Chloroplast division and morphology are differentially affected by overexpression of FtsZ1 and FtsZ2 genes in Arabidopsis

Transcriptomic study of the role of MeFtsZ2-1 in pigment accumulation in cassava leaves

MeFtsZ2-1 is a key gene for plant plastid division, but the mechanism by which MeFtsZ2-1 affects pigment accumulation in cassava (Manihot esculenta Crantz) through plastids remains unclear. We found that MeFtsZ2-1 overexpression in cassava (OE) exhibited darker colors of leaves, with increased levels of anthocyanins and carotenoids. Further observation via Transmission Electron Microscopy (TEM) revealed no apparent defects in chloroplast structure but an increase in the number of plastoglobule in OE leaves. RNA-seq results showed 1582 differentially expressed genes (DEGs) in leaves of OE. KEGG pathway analysis indicated that these DEGs were enriched in pathways related to flavonoid, anthocyanin, and carotenoid biosynthesis. This study reveals the role of MeFtsZ2-1 in cassava pigment accumulation from a physiological and transcriptomic perspective, providing a theoretical basis for improving cassava quality.

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

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

Engineering the plant metabolic system by exploiting metabolic regulation

Plants are the most sophisticated biofactories and sources of food and biofuels present in nature. By engineering plant metabolism, the production of desired compounds can be increased and the nutritional or commercial value of the plant species can be improved. However, this can be challenging because of the complexity of the regulation of multiple genes and the involvement of different protein interactions. To improve metabolic engineering (ME) capabilities, different tools and strategies for rerouting the metabolic pathways have been developed, including genome editing and transcriptional regulation approaches. In addition, cutting-edge technologies have provided new methods for understanding uncharacterized biosynthetic pathways, protein degradation mechanisms, protein-protein interactions, or allosteric feedback, enabling the design of novel ME approaches.

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.

Meeting in the Middle: Lessons and Opportunities from Studying C3-C4 Intermediates

The discovery of C₃-C₄ intermediate species nearly 50 years ago opened up a new avenue for studying the evolution of photosynthetic pathways. Intermediate species exhibit anatomical, biochemical, and physiological traits that range from C₃ to C₄. A key feature of C₃-C₄ intermediates that utilize C₂ photosynthesis is the improvement in photosynthetic efficiency compared with C₃ species. Although the recruitment of some core enzymes is shared across lineages, there is significant variability in gene expression patterns, consistent with models that suggest numerous evolutionary paths from C₃ to C₄ photosynthesis. Despite the many evolutionary trajectories, the recruitment of glycine decarboxylase for C₂ photosynthesis is likely required. As technologies enable high-throughput genotyping and phenotyping, the discovery of new C₃-C₄ intermediates species will enrich comparisons between evolutionary lineages. The investigation of C₃-C₄ intermediate species will enhance our understanding of photosynthetic mechanisms and evolutionary processes and will potentially aid in crop improvement.

A novel amphiphilic motif at the C-terminus of FtsZ1 facilitates chloroplast division

In bacteria and chloroplasts, the GTPase filamentous temperature-sensitive Z (FtsZ) is essential for division and polymerizes to form rings that mark the division site. Plants contain two FtsZ subfamilies (FtsZ1 and FtsZ2) with different assembly dynamics. FtsZ1 lacks the C-terminal domain of a typical FtsZ protein. Here, we show that the conserved short motif FtsZ1Carboxyl-terminus (Z1C) (consisting of the amino acids RRLFF) with weak membrane-binding activity is present at the C-terminus of FtsZ1 in angiosperms. For a polymer-forming protein such as FtsZ, this activity is strong enough for membrane tethering. Arabidopsis thaliana plants with mutated Z1C motifs contained heterogeneously sized chloroplasts and parallel FtsZ rings or long FtsZ filaments, suggesting that the Z1C motif plays an important role in regulating FtsZ ring dynamics. Our findings uncover a type of amphiphilic beta-strand motif with weak membrane-binding activity and point to the importance of this motif for the dynamic regulation of protein complex formation.

