has acquired control over the process. The plastid division apparatus is spatially separated between the stromal and the cytosolic space but where clear coordination mechanisms exist between the two machineries. Our knowledge of the plastid division process has increased dramatically during the past decade and recent findings have not only shed light on plastid division enzymology and the formation of plastid division complexes but also on the integration of the division process into a multicellular context. This review summarises our current knowledge of plastid division with an emphasis on biochemical features, the functional assembly of protein complexes and regulatory features of the overall process.

FtsHi1/ARC1 is an essential gene in Arabidopsis that links chloroplast biogenesis and division

The 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.

Expression of Brassica oleracea FtsZ1-1 and MinD alters chloroplast division in Nicotiana tabacum generating macro- and mini-chloroplasts

FtsZ1-1 and MinD plastid division-related genes were identified and cloned from Brassica oleracea var. botrytis. Transgenic tobacco plants expressing BoFtsZ1-1 or BoMinD exhibited cells with either fewer but abnormally large chloroplasts or more but smaller chloroplasts relative to wild-type tobacco plants. An abnormal chloroplast phenotype in guard cells was found in BoMinD transgenic tobacco plants but not in BoFtsZ1-1 transgenic tobacco plants. Transgenic tobacco plants bearing the macro-chloroplast phenotype had 10 to 20-fold increased levels of total FtsZ1-1 or MinD, whilst the transgenic tobacco plants bearing the mini-chloroplast phenotype had lower increased FtsZ1-1 or absence of detectable MinD. We also described for the first time, plastid transformation of macro-chloroplast bearing tobacco shoots with a gene cassette allowing for expression of green fluorescent protein (GFP). Homoplasmic plastid transformants from normal chloroplast and macro-chloroplast tobacco plants expressing GFP were obtained. Both types of transformants accumulated GFP at ~6% of total soluble protein, thus indicating that cells containing macro-chloroplasts can regenerate shoots in tissue culture and can stably integrate and express a foreign gene to similar levels as plant cells containing a normal chloroplast size and number.

Control of starch granule numbers in Arabidopsis chloroplasts

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves. Lack of quantitative information on the extent of genetic, temporal, developmental, and environmental variation in granule numbers is an important limitation in understanding control of starch degradation and the mechanism of granule initiation. Two methods were developed for reliable estimation of numbers of granules per chloroplast. First, direct measurements were made on large series of consecutive sections of mesophyll tissue obtained by focused ion beam-scanning electron microscopy. Second, average numbers were calculated from the starch contents of leaves and chloroplasts and estimates of granule mass based on granule dimensions. Examination of wild-type plants and accumulation and regulation of chloroplast (arc) mutants with few, large chloroplasts provided the following new insights. There is wide variation in chloroplast volumes in cells of wild-type leaves. Granule numbers per chloroplast are correlated with chloroplast volume, i.e. large chloroplasts have more granules than small chloroplasts. Mature leaves of wild-type plants and arc mutants have approximately the same number of granules per unit volume of stroma, regardless of the size and number of chloroplasts per cell. Granule numbers per unit volume of stroma are also relatively constant in immature leaves but are greater than in mature leaves. Granule initiation occurs as chloroplasts divide in immature leaves, but relatively little initiation occurs in mature leaves. Changes in leaf starch content over the diurnal cycle are largely brought about by changes in the volume of a fixed number of granules.

Cytosolic HSP90 cochaperones HOP and FKBP interact with freshly synthesized chloroplast preproteins of Arabidopsis

Most chloroplast and mitochondrial proteins are synthesized in the cytosol of the plant cell and have to be imported into the organelles post-translationally. Molecular chaperones play an important role in preventing protein aggregation of freshly translated preproteins and assist in maintaining the preproteins in an import competent state. Preproteins can associate with HSP70, HSP90, and 14-3-3 proteins in the cytosol. In this study, we analyzed a large set of wheat germ-translated chloroplast preproteins with respect to their chaperone binding. Our results demonstrate that the formation of distinct 14-3-3 or HSP90 containing preprotein complexes is a common feature in post-translational protein transport in addition to preproteins that seem to interact solely with HSP70. We were able to identify a diverse and extensive class of preproteins as HSP90 substrates, thus providing a tool for the investigation of HSP90 client protein association. The analyses of chimeric HSP90 and 14-3-3 binding preproteins with exchanged transit peptides indicate an involvement of both the transit peptide and the mature part of the proteins, in HSP90 binding. We identified two partner components of the HSP90 cycle, which were present in the preprotein containing high-molecular-weight complexes, the HSP70/HSP90 organizing protein HOP, as well as the immunophilin FKBP73. The results establish chloroplast preproteins as a general class of HSP90 client proteins in plants using HOP and FKBP as novel cochaperones.

