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

Genome-Editing of FtsZ1 for Alteration of Starch Granule Size in Potato Tubers

Genome-editing has enabled rapid improvement for staple food crops, such as potato, a key beneficiary of the technology. In potato, starch contained within tubers represents the primary product for use in food and non-food industries. Starch granules are produced in the plastids of tubers with plastid size correlated with the size of starch grana. The division of plastids is controlled by proteins, including the tubulin-like GTPase FtsZ1. The altered expression of FtsZ1 has been shown to disrupt plastid division, leading to the production of "macro-plastid"-containing plants. These macro-chloroplast plants are characterized by cells containing fewer and enlarged plastids. In this work, we utilize CRISPR/Cas9 to generate FtsZ1 edited potato lines to demonstrate that genome-editing can be used to increase the size of starch granules in tubers. Altered plastid morphology was comparable to the overexpression of FtsZ1 in previous work in potato and other crops. Several lines were generated with up to a 1.98-fold increase in starch granule size that was otherwise phenotypically indistinguishable from wild-type plants. Further, starch paste from one of the most promising lines showed a 2.07-fold increase in final viscosity. The advantages of enlarged starch granules and the potential of CRISPR/Cas9-based technologies for food crop improvement are further discussed.

Interactome of Arabidopsis Thaliana

More than 95,000 protein–protein interactions of Arabidopsis thaliana have been published and deposited in databases. This dataset was supplemented by approximately 900 additional interactions, which were identified in the literature from the years 2002–2021. These protein–protein interactions were used as the basis for a Cytoscape network and were supplemented with data on subcellular localization, gene ontologies, biochemical properties and co-expression. The resulting network has been exemplarily applied in unraveling the PPI-network of the plant vacuolar proton-translocating ATPase (V-ATPase), which was selected due to its central importance for the plant cell. In particular, it is involved in cellular pH homeostasis, providing proton motive force necessary for transport processes, trafficking of proteins and, thereby, cell wall synthesis. The data points to regulation taking place on multiple levels: (a) a phosphorylation-dependent regulation by 14-3-3 proteins and by kinases such as WNK8 and NDPK1a, (b) an energy-dependent regulation via HXK1 and the glucose receptor RGS1 and (c) a Ca2+-dependent regulation by SOS2 and IDQ6. The known importance of V-ATPase for cell wall synthesis is supported by its interactions with several proteins involved in cell wall synthesis. The resulting network was further analyzed for (experimental) biases and was found to be enriched in nuclear, cytosolic and plasma membrane proteins but depleted in extracellular and mitochondrial proteins, in comparison to the entity of protein-coding genes. Among the processes and functions, proteins involved in transcription were highly abundant in the network. Subnetworks were extracted for organelles, processes and protein families. The degree of representation of organelles and processes reveals limitations and advantages in the current knowledge of protein–protein interactions, which have been mainly caused by a high number of database entries being contributed by only a few publications with highly specific motivations and methodologies that favor, for instance, interactions in the cytosol and the nucleus.

The Arabidopsis thaliana chloroplast division protein FtsZ1 counterbalances FtsZ2 filament stability in vitro

Bacterial cell and chloroplast division are driven by a contractile “Z ring” composed of the tubulin-like cytoskeletal GTPase FtsZ. Unlike bacterial Z rings, which consist of a single FtsZ, the chloroplast Z ring in plants is composed of two FtsZ proteins, FtsZ1 and FtsZ2. Both are required for chloroplast division in vivo, but their biochemical relationship is poorly understood. We used GTPase assays, light scattering, transmission electron microscopy, and sedimentation assays to investigate the assembly behavior of purified Arabidopsis thaliana (At) FtsZ1 and AtFtsZ2 both individually and together. Both proteins exhibited GTPase activity. AtFtsZ2 assembled relatively quickly, forming protofilament bundles that were exceptionally stable, as indicated by their sustained assembly and slow disassembly. AtFtsZ1 did not form detectable protofilaments on its own. When mixed with AtFtsZ2, AtFtsZ1 reduced the extent and rate of AtFtsZ2 assembly, consistent with its previously demonstrated ability to promote protofilament subunit turnover in living cells. Mixing the two FtsZ proteins did not increase the overall GTPase activity, indicating that the effect of AtFtsZ1 on AtFtsZ2 assembly was not due to a stimulation of GTPase activity. However, the GTPase activity of AtFtsZ1 was required to reduce AtFtsZ2 assembly. Truncated forms of AtFtsZ1 and AtFtsZ2 consisting of only their conserved core regions largely recapitulated the behaviors of the full-length proteins. Our in vitro findings provide evidence that FtsZ1 counterbalances the stability of FtsZ2 filaments in the regulation of chloroplast Z-ring dynamics and suggest that restraining FtsZ2 self-assembly is a critical function of FtsZ1 in chloroplasts.