Residue 49 of AtMinD1 Plays a Key Role in the Guidance of Chloroplast Division by Regulating the ARC6-AtMinD1 Interaction

Chloroplasts evolved from a free-living cyanobacterium through endosymbiosis. Similar to bacterial cell division, chloroplasts replicate by binary fission, which is controlled by the Minicell (Min) system through confining FtsZ ring formation at the mid-chloroplast division site. MinD, one of the most important members of the Min system, regulates the placement of the division site in plants and works cooperatively with MinE, ARC3, and MCD1. The loss of MinD function results in the asymmetric division of chloroplasts. In this study, we isolated one large dumbbell-shaped and asymmetric division chloroplast Arabidopsis mutant Chloroplast Division Mutant 75 (cdm75) that contains a missense mutation, changing the arginine at residue 49 to a histidine (R49H), and this mutant point is located in the N-terminal Conserved Terrestrial Sequence (NCTS) motif of AtMinD1, which is only typically found in terrestrial plants. This study provides sufficient evidence to prove that residues 1-49 of AtMinD1 are transferred into the chloroplast, and that the R49H mutation does not affect the function of the AtMinD1 chloroplast transit peptide. Subsequently, we showed that the point mutation of R49H could remove the punctate structure caused by residues 1-62 of the AtMinD1 sequence in the chloroplast, suggesting that the arginine in residue 49 (Arg49) is essential for localizing the punctate structure of AtMinD1_{1 - 62} on the chloroplast envelope. Unexpectedly, we found that AtMinD1 could interact directly with ARC6, and that the R49H mutation could prevent not only the previously observed interaction between AtMinD1 and MCD1 but also the interaction between AtMinD1 and ARC6. Thus, we believe that these results show that the AtMinD1 NCTS motif is required for their protein interaction. Collectively, our results show that AtMinD1 can guide the placement of the division site to the mid chloroplast through its direct interaction with ARC6 and reveal the important role of AtMinD1 in regulating the AtMinD1-ARC6 interaction.

Generation, analysis, and transformation of macro-chloroplast Potato (Solanum tuberosum) lines for chloroplast biotechnology

Chloroplast biotechnology is a route for novel crop metabolic engineering. The potential bio-confinement of transgenes, the high protein expression and the possibility to organize genes into operons represent considerable advantages that make chloroplasts valuable targets in agricultural biotechnology. In the last 3 decades, chloroplast genomes from a few economically important crops have been successfully transformed. The main bottlenecks that prevent efficient transformation in a greater number of crops include the dearth of proven selectable marker gene-selection combinations and tissue culture methods for efficient regeneration of transplastomic plants. The prospects of increasing organelle size are attractive from several perspectives, including an increase in the surface area of potential targets. As a proof-of-concept, we generated Solanum tuberosum (potato) macro-chloroplast lines overexpressing the tubulin-like GTPase protein gene FtsZ1 from Arabidopsis thaliana. Macro-chloroplast lines exhibited delayed growth at anthesis; however, at the time of harvest there was no significant difference in height between macro-chloroplast and wild-type lines. Macro-chloroplasts were successfully transformed by biolistic DNA-delivery and efficiently regenerated into homoplasmic transplastomic lines. We also demonstrated that macro-chloroplasts accumulate the same amount of heterologous protein than wild-type organelles, confirming efficient usage in plastid engineering. Advantages and limitations of using enlarge compartments in chloroplast biotechnology are discussed.

Allelic Variation in the Chloroplast Division Gene FtsZ2-2 Leads to Natural Variation in Chloroplast Size