Two mechanosensitive channel homologs influence division ring placement in Arabidopsis chloroplasts

Chloroplasts must divide repeatedly to maintain their population during plant growth and development. A number of proteins required for chloroplast division have been identified, and the functional relationships between them are beginning to be elucidated. In both chloroplasts and bacteria, the future site of division is specified by placement of the Filamentous temperature sensitive Z (FtsZ) ring, and the Min system serves to restrict FtsZ ring formation to mid-chloroplast or mid-cell. How the Min system is regulated in response to environmental and developmental factors is largely unstudied. Here, we investigated the role in chloroplast division played by two Arabidopsis thaliana homologs of the bacterial mechanosensitive (MS) channel MscS: MscS-Like 2 (MSL2) and MSL3. Immunofluorescence microscopy and live imaging approaches demonstrated that msl2 msl3 double mutants have enlarged chloroplasts containing multiple FtsZ rings. Genetic analyses indicate that MSL2, MSL3, and components of the Min system function in the same pathway to regulate chloroplast size and FtsZ ring formation. In addition, an Escherichia coli strain lacking MS channels also showed aberrant FtsZ ring assembly. These results establish MS channels as components of the chloroplast division machinery and suggest that their role is evolutionarily conserved.

Multiple FtsZ2 isoforms involved in chloroplast division and biogenesis are developmentally associated with thylakoid membranes in Arabidopsis

Seed plants and algae have two distinct FtsZ protein families, FtsZ1 and FtsZ2, involved in plastid division. Distinctively, seed plants and mosses contain two FtsZ2 family members (FtsZ2-1 and FtsZ2-2) thus raising the question of the role of these FtsZ2 paralogs in plants. We show that both FtsZ2 paralogs, in addition to being present in the stroma, are associated with the thylakoid membranes and that association is developmentally regulated. We also show that several FtsZ2-1 isoforms are present with distinct intra-plastidial localization. Mutant analyses show that FtsZ2-1 is essential for chloroplast division and that FtsZ2-2 plays a specific role in chloroplast morphology and internal organisation in addition to participating in chloroplast partition.

Characteristics of the tomato chromoplast revealed by proteomic analysis

Chromoplasts are non-photosynthetic specialized plastids that are important in ripening tomato fruit (Solanum lycopersicum) since, among other functions, they are the site of accumulation of coloured compounds. Analysis of the proteome of red fruit chromoplasts revealed the presence of 988 proteins corresponding to 802 Arabidopsis unigenes, among which 209 had not been listed so far in plastidial databanks. These data revealed several features of the chromoplast. Proteins of lipid metabolism and trafficking were well represented, including all the proteins of the lipoxygenase pathway required for the synthesis of lipid-derived aroma volatiles. Proteins involved in starch synthesis co-existed with several starch-degrading proteins and starch excess proteins. Chromoplasts lacked proteins of the chlorophyll biosynthesis branch and contained proteins involved in chlorophyll degradation. None of the proteins involved in the thylakoid transport machinery were discovered. Surprisingly, chromoplasts contain the entire set of Calvin cycle proteins including Rubisco, as well as the oxidative pentose phosphate pathway (OxPPP). The present proteomic analysis, combined with available physiological data, provides new insights into the metabolic characteristics of the tomato chromoplast and enriches our knowledge of non-photosynthetic plastids.