Genome-Wide Analysis of Tubulin Gene Family in Cassava and Expression of Family Member FtsZ2-1 during Various Stress

Filamentous temperature-sensitive protein Z (Tubulin/FtsZ) family is a group of conserved GTP-binding (guanine nucleotide-binding) proteins, which are closely related to plant tissue development and organ formation as the major component of the cytoskeleton. According to the published genome sequence information of cassava (Manihot esculenta Crantz), 23 tubulin genes (MeTubulins) were identified, which were divided into four main groups based on their type and phylogenetic characteristics. The same grouping generally has the same or similar motif composition and exon–intron structure. Collinear analysis showed that fragment repetition event is the main factor in amplification of cassava tubulin superfamily gene. The expression profiles of MeTubulin genes in various tissue were analyzed, and it was found that MeTubulins were mainly expressed in leaf, petiole, and stem, while FtsZ2-1 was highly expressed in storage root. The qRT-PCR results of the FtsZ2-1 gene under hormone and abiotic stresses showed that indole-3-acetic acid (IAA) and gibberellin A3 (GA3) stresses could significantly increase the expression of the FtsZ2-1 gene, thereby revealing the potential role of FtsZ2-1 in IAA and GA3 stress-induced responses.

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.

The chloroplast division protein ARC6 acts to inhibit disassembly of GDP-bound FtsZ2

Chloroplasts host photosynthesis and fulfill other metabolic functions that are essential to plant life. They have to divide by binary fission to maintain their numbers throughout cycles of cell division. Chloroplast division is achieved by a complex ring-shaped division machinery located on both the inner (stromal) and the outer (cytosolic) side of the chloroplast envelope. The inner division ring (termed the Z ring) is formed by the assembly of tubulin-like FtsZ1 and FtsZ2 proteins. ARC6 is a key chloroplast division protein that interacts with the Z ring. ARC6 spans the inner envelope membrane, is known to stabilize or maintain the Z ring, and anchors the Z ring to the inner membrane through interaction with FtsZ2. The underlying mechanism of Z ring stabilization is not well-understood. Here, biochemical and structural characterization of ARC6 was conducted using light scattering, sedimentation, and light and transmission EM. The recombinant protein was purified as a dimer. The results indicated that a truncated form of ARC6 (tARC6), representing the stromal portion of ARC6, affects FtsZ2 assembly without forming higher-order structures and exerts its effect via FtsZ2 dynamics. tARC6 prevented GDP-induced FtsZ2 disassembly and caused a significant net increase in FtsZ2 assembly when GDP was present. Single particle analysis and 3D reconstruction were performed to elucidate the structural basis of ARC6 activity. Together, the data reveal that a dimeric form of tARC6 binds to FtsZ2 filaments and does not increase FtsZ polymerization rates but rather inhibits GDP-associated FtsZ2 disassembly.

Bacterial Heterologous Expression System for Reconstitution of Chloroplast Inner Division Ring and Evaluation of Its Contributors

Plant chloroplasts originate from the symbiotic relationship between ancient free-living cyanobacteria and ancestral eukaryotic cells. Since the discovery of the bacterial derivative FtsZ gene—which encodes a tubulin homolog responsible for the formation of the chloroplast inner division ring (Z ring)—in the Arabidopsis genome in 1995, many components of the chloroplast division machinery were successively identified. The knowledge of these components continues to expand; however, the mode of action of the chloroplast dividing system remains unknown (compared to bacterial cell division), owing to the complexities faced in in planta analyses. To date, yeast and bacterial heterologous expression systems have been developed for the reconstitution of Z ring-like structures formed by chloroplast FtsZ. In this review, we especially focus on recent progress of our bacterial system using the model bacterium Escherichia coli to dissect and understand the chloroplast division machinery—an evolutionary hybrid structure composed of both bacterial (inner) and host-derived (outer) components.

