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.     

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.     

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.     

多细胞生物干细胞不对称分裂的内在调节机制研究进展

多细胞生物的发育是从一个受精卵分化成多种类型细胞的过程。细胞多样性形成的基础是不等分裂,不等分裂是干细胞自我更新和自我维持的关键。干细胞不等分裂有细胞内和细胞外两种调节机制。果蝇神经干细胞增殖和分化、植物胚胎发育、表皮气孔形成及根内皮层的分化,是研究不等细胞分裂调节机制最多的发育背景。本综述介绍了果蝇神经干细胞和植物胚胎发育早期、表皮气孔发生及根皮层内皮层中细胞不等分裂内在调节机制的研究进展。     

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.     

The Arabidopsis SMO2, a homologue of yeast TRM112, modulates progression of cell division during organ growth

Cell proliferation is integrated into developmental progression in multicellular organisms, including plants, and the regulation of cell division is of pivotal importance for plant growth and development. Here, we report the identification of an Arabidopsis SMALL ORGAN 2 (SMO2) gene that functions in regulation of the progression of cell division during organ growth. The smo2 knockout mutant displays reduced size of aerial organs and shortened roots, due to the decreased number of cells in these organs. Further analyses reveal that disruption of SMO2 does not alter the developmental timing but reduces the rate of cell production during leaf and root growth. Moreover, smo2 plants exhibit a constitutive activation of cell cycle‐related genes and over‐accumulation of cells expressing CYCB1;1:β‐glucuronidase (CYCB1;1:GUS) during organogenesis, suggesting that smo2 has a defect in G₂–M phase progression in the cell cycle. SMO2 encodes a functional homologue of yeast TRM112, a plurifunctional component involved in a few cellular events, including tRNA and protein methylation. In addition, the mutation of SMO2 does not appear to affect endoreduplication in Arabidopsis leaf cells. Taken together we postulate that Arabidopsis SMO2 is a conserved yeast TRM112 homologue and SMO2‐mediated cellular events are required for proper progression of cell division in plant growth and development.     

Plastid division coordination across a double-membraned structure

Chloroplasts still retain components of the bacterial cell division machinery and research over the past decade has led to an understanding of how these stromal division proteins assemble and function as a complex chloroplast division machinery. However, during evolution plant chloroplasts have acquired a number of cytosolic division proteins, indicating that unlike the cyanobacterial ancestors of plastids, chloroplast division in higher plants require a second division machinery located on the chloroplast outer envelope membrane. Here we review the current understanding of the stromal and cytosolic plastid division machineries and speculate how two protein machineries coordinate their activities across a double-membraned structure.     

衣藻叶绿体分裂基因CrFtsZ1在E.coli中的表达

FtsZ蛋白在细菌的分裂中起着重要作用,能够在分裂位点形成一个环状结构而控制细菌的分裂过程。细胞内FtsZ蛋白浓度的明显降低或异常升高均可阻断正常的细胞分裂过程进而导致丝状菌体的产生。为了研究衣藻叶绿体分裂基因ftsZ的功能,构建了衣藻CrFtsZ1的原核表达重组质粒。试验结果表明,衣藻ftsZ的表达严重影响了大肠杆菌的分裂,初步证明衣藻FtsZ蛋白不仅与E．coli FtsZ蛋白在序列上相似,而且也有着相似的功能,同时这一结果也为真核细胞中质体的内共生起源提供了直接的证据。     

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.     

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.     

在细胞含质体数目不均、分裂不同步条件下突变质体随细胞分裂而传递的模式

本文在细胞质体数目不均等、分裂不同步条件下建立了突变质体随细胞分裂而传递的数学表达式,并在具体给定条件下给出了具体计算原始细胞分裂n次后其子细胞含有x个质体,且其中已有j个质体发生突变的可能性（即概率）的公式。     

The ultrastructural features and division of secondary plastids

Plastids in heterokonts, cryptophytes, haptophytes, dinoflagellates, chlorarachniophytes, euglenoids, and apicomplexan parasites derive from secondary symbiogenesis. These plastids are surrounded by one or two additional membranes covering the plastid-envelope double membranes. Consequently, nuclear-encoded plastid division proteins have to be targeted into the division site through the additional surrounding membranes. Electron microscopic observations suggest that the additional surrounding membranes are severed by mechanisms distinct from those for the division of the plastid envelope. In heterokonts, cryptophytes and haptophytes, the outermost surrounding membrane (epiplastid rough endoplasmic reticulum, EPrER) is studded with cytoplasmic ribosomes and connected to the rER and the outer nuclear envelope. In monoplastidic species belonging to these three groups, the EPrER and the outer nuclear envelope are directly connected to form a sac enclosing the plastid and the nucleus. This nuclear-plastid connection, referred to as the nucleus-plastid consortium (NPC), may be significant to ensure the transmission of the plastids during cell division. The plastid dividing-ring (PD-ring) is a conserved component of the division machinery for both primary and secondary plastids. Also, homologues of the bacterial cell division protein, FtsZ, may be involved in the division of secondary plastids as well as primary plastids, though in secondary plastids they have not yet been localized to the division site. It remains to be examined whether or not dynamin-like proteins and other protein components known to function in the division of primary plastids are used also in secondary plastids. The nearly completed sequencing of the nuclear genome of the diatom Thalassiosira pseudonana will give impetus to molecular and cell biological studies on the division of secondary plastids.     