GTP-dependent heteropolymer formation and bundling of chloroplast FtsZ1 and FtsZ2

Bacteria and chloroplasts require the ring-forming cytoskeletal protein FtsZ for division. Although bacteria accomplish division with a single FtsZ, plant chloroplasts require two FtsZ types for division, FtsZ1 and FtsZ2. These proteins colocalize to a mid-plastid Z ring, but their biochemical relationship is poorly understood. We investigated the in vitro behavior of recombinant FtsZ1 and FtsZ2 separately and together. Both proteins bind and hydrolyze GTP, although GTPase activities are low compared with the activity of Escherichia coli FtsZ. Each protein undergoes GTP-dependent assembly into thin protofilaments in the presence of calcium as a stabilizing agent, similar to bacterial FtsZ. In contrast, when mixed without calcium, FtsZ1 and FtsZ2 exhibit slightly elevated GTPase activity and coassembly into extensively bundled protofilaments. Coassembly is enhanced by FtsZ1, suggesting that it promotes lateral interactions between protofilaments. Experiments with GTPase-deficient mutants reveal that FtsZ1 and FtsZ2 form heteropolymers. Maximum coassembly occurs in reactions containing equimolar FtsZ1 and FtsZ2, but significant coassembly occurs at other stoichiometries. The FtsZ1:FtsZ2 ratio in coassembled structures mirrors their input ratio, suggesting plasticity in protofilament and/or bundle composition. This behavior contrasts with that of alpha- and beta-tubulin and the bacterial tubulin-like proteins BtubA and BtubB, which coassemble in a strict 1:1 stoichiometry. Our findings raise the possibility that plasticity in FtsZ filament composition and heteropolymerization-induced bundling could have been a driving force for the coevolution of FtsZ1 and FtsZ2 in the green lineage, perhaps arising from an enhanced capacity for the regulation of Z ring composition and activity in vivo.

Arabidopsis FtsZ2-1 and FtsZ2-2 are functionally redundant, but FtsZ-based plastid division is not essential for chloroplast partitioning or plant growth and development

FtsZ1 and FtsZ2 are phylogenetically distinct families of FtsZ in plants that co-localize to mid-plastid rings and facilitate division of chloroplasts. In plants, altered levels of either FtsZ1 or FtsZ2 cause dose-dependent defects in chloroplast division; thus, studies on the functional relationship between FtsZ genes require careful manipulation of FtsZ levels in vivo. To define the functional relationship between the two FtsZ2 genes in Arabidopsis thaliana, FtsZ2-1 and FtsZ2-2, we expressed FtsZ2-1 in an ftsZ2-2 null mutant, and vice versa, and determined whether the chloroplast division defects were rescued in plants expressing different total levels of FtsZ2. Full rescue was observed when either the FtsZ2-1 or FtsZ2-2 level approximated total FtsZ2 levels in wild-type (WT). Additionally, FtsZ2-2 interacts with ARC6, as shown previously for FtsZ2-1. These data indicate that FtsZ2-1 and FtsZ2-2 are functionally redundant for chloroplast division in Arabidopsis. To rigorously validate the requirement of each FtsZ family for chloroplast division, we replaced FtsZ1 with FtsZ2 in vivo, and vice versa, while maintaining the FtsZ level in the transgenic plants equal to that of the total level in WT. Chloroplast division defects were not rescued, demonstrating conclusively that FtsZ1 and FtsZ2 are non-redundant for maintenance of WT chloroplast numbers. Finally, we generated ftsZ triple null mutants and show that plants completely devoid of FtsZ protein are viable and fertile. As plastids are presumably essential organelles, these findings suggest that an FtsZ-independent mode of plastid partitioning may occur in higher plants.

Exploring the mechanism of Physcomitrella patens desiccation tolerance through a proteomic strategy

The moss Physcomitrella patens has been shown to tolerate abiotic stresses, including salinity, cold, and desiccation. To better understand this plant's mechanism of desiccation tolerance, we have applied cellular and proteomic analyses. Gametophores were desiccated over 1 month to 10% of their original fresh weight. We report that during the course of dehydration, several related processes are set in motion: plasmolysis, chloroplast remodeling, and microtubule depolymerization. Despite the severe desiccation, the membrane system maintains integrity. Through two-dimensional gel electrophoresis and image analysis, we identified 71 proteins as desiccation responsive. Following identification and functional categorization, we found that a majority of the desiccation-responsive proteins were involved in metabolism, cytoskeleton, defense, and signaling. Degradation of cytoskeletal proteins might result in cytoskeletal disassembly and consequent changes in the cell structure. Late embryogenesis abundant proteins and reactive oxygen species-scavenging enzymes are both prominently induced, and they might help to diminish the damage brought by desiccation.