Variations in chloroplast movement and chlorophyll fluorescence among chloroplast division mutants under light stress

Chloroplasts divide to maintain consistent size, shape, and number in leaf mesophyll cells. Altered expression of chloroplast division proteins in Arabidopsis results in abnormal chloroplast morphology. To better understand the influence of chloroplast morphology on chloroplast movement and photosynthesis, we compared the chloroplast photorelocation and photosynthetic responses of a series of Arabidopsis chloroplast division mutants with a wide variety of chloroplast phenotypes. Chloroplast movement was monitored by red light reflectance imaging of whole plants under increasing intensities of white light. The accumulation and avoidance responses were differentially affected in different mutants and depended on both chloroplast number and morphological heterogeneity. Chlorophyll fluorescence measurements during 5 d light experiments demonstrated that mutants with large-chloroplast phenotypes generally exhibited greater PSII photodamage than those with intermediate phenotypes. No abnormalities in photorelocation efficiency or photosynthetic capacity were observed in plants with small-chloroplast phenotypes. Simultaneous measurement of chloroplast movement and chlorophyll fluorescence indicated that the energy-dependent (qE) and long-lived components of non-photochemical quenching that reflect photoinhibition are affected differentially in different division mutants exposed to high or fluctuating light intensities. We conclude that chloroplast division mutants with abnormal chloroplast morphologies differ markedly from the wild type in their light adaptation capabilities, which may decrease their relative fitness in nature.

ARC6-mediated Z ring-like structure formation of prokaryote-descended chloroplast FtsZ in Escherichia coli

Plant chloroplasts proliferate through binary fission, and the stromal-side molecules that are involved in chloroplast division are bacterial derivatives. As in bacteria, the prokaryotic tubulin homolog FtsZ assembles into a ring-like structure (Z ring) at mid-chloroplast, and this process is followed by constriction. However, the properties of chloroplast FtsZs remain unclarified. Here, we employed Escherichia coli as a novel heterologous system for expressing chloroplast FtsZs and their regulatory components. Fluorescently labelled Arabidopsis FtsZ2 efficiently assembled into long filaments in E. coli cells, and artificial membrane tethering conferred FtsZ2 filaments with the ability to form Z ring-like structures resembling the bacterial Z ring. A negative regulator of chloroplast FtsZ assembly, ARC3, retained its inhibitory effects on FtsZ2 filamentation and Z ring-like structure formation in E. coli cells. Thus, we provide a novel heterologous system by using bacterial cells to study the regulation of the chloroplast divisome. Furthermore, we demonstrated that the FtsZ2-interacting protein ARC6, which is a potential candidate for Z ring tethering to the chloroplast inner envelope membrane, genuinely targeted FtsZ2 to the membrane components and supported its morphological shift from linear filaments to Z ring-like structures in a manner dependent on the C-terminal ARC6-interacting domain of FtsZ2.

Membrane Proteomics of Arabidopsis Glucosinolate Mutants cyp79B2/B3 and myb28/29

Glucosinolates (Gls) constitute a major group of natural metabolites represented by three major classes (aliphatic, indolic and aromatic) of more than 120 chemical structures. In our previous work, soluble proteins and metabolites in Arabidopsis mutants deficient of aliphatic (myb28/29) and indolic Gls (cyp79B2B3) were analyzed. Here we focus on investigating the changes at the level of membrane proteins in these mutants. Our LC/MS-MS analyses of tandem mass tag (TMT) labeled peptides derived from the cyp79B2/B3 and myb28/29 relative to wild type resulted in the identification of 4,673 proteins, from which 2,171 are membrane proteins. Fold changes and statistical analysis showed 64 increased and 74 decreased in cyp79B2/B3, while 28 increased and 17 decreased in myb28/29. As to the shared protein changes between the mutants, one protein was increased and eight were decreased. Bioinformatics analysis of the changed proteins led to the discovery of three cytochromes in glucosinolate molecular network (GMN): cytochrome P450 86A7 (At1g63710), cytochrome P450 71B26 (At3g26290), and probable cytochrome c (At1g22840). CYP86A7 and CYP71B26 may play a role in hydroxyl-indolic Gls production. In addition, flavone 3'-O-methyltransferase 1 represents an interesting finding as it is likely to participate in the methylation process of the hydroxyl-indolic Gls to form methoxy-indolic Gls. The analysis also revealed additional new nodes in the GMN related to stress and defense activity, transport, photosynthesis, and translation processes. Gene expression and protein levels were found to be correlated in the cyp79B2/B3, but not in the myb28/29.

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.