Chloroplast size varies considerably in nature, but the underlying mechanisms are unknown. By exploiting a near-isogenic line population derived from a cross between the Arabidopsis (Arabidopsis thaliana) accessions Cape Verde Islands (Cvi-1), which has larger chloroplasts, and Landsberg erecta (Ler-0), with smaller chloroplasts, we determined that the large-chloroplast phenotype in Cvi-1 is associated with allelic variation in the gene encoding the chloroplast-division protein FtsZ2-2, a tubulin-related cytoskeletal component of the contractile FtsZ ring inside chloroplasts. Sequencing revealed that the Cvi-1 FtsZ2-2 allele encodes a C-terminally truncated protein lacking a region required for FtsZ2-2 interaction with inner-envelope proteins, and functional complementation experiments in a Columbia-0 ftsZ2-2 null mutant confirmed this allele as causal for the increased chloroplast size in Cvi-1. Comparison of FtsZ2-2 coding sequences in the 1001 Genomes database showed that the Cvi-1 allele is rare and identified additional rare loss-of-function alleles, including a natural null allele, in three other accessions, all of which had enlarged-chloroplast phenotypes. The ratio of nonsynonymous to synonymous substitutions was higher among the FtsZ2-2 genes than among the two other FtsZ family members in Arabidopsis, FtsZ2-1, a close paralog of FtsZ2-2, and the functionally distinct FtsZ1-1, indicating more relaxed constraint on the FtsZ2-2 coding sequence than on those of FtsZ2-1 or FtsZ1-1 Our results establish that allelic variation in FtsZ2-2 contributes to natural variation in chloroplast size in Arabidopsis, and they also demonstrate that natural variation in Arabidopsis can be used to decipher the genetic basis of differences in fundamental cell biological traits, such as organelle size.

Arabidopsis thaliana Leaf Epidermal Guard Cells: A Model for Studying Chloroplast Proliferation and Partitioning in Plants

The existence of numerous chloroplasts in photosynthetic cells is a general feature of plants. Chloroplast biogenesis and inheritance involve two distinct mechanisms: proliferation of chloroplasts by binary fission and partitioning of chloroplasts into daughter cells during cell division. The mechanism of chloroplast number coordination in a given cell type is a fundamental question. Stomatal guard cells (GCs) in the plant shoot epidermis generally contain several to tens of chloroplasts per cell. Thus far, chloroplast number at the stomatal (GC pair) level has generally been used as a convenient marker for identifying hybrid species or estimating the ploidy level of a given plant tissue. Here, we report that Arabidopsis thaliana leaf GCs represent a useful system for investigating the unexploited aspects of chloroplast number control in plant cells. In contrast to a general notion based on analyses of leaf mesophyll chloroplasts, a small difference was detected in the GC chloroplast number among three Arabidopsis ecotypes (Columbia, Landsberg erecta, and Wassilewskija). Fluorescence microscopy often detected dividing GC chloroplasts with the FtsZ1 ring not only at the early stage of leaf expansion but also at the late stage. Compensatory chloroplast expansion, a phenomenon well documented in leaf mesophyll cells of chloroplast division mutants and transgenic plants, could take place between paired GCs in wild-type leaves. Furthermore, modest chloroplast number per GC as well as symmetric division of guard mother cells for GC formation suggests that Arabidopsis GCs would facilitate the analysis of chloroplast partitioning, based on chloroplast counting at the individual cell level.

Exposure to High-Intensity Light Systemically Induces Micro-Transcriptomic Changes in Arabidopsis thaliana Roots

In full sunlight, plants often experience a light intensity exceeding their photosynthetic capacity and causing the activation of a set of photoprotective mechanisms. Numerous reports have explained, on the molecular level, how plants cope with light stress locally in photosynthesizing leaves; however, the response of below-ground organs to above-ground perceived light stress is still largely unknown. Since small RNAs are potent integrators of multiple processes including stress responses, here, we focus on changes in the expression of root miRNAs upon high-intensity-light (HL) stress. To achieve this, we used Arabidopsis thaliana plants growing in hydroponic conditions. The expression of several genes that are known as markers of redox changes was examined over time, with the results showing that typical HL stress signals spread to the below-ground organs. Additionally, micro-transcriptomic analysis of systemically stressed roots revealed a relatively limited reaction, with only 17 up-regulated and five down-regulated miRNAs. The differential expression of candidates was confirmed by RT-qPCR. Interestingly, the detected differences in miRNA abundance disappeared when the roots were separated from the shoots before HL treatment. Thus, our results show that the light stress signal is induced in rosettes and travels through the plant to affect root miRNA levels. Although the mechanism of this regulation is unknown, the engagement of miRNA may create a regulatory platform orchestrating adaptive responses to various simultaneous stresses. Consequently, further research on systemically HL-regulated miRNAs and their respective targets has the potential to identify attractive sequences for engineering stress tolerance in plants.

