Preprophasic establishment of division polarity in monoplastidic mitosis of hornworts

Summary Preprophase in the monoplastidic mitotic cells ofPhaeoceros andNotothylas is characterized by the establishment of a division site in the absence of a typical preprophase band. The future cytokinetic plane is predicted by plastid orientation and development of an elaborate preprophasic microtubule system perpendicular to the division plane. Division of the single plastid is initiated early in preprophase and the constricting plastid migrates to a position perpendicular to the future plane of division. Plastid orientation assures that division of the plastid by mid-constriction will result in distribution of a plastid to each daughter cell. Microtubules parallel the long axis of the plastid and are most numerous adjacent to the nucleus which becomes elongated in the future spindle axis. We conclude that the division site is a fundamental component of the cytokinetic apparatus involved in the determination of cleavage plane prior to nuclear division.     

An Arabidopsis homolog of the bacterial cell division inhibitor SulA is involved in plastid division

Plastids have evolved from an endosymbiosis between a cyanobacterial symbiont and a eukaryotic host cell. Their division is mediated both by proteins of the host cell and conserved bacterial division proteins. Here, we identified a new component of the plastid division machinery, Arabidopsis thaliana SulA. Disruption of its cyanobacterial homolog (SSulA) in Synechocystis and overexpression of an AtSulA-green fluorescent protein fusion in Arabidopsis demonstrate that these genes are involved in cell and plastid division, respectively. Overexpression of AtSulA inhibits plastid division in planta but rescues plastid division defects caused by overexpression of AtFtsZ1-1 and AtFtsZ2-1, demonstrating that its role in plastid division may involve an interaction with AtFtsZ1-1 and AtFtsZ2-1.     

The genetic control of plastid division in higher plants

The division of plastids is an important part of plastid differentiation and development and in distinct cell types, such as leaf mesophyll cells, results in large populations of chloroplasts. The morphology and population dynamics of plastid division have been well documented, but the molecular controls underlying plastid division are largely unknown. With the isolation of Arabidopsis mutants in which specific aspects of plastid and proplastid division have been disrupted, the potential exists for a detailed knowledge of how plastids divide and what factors control the rate of division in different cell types. It is likely that knowledge of plant homologues of bacterial cell division genes will be essential for understanding this process in full. The processes of plastid division and expansion appear to be mutually independent processes, which are compensatory when either division or expansion are disrupted genetically. The rate of cell expansion appears to be an important factor in initiating plastid division and several systems involving rapid cell expansion show high levels of plastid division activity. In addition, observation of plastids in different cell types in higher plants shows that cell-specific signals are also important in the overall process in determining not only the differentiation pathway of plastids but also the extent of plastid division. It appears likely that with the exploitation of molecular techniques and mutants, a detailed understanding of the molecular basis of plastid division may soon be a reality.     

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.     

The molecular biology of plastid division in higher plants

Plastids are essential plant organelles vital for life on earth, responsible not only for photosynthesis but for many fundamental intermediary metabolic reactions. Plastids are not formed de novo but arise by binary fission from pre-existing plastids, and plastid division therefore represents an important process for the maintenance of appropriate plastid populations in plant cells. Plastid division comprises an elaborate pathway of co-ordinated events which include division machinery assembly at the division site, the constriction of envelope membranes, membrane fusion and, ultimately, the separation of the two new organelles. Because of their prokaryotic origin bacterial cell division has been successfully used as a paradigm for plastid division. This has resulted in the identification of the key plastid division components FtsZ, MinD, and MinE, as well as novel proteins with similarities to prokaryotic cell division proteins. Through a combination of approaches involving molecular genetics, cell biology, and biochemistry, it is now becoming clear that these proteins act in concert during plastid division, exhibiting both similarities and differences compared with their bacterial counterparts. Recent efforts in the cloning of the disrupted loci in several of the accumulation and replication of chloroplasts mutants has further revealed that the division of plastids is controlled by a combination of prokaryote-derived and host eukaryote-derived proteins residing not only in the plastid stroma but also in the cytoplasm. Based on the available data to date, a working model is presented showing the protein components involved in plastid division, their subcellular localization, and their protein interaction properties.     