Mesophyll Chloroplast Investment in C3, C4 and C2 Species of the Genus Flaveria

The mesophyll (M) cells of C4 plants contain fewer chloroplasts than observed in related C3 plants; however, it is uncertain where along the evolutionary transition from C3 to C4 that the reduction in M chloroplast number occurs. Using 18 species in the genus Flaveria, which contains C3, C4 and a range of C3-C4 intermediate species, we examined changes in chloroplast number and size per M cell, and positioning of chloroplasts relative to the M cell periphery. Chloroplast number and coverage of the M cell periphery declined in proportion to increasing strength of C4 metabolism in Flaveria, while chloroplast size increased with increasing C4 cycle strength. These changes increase cytosolic exposure to the cell periphery which could enhance diffusion of inorganic carbon to phosphenolpyruvate carboxylase (PEPC), a cytosolic enzyme. Analysis of the transcriptome from juvenile leaves of nine Flaveria species showed that the transcript abundance of four genes involved in plastid biogenesis-FtsZ1, FtsZ2, DRP5B and PARC6-was negatively correlated with variation in C4 cycle strength and positively correlated with M chloroplast number per planar cell area. Chloroplast size was negatively correlated with abundance of FtsZ1, FtsZ2 and PARC6 transcripts. These results indicate that natural selection targeted the proteins of the contractile ring assembly to effect the reduction in chloroplast numbers in the M cells of C4 Flaveria species. If so, efforts to engineer the C4 pathway into C3 plants might evaluate whether inducing transcriptome changes similar to those observed in Flaveria could reduce M chloroplast numbers, and thus introduce a trait that appears essential for efficient C4 function.

Prospective function of FtsZ proteins in the secondary plastid of chlorarachniophyte algae

BACKGROUND

Division of double-membraned plastids (primary plastids) is performed by constriction of a ring-like division complex consisting of multiple plastid division proteins. Consistent with the endosymbiotic origin of primary plastids, some of the plastid division proteins are descended from cyanobacterial cell division machinery, and the others are of host origin. In several algal lineages, complex plastids, the "secondary plastids", have been acquired by the endosymbiotic uptake of primary plastid-bearing algae, and are surrounded by three or four membranes. Although homologous genes for primary plastid division proteins have been found in genome sequences of secondary plastid-bearing organisms, little is known about the function of these proteins or the mechanism of secondary plastid division.

RESULTS

To gain insight into the mechanism of secondary plastid division, we characterized two plastid division proteins, FtsZD-1 and FtsZD-2, in chlorarachniophyte algae. FtsZ homologs were encoded by the nuclear genomes and carried an N-terminal plastid targeting signal. Immunoelectron microscopy revealed that both FtsZD-1 and FtsZD-2 formed a ring-like structure at the midpoint of bilobate plastids with a projecting pyrenoid in Bigelowiella natans. The ring was always associated with a shallow plate-like invagination of the two innermost plastid membranes. Furthermore, gene expression analysis confirmed that transcripts of ftsZD genes were periodically increased soon after cell division during the B. natans cell cycle, which is not consistent with the timing of plastid division.

CONCLUSIONS

Our findings suggest that chlorarachniophyte FtsZD proteins are involved in partial constriction of the inner pair of plastid membranes, but not in the whole process of plastid division. It is uncertain how the outer pair of plastid membranes is constricted, and as-yet-unknown mechanism is required for the secondary plastid division in chlorarachniophytes.

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.

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.

Analysis of the chloroplast proteome in arc mutants and identification of novel protein components associated with FtsZ2

Chloroplasts are descendants of cyanobacteria and divide by binary fission. The number of chloroplasts is regulated in a cell type-specific manner to ensure that specialized cell types can perform their functions optimally. Several protein components of the chloroplast division apparatus have been identified in the past several years, but how this process is regulated in response to developmental status, environmental signals and stress is still unknown. To begin to address this we undertook a proteomic analysis of three accumulation and replication of chloroplasts mutants that show a spectrum of plastid division perturbations. We show that defects in the chloroplast division process results in changes in the abundance of proteins when compared to wild type, but that the profile of the native stromal and membrane complexes remains unchanged. Furthermore, by combining BN-PAGE with protein interaction assays we show that AtFtsZ2-1 and AtFtsZ2-2 assemble together with rpl12A and EF-Tu into a novel chloroplast membrane complex.

Divide and shape: an endosymbiont in action

The endosymbiotic evolution of the plastid within the host cell required development of a mechanism for efficient division of the plastid. Whilst a model for the mechanism of chloroplast division has been constructed, little is known of how other types of plastids divide, especially the proplastid, the progenitor of all plastid types in the cell. It has become clear that plastid shape is highly heterogeneous and dynamic, especially stromules. This article considers how such variation in morphology might be controlled and how such plastids might divide efficiently.