ARC3 Activation by PARC6 Promotes FtsZ-Ring Remodeling at the Chloroplast Division Site

Chloroplast division is initiated by assembly of the stromal Z ring, composed of cytoskeletal Filamenting temperature-sensitive Z (FtsZ) proteins. Midplastid Z-ring positioning is governed by the chloroplast Min (Minicell) system, which inhibits Z-ring assembly everywhere except the division site. The central Min-system player is the FtsZ-assembly inhibitor ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3). Here, we report Arabidopsis (Arabidopsis thaliana) chloroplasts contain two pools of ARC3: one distributed throughout the stroma, which presumably fully inhibits Z-ring assembly at nondivision sites, and the other localized to a midplastid ring-like structure. We show that ARC3 is recruited to the middle of the plastid by the inner envelope membrane protein PARALOG OF ARC6 (PARC6). ARC3 bears a C-terminal Membrane Occupation and Recognition Nexus (MORN) domain; previous yeast two-hybrid experiments with full-length and MORN-truncated ARC3 showed the MORN domain mediates ARC3-PARC6 interaction but prevents ARC3-FtsZ interaction. Using yeast three-hybrid experiments, we demonstrate that the MORN-dependent ARC3-PARC6 interaction enables full-length ARC3 to bind FtsZ. The resulting PARC6/ARC3/FtsZ complex enhances the dynamics of Z rings reconstituted in a heterologous system. Our findings lead to a model whereby activation of midplastid-localized ARC3 by PARC6 facilitates Z-ring remodeling during chloroplast division by promoting Z-ring dynamics and reveal a novel function for MORN domains in regulating protein-protein interactions.

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.

Conserved Dynamics of Chloroplast Cytoskeletal FtsZ Proteins Across Photosynthetic Lineages

The cytoskeletal Filamenting temperature-sensitive Z (FtsZ) ring is critical for cell division in bacteria and chloroplast division in photosynthetic eukaryotes. While bacterial FtsZ rings are composed of a single FtsZ, except in the basal glaucophytes, chloroplast division involves two heteropolymer-forming FtsZ isoforms: FtsZ1 and FtsZ2 in the green lineage and FtsZA and FtsZB in red algae. FtsZ1 and FtsZB probably arose by duplication of the more ancestral FtsZ2 and FtsZA, respectively. We expressed fluorescent fusions of FtsZ from diverse photosynthetic organisms in a heterologous system to compare their intrinsic assembly and dynamic properties. FtsZ2 and FtsZA filaments were morphologically distinct from FtsZ1 and FtsZB filaments. When coexpressed, FtsZ pairs from plants and algae colocalized, consistent with heteropolymerization. Fluorescence recovery after photobleaching experiments demonstrated that subunit exchange was greater from FtsZ1 and FtsZB filaments than from FtsZ2 and FtsZA filaments and that FtsZ1 and FtsZB increased turnover of FtsZ2 and FtsZA, respectively, from heteropolymers. GTPase activity was essential only for turnover of FtsZ2 and FtsZA filaments. Cyanobacterial and glaucophyte FtsZ properties mostly resembled those of FtsZ2 and FtsZA, though the glaucophyte protein exhibited some hybrid features. Our results demonstrate that the more ancestral FtsZ2 and FtsZA have retained functional attributes of their common FtsZ ancestor, while eukaryotic-specific FtsZ1 and FtsZB acquired new but similar dynamic properties, possibly through convergent evolution. Our findings suggest that the evolution of a second FtsZ that could copolymerize with the more ancestral form to enhance FtsZ-ring dynamics may have been essential for plastid evolution in the green and red photosynthetic lineages.