DEVELOPMENT OF STOMATA IN SELAGINELLA: DIVISION POLARITY AND PLASTID MOVEMENTS

The young guard cell of Selaginella inherits a single plastid from the division of the stomatal guard mother cell (GMC). During early stomatal development the single plastid undergoes a complex series of migrations and divisions. The regular pattern of plastid behavior appears to be an expression of the genetic program controlling division plane and cytomorphogenesis. The plastid in the GMC becomes precisely aligned with its midconstriction intersected by the plane of a preprophase band of microtubules (PPB) oriented parallel to the long axis of the leaf. This alignment with respect to the future division plane of the cytoplasm ensures equal plastid distribution to the daughter cells. Cytokinesis occurs in the plane previously marked by the PPB and the plastid in each daughter cell lies between the lateral wall and the newly formed nucleus. Following cytokinesis the plastid in each young guard cell develops a median constriction and migrates to the common ventral wall where the isthmus is associated with a system of microtubules in the vicinity of the developing pore region. Plastid division is completed while the plastid is adjacent to the common ventral wall. Following division, the two daughter plastids move back toward the lateral wall. Each plastid may divide again during guard cell maturation but no further migrations occur.     

The division of pleomorphic plastids with multiple FtsZ rings in tobacco BY-2 cells

Plastids, an essential group of plant cellular organelles, proliferate by division to maintain continuity through cell lineages in plants. In recent years, it was revealed that the bacterial cell division protein FtsZ is encoded in the nuclear genome of plant cells, and plays a major role in the plastid division process forming a ring along the center of plastids. Although the best-characterized type of plastid division so far is the division with a single FtsZ ring at the plastid midpoint, it was recently reported that in some plant organs and tissues, plastids are pleomorphic and form multiple FtsZ rings. However, the pleomorphic plastid division mechanism, such as the formation of multiple FtsZ rings, the constriction of plastids and the behavior of plastid (pt) nucleoids, remains totally unclear. To elucidate these points, we used the cultured cell line, tobacco (Nicotiana tabacum L.) Bright Yellow-2, in which plastids are pleomorphic and show dynamic morphological changes during culture. As a result, it was revealed that as the plastid elongates from an ellipsoid shape to a string shape after medium renewal, FtsZ rings are multiplied almost orderly and perpendicularly to the long axis of plastids. Active DNA synthesis of pt nucleoids is induced by medium transfer, and the division and the distribution of pt nucleoids occur along with plastid elongation. Although it was thought that the plastid divides with simultaneous multiple constrictions at all the FtsZ ring sites, giving rise to many small plastids, we found that the plastids generally divide constricting at only one FtsZ ring site. Moreover, using electron microscopy, we revealed that plastid-dividing (PD) rings are observed only at the constriction site, and not at swollen regions. These results indicate that in the pleomorphic plastid division with multiple FtsZ rings, the formation of PD rings occurs at a limited FtsZ ring site for one division. Multiplied FtsZ rings seem to localize in advance at the expected sites of division, and the formation of a PD ring at each FtsZ ring site occurs in a certain order, not simultaneously. Based on these results, a novel model for the pleomorphic plastid division with multiple FtsZ rings is proposed.     

Plastid apportionment and preprophase microtubule bands in monoplastidic root meristem cells ofIsoetes andSelaginella

Summary Ultrastructural observations on monoplastidic root tip cells ofIsoetes andSelaginella demonstrate two important phenomena associated with preprophasic preparation for mitotic cell division, 1. the preprophase band and 2. precise orientation of the dividing plastid relative to the preprophase band. Both of these phenomena accurately predict the future plane of cell division. The plastid divides in a plane parallel to the spindle and each cell inherits a single plastid which caps the telophase nucleus. When succesive transverse divisions occur, the plastid migrates prior to prophase from a position near an old transverse wall to a lateral position in the cell. The plastid is oriented with its median constriction precisely intersected by the plane of the preprophase band. When a longitudinal division follows a transverse division, the plastid remains in its position adjacent to an old transverse wall where it is bisected by the plane of the longitudinally oriented preprophase band microtubules.     

The topological specificity factor AtMinE1 is essential for correct plastid division site placement in Arabidopsis

In plant cells, plastids divide by binary fission involving a complex pathway of events. Although there are clear similarities between bacterial and plastid division, limited information exists regarding the mechanism of plastid division in higher plants. Here we demonstrate that AtMinE1, an Arabidopsis homologue of the bacterial MinE topological specificity factor, is an essential integral component of the plastid division machinery. In prokaryotes MinE imparts topological specificity during cell division by blocking division apparatus assembly at sites other than midcell. We demonstrate that overexpression of AtMinE1 in E. coli results in loss of topological specificity and minicell formation suggesting evolutionary conservation of MinE mode of action. We further show that AtMinE1 can indeed act as a topological specificity factor during plastid division revealing that AtMinE1 overexpression in Arabidopsis seedlings results in division site misplacement giving rise to multiple constrictions along the length of plastids. In agreement with cell division studies in bacteria, AtMinE1 and AtMinD1 show distinct intraplastidic localisation patterns suggestive of dynamic localisation behaviour. Taken together our findings demonstrate that AtMinE1 is an evolutionary conserved topological specificity factor, most probably acting in concert with AtMinD1, required for correct plastid division in Arabidopsis.     