Genetic mapping and isolation of two arc3 alleles in Arabidopsis

KEY MESSAGE : Two new alleles of arc3 in Arabidopsis thaliana, arc3-4 and arc3-5, were isolated in the Columbia-0 ecotype. The mutants were characterized in detail using microscopy and molecular techniques. Chloroplasts are essential organelles for photosynthesis in plant cells. Division of chloroplasts is coordinated by the internal division machinery (mainly the tubulin-like FtsZ ring) and the external division machinery (mainly the dynamin-like ARC5 ring). Accumulation and replication of chloroplasts3 (ARC3) is important for the correct positioning of chloroplast division machinery. During evolution, ARC3 has probably replaced minicellC (MinC), an important factor involved in positioning of the division site in bacteria. However, the working mechanism of ARC3 is still unclear. Using forward genetic approaches, we isolated two new alleles of arc3 in Arabidopsis thaliana, arc3-4 and arc3-5, in which mutant loci differed from those of previously reported arc3 mutants. Microscopy analyses showed more detailed, and some new, phenotypes of arc3 mutants. Reverse-transcription polymerase chain reaction (RT-PCR) and real-time quantitative RT-PCR (qRT-PCR) results indicated that the mRNA of the ARC3 gene was unstable in arc3-4 and arc3-5 mutant plants. Also, RNA secondary structures of the ARC3 gene were predicted to differ between these two arc3 mutants and wild type. Our studies increase our understanding of the function of ARC3 in chloroplast division.

Distinct functions of chloroplast FtsZ1 and FtsZ2 in Z-ring structure and remodeling

FtsZ, a cytoskeletal GTPase, forms a contractile ring for cell division in bacteria and chloroplast division in plants. Whereas bacterial Z rings are composed of a single FtsZ, those in chloroplasts contain two distinct FtsZ proteins, FtsZ1 and FtsZ2, whose functional relationship is poorly understood. We expressed fluorescently tagged FtsZ1 and FtsZ2 in fission yeast to investigate their intrinsic assembly and dynamic properties. FtsZ1 and FtsZ2 formed filaments with differing morphologies when expressed separately. FRAP showed that FtsZ2 filaments were less dynamic than FtsZ1 filaments and that GTPase activity was essential for FtsZ2 filament turnover but may not be solely responsible for FtsZ1 turnover. When coexpressed, the proteins colocalized, consistent with coassembly, but exhibited an FtsZ2-like morphology. However, FtsZ1 increased FtsZ2 exchange into coassembled filaments. Our findings suggest that FtsZ2 is the primary determinant of chloroplast Z-ring structure, whereas FtsZ1 facilitates Z-ring remodeling. We also demonstrate that ARC3, a regulator of chloroplast Z-ring positioning, functions as an FtsZ1 assembly inhibitor.

Proteomic analysis of chloroplast-to-chromoplast transition in tomato reveals metabolic shifts coupled with disrupted thylakoid biogenesis machinery and elevated energy-production components

A comparative proteomic approach was performed to identify differentially expressed proteins in plastids at three stages of tomato (Solanum lycopersicum) fruit ripening (mature-green, breaker, red). Stringent curation and processing of the data from three independent replicates identified 1,932 proteins among which 1,529 were quantified by spectral counting. The quantification procedures have been subsequently validated by immunoblot analysis of six proteins representative of distinct metabolic or regulatory pathways. Among the main features of the chloroplast-to-chromoplast transition revealed by the study, chromoplastogenesis appears to be associated with major metabolic shifts: (1) strong decrease in abundance of proteins of light reactions (photosynthesis, Calvin cycle, photorespiration) and carbohydrate metabolism (starch synthesis/degradation), mostly between breaker and red stages and (2) increase in terpenoid biosynthesis (including carotenoids) and stress-response proteins (ascorbate-glutathione cycle, abiotic stress, redox, heat shock). These metabolic shifts are preceded by the accumulation of plastid-encoded acetyl Coenzyme A carboxylase D proteins accounting for the generation of a storage matrix that will accumulate carotenoids. Of particular note is the high abundance of proteins involved in providing energy and in metabolites import. Structural differentiation of the chromoplast is characterized by a sharp and continuous decrease of thylakoid proteins whereas envelope and stroma proteins remain remarkably stable. This is coincident with the disruption of the machinery for thylakoids and photosystem biogenesis (vesicular trafficking, provision of material for thylakoid biosynthesis, photosystems assembly) and the loss of the plastid division machinery. Altogether, the data provide new insights on the chromoplast differentiation process while enriching our knowledge of the plant plastid proteome.

