The plastid-dividing machinery: formation, constriction and fission

Increasing amyloplast size in wheat endosperm through mutation of PARC6 affects starch granule morphology

The determination of starch granule morphology in plants is poorly understood. The amyloplasts of wheat endosperm contain large discoid A-type granules and small spherical B-type granules. To study the influence of amyloplast structure on these distinct morphological types, we isolated a mutant in durum wheat (Triticum turgidum) defective in the plastid division protein PARC6, which had giant plastids in both leaves and endosperm. Endosperm amyloplasts of the mutant contained more A- and B-type granules than those of the wild-type. The mutant had increased A- and B-type granule size in mature grains, and its A-type granules had a highly aberrant, lobed surface. This morphological defect was already evident at early stages of grain development and occurred without alterations in polymer structure and composition. Plant growth and grain size, number and starch content were not affected in the mutants despite the large plastid size. Interestingly, mutation of the PARC6 paralog, ARC6, did not increase plastid or starch granule size. We suggest TtPARC6 can complement disrupted TtARC6 function by interacting with PDV2, the outer plastid envelope protein that typically interacts with ARC6 to promote plastid division. We therefore reveal an important role of amyloplast structure in starch granule morphogenesis in wheat.

Three Diverse Granule Preparation Methods for Proteomic Analysis of Mature Rice (Oryza sativa L.) Starch Grain

Starch is the primary form of reserve carbohydrate storage in plants. Rice (Oryza sativa L.) is a monocot whose reserve starch is organized into compounded structures within the amyloplast, rather than a simple starch grain (SG). The mechanism governing the assembly of the compound SG from polyhedral granules in apposition, however, remains unknown. To further characterize the proteome associated with these compounded structures, three distinct methods of starch granule preparation (dispersion, microsieve, and flotation) were performed. Phase separation of peptides (aqueous trypsin-shaving and isopropanol solubilization of residual peptides) isolated starch granule-associated proteins (SGAPs) from the distal proteome of the amyloplast and the proximal ‘amylome’ (the amyloplastic proteome), respectively. The term ‘distal proteome’ refers to SGAPs loosely tethered to the amyloplast, ones that can be rapidly proteolyzed, while proximal SGAPs are those found closer to the remnant amyloplast membrane fragments, perhaps embedded therein—ones that need isopropanol solvent to be removed from the mature organelle surface. These two rice starch-associated peptide samples were analyzed using nano-liquid chromatography–tandem mass spectrometry (Nano-HPLC-MS/MS). Known and novel proteins, as well as septum-like structure (SLS) proteins, in the mature rice SG were found. Data mining and gene ontology software were used to categorize these putative plastoskeletal components as a variety of structural elements, including actins, tubulins, tubulin-like proteins, and cementitious elements such as reticulata related-like (RER) proteins, tegument proteins, and lectins. Delineating the plastoskeletal proteome begins by understanding how each starch granule isolation procedure affects observed cytoplasmic and plastid proteins. The three methods described herein show how the technique used to isolate SGs differentially impacts the subsequent proteomic analysis and results obtained. It can thus be concluded that future investigations must make judicious decisions regarding the methodology used in extracting proteomic information from the compound starch granules being assessed, since different methods are shown to yield contrasting results herein. Data are available via ProteomeXchange with identifier PXD032314.

Reassessing the evolution of the 1-deoxy-D-xylulose 5-phosphate synthase family suggests a possible novel function for the DXS class 3 proteins