ARC6 is a J-domain plastid division protein and an evolutionary descendant of the cyanobacterial cell division protein Ftn2

Replication of chloroplasts is essential for achieving and maintaining optimal plastid numbers in plant cells. The plastid division machinery contains components of both endosymbiotic and host cell origin, but little is known about the regulation and molecular mechanisms that govern the division process. The Arabidopsis mutant arc6 is defective in plastid division, and its leaf mesophyll cells contain only one or two grossly enlarged chloroplasts. We show here that arc6 chloroplasts also exhibit abnormal localization of the key plastid division proteins FtsZ1 and FtsZ2. Whereas in wild-type plants, the FtsZ proteins assemble into a ring at the plastid division site, chloroplasts in the arc6 mutant contain numerous short, disorganized FtsZ filament fragments. We identified the mutation in arc6 and show that the ARC6 gene encodes a chloroplast-targeted DnaJ-like protein localized to the plastid envelope membrane. An ARC6-green fluorescent protein fusion protein was localized to a ring at the center of the chloroplasts and rescued the chloroplast division defect in the arc6 mutant. The ARC6 gene product is related closely to Ftn2, a prokaryotic cell division protein unique to cyanobacteria. Based on the FtsZ filament morphology observed in the arc6 mutant and in plants that overexpress ARC6, we hypothesize that ARC6 functions in the assembly and/or stabilization of the plastid-dividing FtsZ ring. We also analyzed FtsZ localization patterns in transgenic plants in which plastid division was blocked by altered expression of the division site-determining factor AtMinD. Our results indicate that MinD and ARC6 act in opposite directions: ARC6 promotes and MinD inhibits FtsZ filament formation in the chloroplast.     

高等植物质体的分裂

质体来源于早期具光合能力的原核生物与原始真核生物的内共生事件。原核起源的蛋白以及真核寄主起源的蛋白共同参与了质体的分裂过程。以原核生物的细胞分裂蛋白为蓝本, 近些年在植物中陆续鉴定出几种主要的原核生物细胞分裂蛋白的同源物, 如FtsZ、MinD和MinE蛋白。然而, 除此之外, 原核细胞大多数分裂相关因子在植物中找不到其同源物, 但却鉴定了许多真核寄主来源的分裂相关蛋白。当前研究的重点是剖析各种质体分裂蛋白协同作用的机制, 业已证明MinD和MinE的协同作用保证了FtsZ(Z)环的正确定位。尽管经典的FtsZ的抑制因子MinC在植物中不存在, 但实验表明ARC3在拟南芥中具有类似MinC的功能。ARC3蛋白与真核起源的蛋白如ARC5、ARTEMIS、FZL和PD环以及其它原核起源的蛋白如ARC6和GC1等共同构成了一个复杂的植物质体分裂调控系统。     

高等植物质体的分裂

质体来源于早期具光合能力的原核生物与原始真核生物的内共生事件。原核起源的蛋白以及真核寄主起源的蛋白共同参与了质体的分裂过程。以原核生物的细胞分裂蛋白为蓝本,近些年在植物中陆续鉴定出几种主要的原核生物细胞分裂蛋白的同源物,如FtsZ、MinD和MinE蛋白。然而,除此之外,原核细胞大多数分裂相关因子在植物中找不到其同源物,但却鉴定了许多真核寄主来源的分裂相关蛋白。当前研究的重点是剖析各种质体分裂蛋白协同作用的机制,业已证明MinD和Mine的协同作用保证了FtsZ（Z）环的正确定位。尽管经典的FtsZ的抑制因子MinC在植物中不存在,但实验表明ARC3在拟南芥中具有类似MinC的功能。ARC3蛋白与真核起源的蛋白如ARC5、ARTEMIS、FZL和PD环以及其它原核起源的蛋白如ARC6和GC1等共同构成了一个复杂的植物质体分裂调控系统。     