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.

Structure, regulation, and evolution of the plastid division machinery

Plastids have evolved from a cyanobacterial endosymbiont, and their continuity is maintained by the plastid division and segregation which is regulated by the eukaryotic host cell. Plastids divide by constriction of the inner- and outer-envelope membranes. Recent studies revealed that this constriction is performed by a large protein and glucan complex at the division site that spans the two envelope membranes. The division complex has retained certain components of the cyanobacterial division complex along with components developed by the host cell. Based on the information on the division complex at the molecular level, we are beginning to understand how the division complex has evolved and how it is assembled, constricted, and regulated in the host cell. This chapter reviews the current understanding of the plastid division machinery and some of the questions that will be addressed in the near future.

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.

Immunofluorescence microscopy for localization of Arabidopsis chloroplast proteins

Immunofluorescence microscopy reveals localization of proteins in cells and tissues by means of highly specific, fluorescently labeled antibodies. This technique is an important complement to localization methods that use genetically encoded fluorescent tags. This chapter describes the five stages of immunofluorescence localization of proteins in plant chloroplasts in sectioned leaf tissue: (1) fixation, (2) tissue embedding and sectioning, (3) treatment of sections prior to immunolabeling, (4) immunostaining, and (5) fluorescence microscopy and image capture. Protocols for both cryosectioning and sectioning of low-melting-point wax-embedded samples are described. Immunofluorescence localization in chloroplasts is complicated by their intense autofluorescence background. Measures to suppress nonspecific background staining, confirm specificity of the fluorescence signal, and optimize imaging conditions are described.

Chloroplast division: squeezing the photosynthetic captive

Chloroplasts have evolved from a cyanobacterial endosymbiont and have been retained in eukaryotic cells for more than one billion years via chloroplast division and inheritance by daughter cells during cell division. Recent studies revealed that chloroplast division is performed by a large protein complex at the division site, encompassing both the inside and the outside of the two envelope membranes. The division complex has retained a few components of the cyanobacterial division complex to go along with other components supplied by the host cell. On the basis of the information about the division complex, we are beginning to understand how the division complex evolved, and how eukaryotic host cells regulate chloroplast division during proliferation and differentiation.

Plastid division

BACKGROUND AND AIMS

Plastids undergo a process of binary fission in order to replicate. Plastid replication is required at two distinct stages of plant growth: during cell division to ensure correct plastid segregation, and during cell expansion and development to generate large populations of functional plastids, as in leaf mesophyll cells. This review considers some of the recent advances in the understanding of how plastids undergo binary fission, a process which uses several different proteins, both internal and external to the plastid, which have been derived from the original endosymbiont's genome as well as new proteins that have been recruited from the host genome.

KEY POINTS

Several of the proteins currently used in this process in higher plants have homologues in modern-day bacteria. An alternative mode of replication by a budding-type mechanism also appears to be used in some circumstances. The review also highlights how most of our knowledge of plastid division is centred on the chloroplast developing in leaf mesophyll cells and a role for plastid division during the development of other plastid types is poorly understood. Whilst models for a protein-based mechanism have been devised, exactly how the division process is controlled at the plastid level and at the plastid population level is poorly understood.

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.

Targeted gene knockouts reveal overlapping functions of the five Physcomitrella patens FtsZ isoforms in chloroplast division, chloroplast shaping, cell patterning, plant development, and gravity sensing

Chloroplasts and bacterial cells divide by binary fission. The key protein in this constriction division is FtsZ, a self-assembling GTPase similar to eukaryotic tubulin. In prokaryotes, FtsZ is almost always encoded by a single gene, whereas plants harbor several nuclear-encoded FtsZ homologs. In seed plants, these proteins group in two families and all are exclusively imported into plastids. In contrast, the basal land plant Physcomitrella patens, a moss, encodes a third FtsZ family with one member. This protein is dually targeted to the plastids and to the cytosol. Here, we report on the targeted gene disruption of all ftsZ genes in P. patens. Subsequent analysis of single and double knockout mutants revealed a complex interaction of the different FtsZ isoforms not only in plastid division, but also in chloroplast shaping, cell patterning, plant development, and gravity sensing. These results support the concept of a plastoskeleton and its functional integration into the cytoskeleton, at least in the moss P. patens.