The methylerythritol 4-phosphate (MEP) pathway is of paramount importance for generating plastidial isoprenoids. The first enzyme of the MEP pathway, 1-deoxy-D-xylulose-5-phosphate synthase (DXS), catalyzes a flux-controlling step. In plants the DXS gene family is composed of three distinct classes with non-redundant functions. Although the DXS1 and DXS2 subfamilies have been well characterized, the DXS3 subfamily has been considerably understudied. Here, we carried out in silico and functional analyses to better understand the DXS3 class. Our phylogenetic analysis showed high variation in copy number among the different DXS classes, with the apparent absence of DXS1 class in some species. We found that DXS3 subfamily emerged later than DXS1 and DXS2 and it is under less intense purifying selection. Furthermore, in the DXS3 subfamily critical amino acids positions in the thiamine pyrophosphate binding pocket are not conserved. We demonstrated that the DXS3 proteins from Arabidopsis, Maize, and Rice lack functional DXS activity. Moreover, the Arabidopsis DXS3 protein displayed distinctive sub-organellar chloroplast localization not observed in any DXS1 or DXS2 proteins. Co-expression analysis of the DXS3 from Arabidopsis showed that, unlike DXS1 and DXS2 proteins, it co-expresses with genes related to post-embryonic development and reproduction and not with primary metabolism and isoprenoid synthesis.

Effects of Salinity Stress on Chloroplast Structure and Function

Salinity is a growing problem affecting soils and agriculture in many parts of the world. The presence of salt in plant cells disrupts many basic metabolic processes, contributing to severe negative effects on plant development and growth. This review focuses on the effects of salinity on chloroplasts, including the structures and function of these organelles. Chloroplasts house various important biochemical reactions, including photosynthesis, most of which are considered essential for plant survival. Salinity can affect these reactions in a number of ways, for example, by changing the chloroplast size, number, lamellar organization, lipid and starch accumulation, and interfering with cross-membrane transportation. Research has shown that maintenance of the normal chloroplast physiology is necessary for the survival of the entire plant. Many plant species have evolved different mechanisms to withstand the harmful effects of salt-induced toxicity on their chloroplasts and its machinery. The differences depend on the plant species and growth stage and can be quite different between salt-sensitive (glycophyte) and salt-tolerant (halophyte) plants. Salt stress tolerance is a complex trait, and many aspects of salt tolerance in plants are not entirely clear yet. In this review, we discuss the different mechanisms of salt stress tolerance in plants with a special focus on chloroplast structure and its functions, including the underlying differences between glycophytes and halophytes.

Electron Tomography Analysis of Thylakoid Assembly and Fission in Chloroplasts of a Single-Cell C4 plant, Bienertia sinuspersici

Bienertia sinuspersici is a single-cell C4 plant species of which chlorenchyma cells have two distinct groups of chloroplasts spatially segregated in the cytoplasm. The central vacuole encloses most chloroplasts at the cell center and confines the rest of the chloroplasts near the plasma membrane. Young chlorenchyma cells, however, do not have large vacuoles and their chloroplasts are homogenous. Therefore, maturing Bienertia chlorenchyma cells provide a unique opportunity to investigate chloroplast proliferation in the central cluster and the remodeling of chloroplasts that have been displaced by the vacuole to the cell periphery. Chloroplast numbers and sizes increased, more notably, during later stages of maturation than the early stages. Electron tomography analyses indicated that chloroplast enlargement is sustained by thylakoid growth and that invaginations from the inner envelope membrane contributed to thylakoid assembly. Grana stacks acquired more layers, differentiating them from stroma thylakoids as central chloroplasts matured. In peripheral chloroplasts, however, grana stacks stretched out to a degree that the distinction between grana stacks and stroma thylakoids was obscured. In central chloroplasts undergoing division, thylakoids inside the cleavage furrow were kinked and severed. Grana stacks in the division zone were disrupted, and large complexes in their membranes were dislocated, suggesting the existence of a thylakoid fission machinery.

How do plastids and mitochondria divide?

Plastids and mitochondria are thought to have originated from free-living cyanobacterial and alpha-proteobacterial ancestors, respectively, via endosymbiosis. Their evolutionary origins dictate that these organelles do not multiply de novo but through the division of pre-existing plastids and mitochondria. Over the past three decades, studies have shown that plastid and mitochondrial division are performed by contractile ring-shaped structures, broadly termed the plastid and mitochondrial-division machineries. Interestingly, the division machineries are hybrid forms of the bacterial cell division system and eukaryotic membrane fission system. The structure and function of the plastid and mitochondrial-division machineries are similar to each other, implying that the division machineries evolved in parallel since their establishment in primitive eukaryotes. Compared with our knowledge of their structures, our understanding of the mechanical details of how these division machineries function is still quite limited. Here, we review and compare the structural frameworks of the plastid and mitochondrial-division machineries in both lower and higher eukaryotes. Then, we highlight fundamental issues that need to be resolved to reveal the underlying mechanisms of plastid and mitochondrial division. Finally, we highlight related studies that point to an exciting future for the field.