Plastid division: across time and space

Plastid division is executed by the coordinated action of at least two molecular machineries--an internal machinery situated on the stromal side of the inner envelope membrane that was contributed by the cyanobacterial endosymbiont from which plastids evolved, and an external machinery situated on the cytosolic side of the outer envelope membrane that was contributed by the host. Here we review progress in defining the components of the plastid division complex and understanding the mechanisms of envelope constriction and division-site placement in plants. We also highlight recent work identifying the first molecular linkage between the internal and external division machineries, shedding light on how their mid-plastid positioning is coordinated across the envelope membranes. Little is known about the mechanisms that regulate plastid division in plant cells, but recent studies have begun to hint at potential mechanisms.

Arabidopsis ARC6 coordinates the division machineries of the inner and outer chloroplast membranes through interaction with PDV2 in the intermembrane space

Chloroplasts arose from a free-living cyanobacterial endosymbiont and divide by binary fission. Division involves the assembly and constriction of the endosymbiont-derived, tubulin-like FtsZ ring on the stromal surface of the inner envelope membrane and the host-derived, dynamin-like ARC5 ring on the cytosolic surface of the outer envelope membrane. Despite the identification of many proteins required for plastid division, the factors coordinating the internal and external division machineries are unknown. Here, we provide evidence that this coordination is mediated in Arabidopsis thaliana by an interaction between ARC6, an FtsZ assembly factor spanning the inner envelope membrane, and PDV2, an ARC5 recruitment factor spanning the outer envelope membrane. ARC6 and PDV2 interact via their C-terminal domains in the intermembrane space, consistent with their in vivo topologies. ARC6 acts upstream of PDV2 to localize PDV2 (and hence ARC5) to the division site. We present a model whereby ARC6 relays information on stromal FtsZ ring positioning through PDV2 to the chloroplast surface to specify the site of ARC5 recruitment. Because orthologs of ARC6 occur in land plants, green algae, and cyanobacteria but PDV2 occurs only in land plants, the connection between ARC6 and PDV2 represents the evolution of a plant-specific adaptation to coordinate the assembly and activity of the endosymbiont- and host-derived plastid division components.

In vivo quantitative relationship between plastid division proteins FtsZ1 and FtsZ2 and identification of ARC6 and ARC3 in a native FtsZ complex

FtsZ1 and FtsZ2 are phylogenetically distinct homologues of the tubulin-like bacterial cell division protein FtsZ that play major roles in the initiation and progression of plastid division in plant cells. Both proteins are components of a mid-plastid ring, the Z-ring, which functions as a contractile ring on the stromal surface of the chloroplast IEM (inner envelope membrane). FtsZ1 and FtsZ2 have been shown to interact, but their in vivo biochemical properties are largely unknown. To gain insight into the in vivo biochemical relationship between FtsZ1 and FtsZ2, in the present study we investigated their molecular levels in wild-type Arabidopsis thaliana plants and endogenous interactions in Arabidopsis and pea. Quantitative immunoblotting and morphometric analysis showed that the average total FtsZ concentration in chloroplasts of 3-week-old Arabidopsis plants is comparable with that in Escherichia coli. FtsZ levels declined as plants matured, but the molar ratio between FtsZ1 and FtsZ2 remained constant at approx. 1:2, suggesting that this stoichiometry is regulated and functionally important. Density-gradient centrifugation, native gel electrophoresis, gel filtration and co-immunoprecipitation experiments showed that a portion of the FtsZ1 and FtsZ2 in Arabidopsis and pea chloroplasts is stably associated in a complex of approximately 200-245 kDa. This complex also contains the FtsZ2-interacting protein ARC6 (accumulation and replicatioin of chloroplasts 6), an IEM protein, and analysis of density-gradient fractions suggests the presence of the FtsZ1-interacting protein ARC3. Based on the mid-plastid localization of ARC6 and ARC3 and their postulated roles in promoting and inhibiting chloroplast FtsZ polymer formation respectively, we hypothesize that the FtsZ1-FtsZ2-ARC3-ARC6 complex represents an unpolymerized IEM-associated pool of FtsZ that contributes to the dynamic regulation of Z-ring assembly and remodelling at the plastid division site in vivo.