Manipulation of starch granule size distribution in potato tubers by modulation of plastid division

Starch granule size is an important parameter for starch applications in industry. Starch granules are formed in amyloplasts, which are, like chloroplasts, derived from proplastids. Division processes and associated machinery are likely to be similar for all plastids. Essential roles for FtsZ proteins in plastid division in land plants have been revealed. FtsZ forms the so-called Z ring which, together with inner and outer plastid division rings, brings about constriction of the plastid. It has been shown that modulation of the expression level of FtsZ may result in altered chloroplast size and number. To test whether FtsZ is also involved in amyloplast division and whether this, in turn, may affect the starch granule size in crop plants, FtsZ protein levels were either reduced or increased in potato. As shown previously in other plant species, decreased StFtsZ1 protein levels in leaves resulted in a decrease in the number of chloroplasts in guard cells. More interestingly, plants with increased StFtsZ1 protein levels in tubers resulted in less, but larger, starch granules. This suggests that the stoichiometry between StFtsZ1 and other components of the plastid division machinery is important for its function. Starch from these tubers also had altered pasting properties and phosphate content. The importance of our results for the starch industry is discussed.     

Dominant negative mutants of the Cdc2 kinase uncouple cell division from iterative plant development.

Because plant cells do not move and are surrounded by a rigid cell wall, cell division rates and patterns are believed to be directly responsible for generating new structures throughout development. To study the relationship between cell division and morphogenesis, transgenic tobacco and Arabidopsis plants were constructed expressing dominant mutations in a key regulator of the Arabidopsis cell cycle, the Cdc2a kinase. Plants constitutively overproducing the wild-type Cdc2a or the mutant form predicted to accelerate the cell cycle did not exhibit a significantly altered development. In contrast, a mutation expected to arrest the cell cycle abolished cell division when expressed in Arabidopsis, whereas some tobacco plants constitutively producing this mutant protein were recovered. These plants had a reduced histone H1 kinase activity and contained considerably fewer cells. These cells were, however, much larger and underwent normal differentiation. Morphogenesis, histogenesis and developmental timing were unaffected. The results indicate that, in plants, the developmental controls defining shape can act independently from cell division rates.     

A homologue of the bacterial cell division site-determining factor MinD mediates placement of the chloroplast division apparatus

BACKGROUND: Chloroplast division in plant cells occurs by binary fission, yielding two daughter plastids of equal size. Previously, we reported that two Arabidopsis homologues of FtsZ, a bacterial protein that forms a cytokinetic ring during cell division, are essential for plastid division in plants, and may be involved in the formation of plastid-dividing rings on both the stromal and cytosolic surfaces of the chloroplast envelope membranes. In bacteria, positioning of the FtsZ ring at the center of the cell is mediated in part by the protein MinD. Here, we identified AtMinD1, an Arabidopsis homologue of MinD, and investigated whether positioning of the plastid-division apparatus at the plastid midpoint might involve a mechanism similar to that in bacteria. RESULTS: Sequence analysis and in vitro chloroplast import experiments indicated that AtMinD1 contains a transit peptide that targets it to the chloroplast. Transgenic Arabidopsis plants with reduced AtMinD1 expression exhibited variability in chloroplast size and number and asymmetrically constricted chloroplasts, strongly suggesting that the plastid-division machinery is misplaced. Overexpression of AtMinD1 inhibited chloroplast division. These phenotypes resemble those of bacterial mutants with altered minD expression. CONCLUSIONS: Placement of the plastid-division machinery at the organelle midpoint requires a plastid-targeted form of MinD. The results are consistent with a model whereby assembly of the division apparatus is initiated inside the chloroplast by the plastidic form of FtsZ, and suggest that positioning of the cytosolic components of the apparatus is specified by the position of the plastidic components.     

Abscisic acid switches cell division modes of asymmetric cell division and symmetric cell division in stem cells of protonemal filaments in the moss Physcomitrium patens

Multicellular organisms regulate cell numbers and cell fate by using asymmetric cell division (ACD) and symmetric cell division (SCD) during their development and to adapt to unfavorable environmental conditions. A stem cell self-renews and generates differentiated cells. In plants, various types of cells are produced by ACD or SCD; however, the molecular mechanisms of ACD or SCD and the cell division mode switch are largely unknown. The moss Physcomitrium (Physcomitrella) patens is a suitable model to study plant stem cells due to its simple anatomy. Here, we report the cell division mode switch induced by abscisic acid (ABA) in P. patens. ABA is synthesized in response to abiotic stresses and induces round-shape cells, called brood cells, from cylindrical protonemal cells. Although two daughter cells with distinct sizes were produced by ACD in a protonemal stem cell on ABA-free media, the sizes of two daughter cells became similar with ABA treatment. Actin microfilaments were spatially localized on the apices of apical stem cells in protonemata on ABA-free media, but the polar accumulation was lost under the condition of ABA treatment. Moreover, ABA treatment conferred an identical cell fate to the daughter cells in terms of cell division activity. Collectively, the results indicate ABA may suppress the ACD characteristics but evoke SCD in cells. We also noticed that ABA-induced brood cells not only self-renewed but regenerated protonemal cells when ABA was removed from the media, suggesting that brood cells are novel stem cells that are induced by environmental signals in P. patens.