Chloroplast vesicle transport

Photosynthesis is a well-known process that has been intensively investigated, but less is known about the biogenesis of the thylakoid membrane that harbors the photosynthetic machinery. Thylakoid membranes are constituted by several components, the major ones being proteins and lipids. However, neither of these two are produced in the thylakoid membranes themselves but are targeted there by different mechanisms. The interior of the chloroplast, the stroma, is an aqueous compartment that prevents spontaneous transport of single lipids and/or membrane proteins due to their hydrophobicities. Thylakoid targeted proteins are encoded either in the nucleus or plastid, and thus some cross the envelope membrane before entering one of the identified thylakoid targeting pathways. However, the pathway for all thylakoid proteins is not known. Lipids are produced at the envelope membrane and have been proposed to reach the thylakoid membrane by different means: invaginations of the envelope membrane, direct contact sites between these membranes, or through vesicles. Vesicles have been observed in chloroplasts but not much is yet known about the mechanism or regulation of their formation. The question of whether proteins can also make use of vesicles as one mechanism of transport remains to be answered. Here we discuss the presence of vesicles in chloroplasts and their potential role in transporting lipids and proteins. We additionally discuss what is known about the proteins involved in the vesicle transport and the gaps in knowledge that remain to be filled.

Overexpression of MinE gene affects the plastid division in cassava

The MinE protein plays an important role in plastid division. In this study, the MinE gene was isolated from the cassava (Manihot esculenta Crantz) genome. We isolated high quality and quantity protoplasts and succeed in performing the transient expression of the GFP-fused Manihot esculenta MinE (MeMinE) protein in cassava mesophyll protoplasts. The transient expression of MeMinE-GFP in cassava protoplasts showed that the MeMinE protein was located in the chloroplast. Due to the abnormal division of chloroplasts, overexpression of MeMinE proteins in cassava mesophyll protoplasts could result in fewer and smaller chloroplasts. Overexpression of MeMinE proteins also showed abnormal cell division characteristics and minicell occurrence in Escherichia coli caused by aberrant septation events in the cell poles.

The cellular machineries responsible for the division of endosymbiotic organelles

Chloroplasts (plastids) and mitochondria evolved from endosymbiotic bacteria. These organelles perform vital functions in photosynthetic eukaryotes, such as harvesting and converting energy for use in biological processes. Consistent with their evolutionary origins, plastids and mitochondria proliferate by the binary fission of pre-existing organelles. Here, I review the structures and functions of the supramolecular machineries driving plastid and mitochondrial division, which were discovered and first studied in the primitive red alga Cyanidioschyzon merolae. In the past decade, intact division machineries have been isolated from plastids and mitochondria and examined to investigate their underlying structure and molecular mechanisms. A series of studies has elucidated how these division machineries assemble and transform during the fission of these organelles, and which of the component proteins generate the motive force for their contraction. Plastid- and mitochondrial-division machineries have important similarities in their structures and mechanisms despite sharing no component proteins, implying that these division machineries evolved in parallel. The establishment of these division machineries might have enabled the host eukaryotic ancestor to permanently retain these endosymbiotic organelles by regulating their binary fission and the equal distribution of resources to daughter cells. These findings provide key insights into the establishment of endosymbiotic organelles and have opened new avenues of research into their evolution and mechanisms of proliferation.

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.