Amyloplast division progresses simultaneously at multiple sites in the endosperm of rice

The amyloplast, a form of differentiated plastid, proliferates in sink tissues, where it synthesizes and stores starch granules. Little is known about the molecular mechanism for amyloplast division and development. The rice (Oryza sativa) endosperm provides an excellent model system for studying molecular mechanisms involved in amyloplast division and starch synthesis. We compared amyloplast division processes in the endosperm of wild type and a mutant of ARC5, a member of the dynamin superfamily. Plant growth and fertility of arc5 were not significantly different from the wild type. Unlike binary fission of chloroplast in the leaf, small amyloplasts in the endosperm of wild type divide simultaneously at multiple sites, generating a beads-on-a-string structure. In addition, large amyloplasts divide by budding-type division, giving rise to small amyloplasts attached to their surfaces. ARC5 and FtsZ2-1 fused to fluorescent proteins were targeted to the constriction sites in dividing amyloplasts. Both the loss of function of ARC5 and overexpression of ARC5 fusion proteins in the endosperm did not produce spherical amyloplasts with increased diameter, but produced either fused amyloplasts with thick connections or pleomorphic types, suggesting that proper stoichiometry between ARC5 and other components in the amyloplast division machinery is necessary for the completion of the late stage of amyloplast division. The size distribution of starch granules purified from arc5 was shifted to small and the starch gelatinization peak temperature was significantly higher than for wild-type starch, suggesting that amyloplast division processes have a significant effect on starch synthesis.

PARC6, a novel chloroplast division factor, influences FtsZ assembly and is required for recruitment of PDV1 during chloroplast division in Arabidopsis

Chloroplast division in plant cells is accomplished through the coordinated action of the tubulin-like FtsZ ring inside the organelle and the dynamin-like ARC5 ring outside the organelle. This coordination is facilitated by ARC6, an inner envelope protein required for both assembly of FtsZ and recruitment of ARC5. Recently, we showed that ARC6 specifies the mid-plastid positioning of the outer envelope proteins PDV1 and PDV2, which have parallel functions in dynamin recruitment. PDV2 positioning involves direct ARC6-PDV2 interaction, but PDV1 and ARC6 do not interact indicating that an additional factor functions downstream of ARC6 to position PDV1. Here, we show that PARC6 (paralog of ARC6), an ARC6-like protein unique to vascular plants, fulfills this role. Like ARC6, PARC6 is an inner envelope protein with its N-terminus exposed to the stroma and Arabidopsis parc6 mutants exhibit defects of chloroplast and FtsZ filament morphology. However, whereas ARC6 promotes FtsZ assembly, PARC6 appears to inhibit FtsZ assembly, suggesting that ARC6 and PARC6 function as antagonistic regulators of FtsZ dynamics. The FtsZ inhibitory activity of PARC6 may involve its interaction with the FtsZ-positioning factor ARC3. A PARC6-GFP fusion protein localizes both to the mid-plastid and to a single spot at one pole, reminiscent of the localization of ARC3, PDV1 and ARC5. Although PARC6 localizes PDV1, it is not required for PDV2 localization or ARC5 recruitment. Our findings indicate that PARC6, like ARC6, plays a role in coordinating the internal and external components of the chloroplast division complex, but that PARC6 has evolved distinct functions in the division process.

Plastid chaperonin proteins Cpn60 alpha and Cpn60 beta are required for plastid division in Arabidopsis thaliana

BACKGROUND

Plastids arose from a free-living cyanobacterial endosymbiont and multiply by binary division as do cyanobacteria. Plastid division involves nucleus-encoded homologs of cyanobacterial division proteins such as FtsZ, MinD, MinE, and ARC6. However, homologs of many other cyanobacterial division genes are missing in plant genomes and proteins of host eukaryotic origin, such as a dynamin-related protein, PDV1 and PDV2 are involved in the division process. Recent identification of plastid division proteins has started to elucidate the similarities and differences between plastid division and cyanobacterial cell division. To further identify new proteins that are required for plastid division, we characterized previously and newly isolated plastid division mutants of Arabidopsis thaliana.