Pattern of the plastid division in spore mother cells of the hornwortAnthoceros punctatus

Relative changes in plastid DNA content in each stage of plastid division were investigated in order to better understand the division cycle of plastids in spore mother cells in the horwortAnthoceros punctatus. Samples of cells stained with DAPI were observed with epifluorescence microscopy and CHIAS. In spore mother cells of this species, plastids duplicated their own DNA prior to the plastidkinesis of the first plastid division, but did not replicate plastid DNA prior to the plastidkinesis of the second plastid division. Therefore, the DNA content of those plastids in which division had been completed was reduced to half its initial value. This indicates that the DNA replication pattern of plastids in spore mother cells corresponds to that of cell nuclei during premeiosis and meiosis inA. punctatus.     

THE QUADRIPOLAR MICROTUBULE SYSTEM AND MEIOTIC SPINDLE ONTOGENY IN HORNWORTS (BRYOPHYTA: ANTHOCEROTAE)

Ontogeny of the meiotic spindle in hornworts was studied by light microscopy of live materials, transmission electron microscopy, and indirect immunofluorescence microscopy. As in monoplastidic meiosis of mosses and Isoetes, the single plastid divides twice, and the four resultant plastids migrate into the future spore domains where they organize a quadripolar microtubule system (QMS). Additionally, a unique axial microtubule system (AMS) was found to parallel the plastid isthmus at each division in meiosis, much as in the single plastid division of mitosis. This finding is used to make a novel comparison of mitotic and meiotic spindle development. The AMS contributes directly to development of the mitotic spindle, whereas ontogeny of the meiotic spindle is more complex. Nuclear division in meiosis is delayed until after the second plastid division; the first AMS disappears without spindle formation, and the two AMSs of the second plastid division contribute to development of the QMS. Proliferation of microtubules at each plastid results in the QMS consisting of four cones of microtubules interconnecting the plastids and surrounding the nucleus. The QMS contributes to the development of a functionally bipolar spindle. The meiotic spindle is comparable to a merger of two mitotic spindles. However, the first division spindle does not terminate in what would be the poles of mitosis; instead the poles converge to orient the spindle axis midway between pairs of non-sister plastids.     

Plastid division: evidence for a prokaryotically derived mechanism

Plastid division is a critical process in plant cell biology but it is poorly understood. Recent studies combining mutant analysis, gene cloning, and exploitation of genomic resources have revealed that the molecular machinery associated with plastid division is derived evolutionarily from the bacterial cell division apparatus. Comparison of the two processes provides a basis for identifying new components of the plastid division mechanism, but also serves to highlight the differences, not least of which is the nuclear control of the plastid division process.     

Plastid division is driven by a complex mechanism that involves differential transition of the bacterial and eukaryotic division rings

During plastid division, two structures have been detected at the division site in separate analyses. The plastid-dividing ring can be detected by transmission electron microscopy as two (or three) electron-dense rings: an outer ring on the cytosolic face of the outer envelope, occasionally a middle ring in the intermembrane space, and an inner ring on the stromal face of the inner envelope. The FtsZ ring, which plays a central role in bacterial division, also is involved in plastid division and is believed to have descended to plastids from cyanobacterial endosymbiosis. The relationship between the two structures is not known, although there is discussion regarding whether they are identical. Biochemical and immunocytochemical investigations, using synchronized chloroplasts of the red alga Cyanidioschyzon merolae, showed that the plastid FtsZ ring is distinct and separable from the plastid-dividing ring. The FtsZ ring localizes in stroma and faces the inner plastid-dividing ring at the far side from the inner envelope. The FtsZ ring and the inner and outer plastid-dividing rings form in that order before plastid division. The FtsZ ring disappears at the late stage of constriction before dissociation of the plastid-dividing ring, when the constriction is still in progress. Our results suggest that the FtsZ ring;-based system, which originated from a plastid ancestor, cyanobacteria, and the plastid-dividing ring;-based system, which probably originated from host eukaryotic cells, form a complex and are involved in plastid division by distinct modes.     