Insights into the Mechanisms of Chloroplast Division

The endosymbiosis of a free-living cyanobacterium into an ancestral eukaryote led to the evolution of the chloroplast (plastid) more than one billion years ago. Given their independent origins, plastid proliferation is restricted to the binary fission of pre-existing plastids within a cell. In the last 25 years, the structure of the supramolecular machinery regulating plastid division has been discovered, and some of its component proteins identified. More recently, isolated plastid-division machineries have been examined to elucidate their structural and mechanistic details. Furthermore, complex studies have revealed how the plastid-division machinery morphologically transforms during plastid division, and which of its component proteins play a critical role in generating the contractile force. Identifying the three-dimensional structures and putative functional domains of the component proteins has given us hints about the mechanisms driving the machinery. Surprisingly, the mechanisms driving plastid division resemble those of mitochondrial division, indicating that these division machineries likely developed from the same evolutionary origin, providing a key insight into how endosymbiotic organelles were established. These findings have opened new avenues of research into organelle proliferation mechanisms and the evolution of organelles.

The Arabidopsis arc5 and arc6 mutations differentially affect plastid morphology in pavement and guard cells in the leaf epidermis

Chloroplasts, or photosynthetic plastids, multiply by binary fission, forming a homogeneous population in plant cells. In Arabidopsis thaliana, the division apparatus (or division ring) of mesophyll chloroplasts includes an inner envelope transmembrane protein ARC6, a cytoplasmic dynamin-related protein ARC5 (DRP5B), and members of the FtsZ1 and FtsZ2 families of proteins, which co-assemble in the stromal mid-plastid division ring (FtsZ ring). FtsZ ring placement is controlled by several proteins, including a stromal factor MinE (AtMinE1). During leaf mesophyll development, ARC6 and AtMinE1 are necessary for FtsZ ring formation and thus plastid division initiation, while ARC5 is essential for a later stage of plastid division. Here, we examined plastid morphology in leaf epidermal pavement cells (PCs) and stomatal guard cells (GCs) in the arc5 and arc6 mutants using stroma-targeted fluorescent proteins. The arc5 PC plastids were generally a bit larger than those of the wild type, but most had normal shapes and were division-competent, unlike mutant mesophyll chloroplasts. The arc6 PC plastids were heterogeneous in size and shape, including the formation of giant and mini-plastids, plastids with highly developed stromules, and grape-like plastid clusters, which varied on a cell-by-cell basis. Moreover, unique plastid phenotypes for stomatal GCs were observed in both mutants. The arc5 GCs rarely lacked chlorophyll-bearing plastids (chloroplasts), while they accumulated minute chlorophyll-less plastids, whereas most GCs developed wild type-like chloroplasts. The arc6 GCs produced large chloroplasts and/or chlorophyll-less plastids, as previously observed, but unexpectedly, their chloroplasts/plastids exhibited marked morphological variations. We quantitatively analyzed plastid morphology and partitioning in paired GCs from wild-type, arc5, arc6, and atminE1 plants. Collectively, our results support the notion that ARC5 is dispensable in the process of equal division of epidermal plastids, and indicate that dysfunctions in ARC5 and ARC6 differentially affect plastid replication among mesophyll cells, PCs, and GCs within a single leaf.

Chloroplast function and ion regulation in plants growing on saline soils: lessons from halophytes

Salt stress impacts multiple aspects of plant metabolism and physiology. For instance it inhibits photosynthesis through stomatal limitation, causes excessive accumulation of sodium and chloride in chloroplasts, and disturbs chloroplast potassium homeostasis. Most research on salt stress has focused primarily on cytosolic ion homeostasis with few studies of how salt stress affects chloroplast ion homeostasis. This review asks the question whether membrane-transport processes and ionic relations are differentially regulated between glycophyte and halophyte chloroplasts and whether this contributes to the superior salt tolerance of halophytes. The available literature indicates that halophytes can overcome stomatal limitation by switching to CO2 concentrating mechanisms and increasing the number of chloroplasts per cell under saline conditions. Furthermore, salt entry into the chloroplast stroma may be critical for grana formation and photosystem II activity in halophytes but not in glycophytes. Salt also inhibits some stromal enzymes (e.g. fructose-1,6-bisphosphatase) to a lesser extent in halophyte species. Halophytes accumulate more chloride in chloroplasts than glycophytes and appear to use sodium in functional roles. We propose the molecular identities of candidate transporters that move sodium, chloride and potassium across chloroplast membranes and discuss how their operation may regulate photochemistry and photosystem I and II activity in chloroplasts.