RESULTS

Leaf cells of two mutants, br04 and arc2, contain fewer, larger chloroplasts than those of wild type. We found that ARC2 and BR04 are identical to nuclear genes encoding the plastid chaperonin 60 alpha (ptCpn60alpha) and chaperonin 60 beta (ptCpn60beta) proteins, respectively. In both mutants, plastid division FtsZ ring formation was partially perturbed though the level of FtsZ2-1 protein in plastids of ptcpn60beta mutants was similar to that in wild type. Phylogenetic analyses showed that both ptCpn60 proteins are derived from ancestral cyanobacterial proteins. The A. thaliana genome encodes two members of ptCpn60alpha family and four members of ptCpn60beta family respectively. We found that a null mutation in ptCpn60alpha abolished greening of plastids and resulted in an albino phenotype while a weaker mutation impairs plastid division and reduced chlorophyll levels. The functions of at least two ptCpn60beta proteins are redundant and the appearance of chloroplast division defects is dependent on the number of mutant alleles.

CONCLUSION

Our results suggest that both ptCpn60alpha and ptCpn60beta are required for the formation of a normal plastid division apparatus, as the prokaryotic counterparts are required for assembly of the cell division apparatus. Since moderate reduction of ptCpn60 levels impaired normal FtsZ ring formation but not import of FtsZ into plastids, it is suggested that the proper levels of ptCpn60 are required for folding of stromal plastid division proteins and/or regulation of FtsZ polymer dynamics.

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.

MULTIPLE FtsZ RING FORMATION AND REDUPLICATED CHLOROPLAST DNA IN NANNOCHLORIS BACILLARIS (CHLOROPHYTA, TREBOUXIOPHYCEAE) UNDER PHOSPHATE-ENRICHED CULTURE(1)

We examined the effects of phosphate enrichment on chloroplasts of the unicellular green alga Nannochloris bacillaris Naumann. The doubling time of cells was similar in phosphate-limited (no β-glycerophosphate) and phosphate-enriched (2 mM β-glycerophosphate) media. The lengths of cells and chloroplasts were similar, regardless of phosphate concentration. The relationship between the ring formation of the prokaryote-derived chloroplast division protein FtsZ and phosphate concentration was examined using indirect fluorescent antibody staining. The number of FtsZ rings increased as the phosphate concentration of the medium increased. Multiple FtsZ rings were formed in cells in phosphate-enriched medium; up to six FtsZ rings per chloroplast were observed. The number of FtsZ rings increased as the chloroplast grew. The FtsZ ring located near the center of the chloroplast had the strongest fluorescence. The FtsZ ring at the relative center of all FtsZ rings was used for division. Plastid division rings did not multiply in phosphate-enriched culture. The chloroplast DNA content was 2.3 times greater in phosphate-enriched than in phosphate-limited culture and decreased in cells cultured in phosphate-enriched medium containing 5-fluorodeoxyuridine (FdUr). In the presence of FdUr, only one FtsZ ring formed, even under phosphate enrichment. This finding suggests that excessive chloroplast DNA replication induces multiple FtsZ ring formation in phosphate-enriched culture. We propose a multiple FtsZ ring formation model under phosphate enrichment.

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.

Effects of arc3, arc5 and arc6 mutations on plastid morphology and stromule formation in green and nongreen tissues of Arabidopsis thaliana

Mutations in the ARC3, ARC5 and ARC6 genes of Arabidopsis thaliana affect chloroplast division. We investigated whether ARC3, ARC5 and ARC6 are also involved in determining plastid morphology in nongreen tissues, where stromules, stroma-filled tubular extensions of the plastid envelope membrane, are more abundant than in mesophyll cells. Using plastid-targeted green fluorescent protein to observe plastids throughout the organs of these mutants, we have discovered a number of new mutant phenotypes. The size of arc3 plastids was heterogeneous in various tissues. arc5 plastids appeared wild-type in the majority of nongreen tissues examined. However, in cells of stamen filaments, the arc5 mutant showed an increase in the frequency of stromules. Increased stromule frequency was observed for a number of organs in the arc6 mutant. Some arc6 cells contained heterogeneous mixtures of plastids; epidermal cells of hypocotyls, stamen filaments and the bases of petals possessed both very large chloroplasts as well as much smaller nongreen plastids. Quantitative analysis in hypocotyl cells revealed that the alteration in stromule length in arc3 and arc6 mutants occurred despite wild-type plastid densities. Thus, in hypocotyls, the effects of the arc3 and arc6 mutations on stromule length and frequency are independent of changes in plastid division. Although electron micrographs of stromules emanating from chloroplasts have rarely been reported, within the arc3 mutant, narrow, 40-50 nm diameter, recoiled stromules could be followed for about 10 microm in electron micrographs of leaf tissue.