Genome-wide gene expression profiles in response to plastid division perturbations

Plastids are vital organelles involved in important metabolic functions that directly affect plant growth and development. Plastids divide by binary fission involving the coordination of numerous protein components. A tight control of the plastid division process ensures that: there is a full plastid complement during and after cell division, specialized cell types have optimal plastid numbers; the division rate is modulated in response to stress, metabolic fluxes and developmental status. However, how this control is exerted by the host nucleus is unclear. Here, we report a genome-wide microarray analysis of three accumulation and replication of chloroplasts (arc) mutants that show a spectrum of altered plastid division characteristics. To ensure a comprehensive data set, we selected arc3, arc5 and arc11 because they harbour mutations in protein components of both the stromal and cytosolic division machinery, are of different evolutionary origin and display different phenotypic severities in terms of chloroplast number, size and volume. We show that a surprisingly low number of genes are affected by altered plastid division status, but that the affected genes encode proteins important for a variety of fundamental plant processes.     

ARC3 is a stromal Z-ring accessory protein essential for plastid division

In plants, chloroplast division is an integral part of development, and these vital organelles arise by binary fission from pre-existing cytosolic plastids. Chloroplasts arose by endosymbiosis and although they have retained elements of the bacterial cell division machinery to execute plastid division, they have evolved to require two functionally distinct forms of the FtsZ protein and have lost elements of the Min machinery required for Z-ring placement. Here, we analyse the plastid division component accumulation and replication of chloroplasts 3 (ARC3) and show that ARC3 forms part of the stromal plastid division machinery. ARC3 interacts specifically with AtFtsZ1, acting as a Z-ring accessory protein and defining a unique function for this family of FtsZ proteins. ARC3 is involved in division site placement, suggesting that it might functionally replace MinC, representing an important advance in our understanding of the mechanism of chloroplast division and the evolution of the chloroplast division machinery.     

GIANT CHLOROPLAST 1 is essential for correct plastid division in Arabidopsis

Plastids are vital plant organelles involved in many essential biological processes. Plastids are not created de novo but divide by binary fission mediated by nuclear-encoded proteins of both prokaryotic and eukaryotic origin. Although several plastid division proteins have been identified in plants, limited information exists regarding possible division control mechanisms. Here, we describe the identification of GIANT CHLOROPLAST 1 (GC1), a new nuclear-encoded protein essential for correct plastid division in Arabidopsis. GC1 is plastid-localized and is anchored to the stromal surface of the chloroplast inner envelope by a C-terminal amphipathic helix. In Arabidopsis, GC1 deficiency results in mesophyll cells harbouring one to two giant chloroplasts, whilst GC1 overexpression has no effect on division. GC1 can form homodimers but does not show any interaction with the Arabidopsis plastid division proteins AtFtsZ1-1, AtFtsZ2-1, AtMinD1, or AtMinE1. Analysis reveals that GC1-deficient giant chloroplasts contain densely packed wild-type-like thylakoid membranes and that GC1-deficient leaves exhibit lower rates of CO(2) assimilation compared to wild-type. Although GC1 shows similarity to a putative cyanobacterial SulA cell division inhibitor, our findings suggest that GC1 does not act as a plastid division inhibitor but, rather, as a positive factor at an early stage of the division process.     

MONOPLASTIDIC CELL DIVISION IN LOWER LAND PLANTS

In many bryophytes and vascular cryptogams mitosis and/or meiosis takes place in cells containing a single plastid. In monoplastidic cell division plastid polarity assures that nuclear and plastid division are infallibly coordinated. The two major components of plastid polarity are morphogenetic plastid migration and microtubule organization at the plastids. Before nuclear division the plastid migrates to a position intersecting the future division plane. This morphogenetic migration is a reliable marker of division polarity in cells with and without a preprophase band of microtubules (PPB). The PPB, which predicts the future division plane before mitosis, is a characteristic feature of land plants and its insertion into the cytokinetic apparatus marks the evolution of a cortical microtubule system and a commitment to meristematic growth. Microtubule systems associated with plastid division, the axial microtubule system (AMS) in mitosis and the quadripolar microtubule system (QMS) in meiosis, contribute to predictive positioning of plastids and participate directly in spindle ontogeny. Division polarity in monoplastidic sporocytes is remarkable in that division sites are selected prior to the two successive nuclear divisions of meiosis. Plastid arrangement prior to meiosis determines the future spore domains in monoplastidic sporocytes, whereas in polyplastidic sporocytes the spore nuclei play a major role in claiming cytoplasmic domains. It is hypothesized that predivision microtubule systems associated with monoplastidic cell division are early forming components of the mitotic apparatus that serve to orient the spindle and insure equal apportionment of nucleus and plastids. “Can it be supposed that cytoplasm would be intrusted with so important a task as the preparation of a chloroplast for each of the four nuclei that are later to preside over the spores before there is any indication that such nuclear division is to take place?” Bradley Moore Davis, 1899