Hierarchal order in the formation of chloroplast division machinery in the red alga Cyanidioschyzon merolae

Chloroplasts have evolved from a cyanobacterial endosymbiont and multiply by dividing. Chloroplast division is performed by constriction of the ring-like protein complex (the PD machinery), which forms at the division site. The PD machinery is composed of cyanobacteria-descended components such as FtsZ and eukaryote-derived proteins such as the dynamin-related protein, DRP5B. In the red alga Cyanidioschyzon merolae, FtsZ ring formation on the stromal side precedes PDR1 and DRP5B ring formation on the cytosolic side. In this study, we impaired FtsZ ring formation in C. merolae by overexpressing FtsZ just before FtsZ ring formation. As a result, PDR1 and DRP5B failed to localize at the chloroplast division site, suggesting that FtsZ ring formation is required for the PDR1 and DRP5B rings. We further found, by expressing a dominant negative form of DRP5B, that DRP5B ring formation begins on the nuclear side of the chloroplast division site. These findings provide insight into how the PD machinery forms in red algae.

Structural insights into the coordination of plastid division by the ARC6-PDV2 complex

Chloroplasts divide by binary fission, which is accomplished by the simultaneous constriction of the FtsZ ring on the stromal side of the inner envelope membrane, and the ARC5 ring on the cytosolic side of the outer envelope membrane. The two rings are connected and coordinated mainly by the interaction between the inner envelope membrane protein ARC6 and the outer envelope membrane protein PDV2 in the intermembrane space. The underlying mechanism of this coordination is unclear to date. Here, we solved the crystal structure of the intermembrane space region of the ARC6-PDV2 complex. The results indicated that PDV2 inserts its carboxy terminus into a pocket formed in ARC6, and this interaction further induces the dimerization of the intermembrane space regions of two ARC6 molecules. A pdv2 mutant attenuating PDV2-induced ARC6 dimerization showed abnormal morphology of ARC6 rings and compromised chloroplast division in plant cells. Together, our data reveal that PDV2-induced dimerization of ARC6 plays a critical role in chloroplast division and provide insights into the coordination mechanism of the internal and external plastid division machineries.

PDV2 has a dosage effect on chloroplast division in Arabidopsis

PDV2 has a dosage effect on chloroplast division in Arabidopsis thaliana , but this effect may vary in different plants. Chloroplasts have to be divided as plants grow to maintain an optimized number in the cell. Chloroplasts are divided by protein complexes across the double membranes from the stroma side to the cytosolic side. PDV2 is a chloroplast division protein on the chloroplast outer membrane. It recruits the dynamin-related GTPase ARC5 to the division site. The C-terminus of PDV2 and the C-terminus of ARC6 interact in the intermembrane space, which is important for the localization of PDV2. Previously, it was shown that overexpression of PDV2 can increase the division of chloroplasts in Arabidopsis and moss, so the authors concluded that PDV2 determines the rate of chloroplast division in land plants. PDV2 was also shown to inhibit the GTPase activity of ARC5 by in vitro experiment. These results look to be contradictory. Here, we identified a null allele of PDV2 in Arabidopsis and studied plants with different levels of PDV2. Our results suggested that the chloroplast division phenotype in Arabidopsis is sensitive to the level of PDV2, while this is not the case for ARC6. The level of PDV2 protein is reduced sharply in fast-growing leaves, while the level of ARC6 is not. The levels of PDV2 and ARC6 in several other plant species at different developmental stages were also investigated. The results indicated that their expression pattern varies in different species. Thus, PDV2 is an important positive factor of chloroplast division with an apparent dosage effect in Arabidopsis, but this effect for different chloroplast division proteins in different plants may vary.

Overview of the Diverse Roles of Bacterial and Archaeal Cytoskeletons

As discovered over the past 25 years, the cytoskeletons of bacteria and archaea are complex systems of proteins whose central components are dynamic cytomotive filaments. They perform roles in cell division, DNA partitioning, cell shape determination and the organisation of intracellular components. The protofilament structures and polymerisation activities of various actin-like, tubulin-like and ESCRT-like proteins of prokaryotes closely resemble their eukaryotic counterparts but show greater diversity. Their activities are modulated by a wide range of accessory proteins but these do not include homologues of the motor proteins that supplement filament dynamics to aid eukaryotic cell motility. Numerous other filamentous proteins, some related to eukaryotic IF-proteins/lamins and dynamins etc, seem to perform structural roles similar to those in eukaryotes.

Chloroplast division checkpoint in eukaryotic algae

Chloroplasts evolved from a cyanobacterial endosymbiont. It is believed that the synchronization of endosymbiotic and host cell division, as is commonly seen in existing algae, was a critical step in establishing the permanent organelle. Algal cells typically contain one or only a small number of chloroplasts that divide once per host cell cycle. This division is based partly on the S-phase-specific expression of nucleus-encoded proteins that constitute the chloroplast-division machinery. In this study, using the red alga Cyanidioschyzon merolae, we show that cell-cycle progression is arrested at the prophase when chloroplast division is blocked before the formation of the chloroplast-division machinery by the overexpression of Filamenting temperature-sensitive (Fts) Z2-1 (Fts72-1), but the cell cycle progresses when chloroplast division is blocked during division-site constriction by the overexpression of either FtsZ2-1 or a dominant-negative form of dynamin-related protein 5B (DRP5B). In the cells arrested in the prophase, the increase in the cyclin B level and the migration of cyclin-dependent kinase B (CDKB) were blocked. These results suggest that chloroplast division restricts host cell-cycle progression so that the cell cycle progresses to the metaphase only when chloroplast division has commenced. Thus, chloroplast division and host cell-cycle progression are synchronized by an interactive restriction that takes place between the nucleus and the chloroplast. In addition, we observed a similar pattern of cell-cycle arrest upon the blockage of chloroplast division in the glaucophyte alga Cyanophora paradoxa, raising the possibility that the chloroplast division checkpoint contributed to the establishment of the permanent organelle.

An improved immunofluorescence staining method for chloroplast proteins

An improved immunofluorescence staining method significantly facilitates the visualization of the subcellular localization of interested proteins in chloroplasts. As an important technical approach, immunofluorescence staining is widely used in the subcellular localization study of interested proteins. During the study of the functions of chloroplast division proteins, immunofluorescence staining was frequently adopted. Previously, a method has been developed to study the localization of a chloroplast division protein, FtsZ. However, it is laborious and time-consuming. In this study, we report a modified immunofluorescence staining method, in which protoplasts were isolated from leaf tissues, and then fixed for immunofluorescence staining. The time of the experiment was significantly reduced to several hours. Furthermore, we used correction pen in the fixation procedure and a new way to coat the slide, which greatly saved the cost of the experiment. With the chloroplast division protein ARC6 as an example, we can get a good fluorescence signal. Moreover, the localization of ARC6 in two chloroplast division mutants, arc3 and arc5, and three other plant species, such as cabbage, radish and pea, was also successfully analyzed with our new method. Overall, the immunofluorescence staining method we reported here is very practical, and it significantly facilitates the visualization of the subcellular localization of interested proteins in plant cells.

Chloroplast FtsZ assembles into a contractible ring via tubulin-like heteropolymerization

Chloroplast division is driven by a ring containing FtsZ1 and FtsZ2 proteins, which originated from bacterial FtsZ, a tubulin-like protein; however, mechanistic details of the chloroplast FtsZ ring remain unclear. Here, we report that FtsZ1 and FtsZ2 can heteropolymerize into a contractible ring ex vivo. Fluorescently labelled FtsZ1 and/or FtsZ2 formed single rings in cells of the yeast Pichia pastoris. Photobleaching experiments indicated that co-assembly of FtsZ1 and FtsZ2 imparts polarity to polymerization. Assembly of FtsZ chimaeras revealed that the protofilaments assemble via heteropolymerization of FtsZ2 and FtsZ1. Contraction of the ring was accompanied by an increase in the filament turnover rate. Our findings suggest that the evolutionary duplication of FtsZ in plants may have increased the mobility and kinetics of FtsZ ring dynamics in chloroplast division. Thus, the gene duplication and heteropolymerization of chloroplast FtsZs may represent convergent evolution with eukaryotic tubulin.

Novel roles for actin in mitochondrial fission

Mitochondrial dynamics, including fusion, fission and translocation, are crucial to cellular homeostasis, with roles in cellular polarity, stress response and apoptosis. Mitochondrial fission has received particular attention, owing to links with several neurodegenerative diseases. A central player in fission is the cytoplasmic dynamin-related GTPase Drp1, which oligomerizes at the fission site and hydrolyzes GTP to drive membrane ingression. Drp1 recruitment to the outer mitochondrial membrane (OMM) is a key regulatory event, which appears to require a pre-constriction step in which the endoplasmic reticulum (ER) and mitochondrion interact extensively, a process termed ERMD (ER-associated mitochondrial division). It is unclear how ER-mitochondrial contact generates the force required for pre-constriction or why pre-constriction leads to Drp1 recruitment. Recent results, however, show that ERMD might be an actin-based process in mammals that requires the ER-associated formin INF2 upstream of Drp1, and that myosin II and other actin-binding proteins might be involved. In this Commentary, we present a mechanistic model for mitochondrial fission in which actin and myosin contribute in two ways; firstly, by supplying the force for pre-constriction and secondly, by serving as a coincidence detector for Drp1 binding. In addition, we discuss the possibility that multiple fission mechanisms exist in mammals.

Mycoplasma pneumoniae, an underutilized model for bacterial cell biology

In recent decades, bacterial cell biology has seen great advances, and numerous model systems have been developed to study a wide variety of cellular processes, including cell division, motility, assembly of macromolecular structures, and biogenesis of cell polarity. Considerable attention has been given to these model organisms, which include Escherichia coli, Bacillus subtilis, Caulobacter crescentus, and Myxococcus xanthus. Studies of these processes in the pathogenic bacterium Mycoplasma pneumoniae and its close relatives have also been carried out on a smaller scale, but this work is often overlooked, in part due to this organism's reputation as minimalistic and simple. In this minireview, I discuss recent work on the role of the M. pneumoniae attachment organelle (AO), a structure required for adherence to host cells, in these processes. The AO is constructed from proteins that generally lack homology to those found in other organisms, and this construction occurs in coordination with cell cycle events. The proteins of the M. pneumoniae AO share compositional features with proteins with related roles in model organisms. Once constructed, the AO becomes activated for its role in a form of gliding motility whose underlying mechanism appears to be distinct from that of other gliding bacteria, including Mycoplasma mobile. Together with the FtsZ cytoskeletal protein, motility participates in the cell division process. My intention is to bring this deceptively complex organism into alignment with the better-known model systems.

Nucleotides and substrates trigger the dynamics of the Toc34 GTPase homodimer involved in chloroplast preprotein translocation

GTPases are molecular switches that control numerous crucial cellular processes. Unlike bona fide GTPases, which are regulated by intramolecular structural transitions, the less well studied GAD-GTPases are activated by nucleotide-dependent dimerization. A member of this family is the translocase of the outer envelope membrane of chloroplast Toc34 involved in regulation of preprotein import. The GTPase cycle of Toc34 is considered a major circuit of translocation regulation. Contrary to expectations, previous studies yielded only marginal structural changes of dimeric Toc34 in response to different nucleotide loads. Referencing PELDOR and FRET single-molecule and bulk experiments, we describe a nucleotide-dependent transition of the dimer flexibility from a tight GDP- to a flexible GTP-loaded state. Substrate binding induces an opening of the GDP-loaded dimer. Thus, the structural dynamics of bona fide GTPases induced by GTP hydrolysis is replaced by substrate-dependent dimer flexibility, which likely represents a general regulatory mode for dimerizing GTPases.

DipM is required for peptidoglycan hydrolysis during chloroplast division

BACKGROUND

Chloroplasts have evolved from a cyanobacterial endosymbiont and their continuity has been maintained over time by chloroplast division, a process which is performed by the constriction of a ring-like division complex at the division site. The division complex has retained certain components of the cyanobacterial division complex, which function inside the chloroplast. It also contains components developed by the host cell, which function outside of the chloroplast and are believed to generate constrictive force from the cytosolic side, at least in red algae and Viridiplantae. In contrast to the chloroplasts in these lineages, those in glaucophyte algae possess a peptidoglycan layer between the two envelope membranes, as do cyanobacteria.

RESULTS

In this study, we show that chloroplast division in the glaucophyte C. paradoxa does not involve any known chloroplast division proteins of the host eukaryotic origin, but rather, peptidoglycan spitting and probably the outer envelope division process rely on peptidoglycan hydrolyzing activity at the division site by the DipM protein, as in cyanobacterial cell division. In addition, we found that DipM is required for normal chloroplast division in the moss Physcomitrella patens.

CONCLUSIONS

These results suggest that the regulation of peptidoglycan splitting was essential for chloroplast division in the early evolution of chloroplasts and this activity is likely still involved in chloroplast division in Viridiplantae.

Biogenesis and homeostasis of chloroplasts and other plastids

Chloroplasts are the organelles that define plants, and they are responsible for photosynthesis as well as numerous other functions. They are the ancestral members of a family of organelles known as plastids. Plastids are remarkably dynamic, existing in strikingly different forms that interconvert in response to developmental or environmental cues. The genetic system of this organelle and its coordination with the nucleocytosolic system, the import and routing of nucleus-encoded proteins, as well as organellar division all contribute to the biogenesis and homeostasis of plastids. They are controlled by the ubiquitin-proteasome system, which is part of a network of regulatory mechanisms that integrate plastid development into broader programmes of cellular and organismal development.

FtsZ-less prokaryotic cell division as well as FtsZ- and dynamin-less chloroplast and non-photosynthetic plastid division

The chloroplast division machinery is a mixture of a stromal FtsZ-based complex descended from a cyanobacterial ancestor of chloroplasts and a cytosolic dynamin-related protein (DRP) 5B-based complex derived from the eukaryotic host. Molecular genetic studies have shown that each component of the division machinery is normally essential for normal chloroplast division. However, several exceptions have been found. In the absence of the FtsZ ring, non-photosynthetic plastids are able to proliferate, likely by elongation and budding. Depletion of DRP5B impairs, but does not stop chloroplast division. Chloroplasts in glaucophytes, which possesses a peptidoglycan (PG) layer, divide without DRP5B. Certain parasitic eukaryotes possess non-photosynthetic plastids of secondary endosymbiotic origin, but neither FtsZ nor DRP5B is encoded in their genomes. Elucidation of the FtsZ- and/or DRP5B-less chloroplast division mechanism will lead to a better understanding of the function and evolution of the chloroplast division machinery and the finding of the as-yet-unknown mechanism that is likely involved in chloroplast division. Recent studies have shown that FtsZ was lost from a variety of prokaryotes, many of which lost PG by regressive evolution. In addition, even some of the FtsZ-bearing bacteria are able to divide when FtsZ and PG are depleted experimentally. In some cases, alternative mechanisms for cell division, such as budding by an increase of the cell surface-to-volume ratio, are proposed. Although PG is believed to have been lost from chloroplasts other than in glaucophytes, there is some indirect evidence for the existence of PG in chloroplasts. Such information is also useful for understanding how non-photosynthetic plastids are able to divide in FtsZ-depleted cells and the reason for the retention of FtsZ in chloroplast division. Here we summarize information to facilitate analyses of FtsZ- and/or DRP5B-less chloroplast and non-photosynthetic plastid division.