Isolation and Characterization of Ftsz Genes in Cassava

The filamenting temperature-sensitive Z proteins (FtsZs) play an important role in plastid division. In this study, three FtsZ genes were isolated from the cassava genome, and named MeFtsZ1, MeFtsZ2-1, and MeFtsZ2-2, respectively. Based on phylogeny, the MeFtsZs were classified into two groups (FtsZ1 and FtsZ2). MeFtsZ1 with a putative signal peptide at N-terminal, has six exons, and is classed to FtsZ1 clade. MeFtsZ2-1 and MeFtsZ2-2 without a putative signal peptide, have seven exons, and are classed to FtsZ2 clade. Subcellular localization found that all the three MeFtsZs could locate in chloroplasts and form a ring in chloroplastids. Structure analysis found that all MeFtsZ proteins contain a conserved guanosine triphosphatase (GTPase) domain in favor of generate contractile force for cassava plastid division. The expression profiles of MeFtsZ genes by quantitative reverse transcription-PCR (qRT-PCR) analysis in photosynthetic and non-photosynthetic tissues found that all of the MeFtsZ genes had higher expression levels in photosynthetic tissues, especially in younger leaves, and lower expression levels in the non-photosynthetic tissues. During cassava storage root development, the expressions of MeFtsZ2-1 and MeFtsZ2-2 were comparatively higher than MeFtsZ1. The transformed Arabidopsis of MeFtsZ2-1 and MeFtsZ2-2 contained abnormally shape, fewer number, and larger volume chloroplasts. Phytohormones were involved in regulating the expressions of MeFtsZ genes. Therefore, we deduced that all of the MeFtsZs play an important role in chloroplast division, and that MeFtsZ2 (2-1, 2-2) might be involved in amyloplast division and regulated by phytohormones during cassava storage root development.

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

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

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

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

Chloroplast division checkpoint in eukaryotic algae

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

Roles of Arabidopsis PARC6 in Coordination of the Chloroplast Division Complex and Negative Regulation of FtsZ Assembly

Chloroplast division is driven by the simultaneous constriction of the inner FtsZ ring (Z ring) and the outer DRP5B ring. The assembly and constriction of these rings in Arabidopsis (Arabidopsis thaliana) are coordinated partly through the inner envelope membrane protein ACCUMULATION AND REPLICATION OF CHLOROPLASTS6 (ARC6). Previously, we showed that PARC6 (PARALOG OF ARC6), also in the inner envelope membrane, negatively regulates FtsZ assembly and acts downstream of ARC6 to position the outer envelope membrane protein PLASTID DIVISION1 (PDV1), which functions together with its paralog PDV2 to recruit DYNAMIN-RELATED PROTEIN 5B (DRP5B) from a cytosolic pool to the outer envelope membrane. However, whether PARC6, like ARC6, also functions in coordination of the chloroplast division contractile complexes was unknown. Here, we report a detailed topological analysis of Arabidopsis PARC6, which shows that PARC6 has a single transmembrane domain and a topology resembling that of ARC6. The newly identified stromal region of PARC6 interacts not only with ARC3, a direct inhibitor of Z-ring assembly, but also with the Z-ring protein FtsZ2. Overexpression of PARC6 inhibits FtsZ assembly in Arabidopsis but not in a heterologous yeast system (Schizosaccharomyces pombe), suggesting that the negative regulation of FtsZ assembly by PARC6 is a consequence of its interaction with ARC3. A conserved carboxyl-terminal peptide in FtsZ2 mediates FtsZ2 interaction with both PARC6 and ARC6. Consistent with its role in the positioning of PDV1, the intermembrane space regions of PARC6 and PDV1 interact. These findings provide new insights into the functions of PARC6 and suggest that PARC6 coordinates the inner Z ring and outer DRP5B ring through interaction with FtsZ2 and PDV1 during chloroplast division.

Arabidopsis FHY3/CPD45 regulates far-red light signaling and chloroplast division in parallel

CPD45 (chloroplast division45),which is also known as FHY3 (far-red elongated hypocotyl3), is a key factor in the far-red light signaling pathway in Arabidopsis. We previously showed that FHY3/CPD45 also regulates chloroplast division. Because light is also a regulator of chloroplast development and division, we sought to clarify the relationship between far-red light signaling and chloroplast division pathways. We found that the chloroplast division mutant arc5-3 had no defect in far-red light sensing, and that constitutive overexpression of ARC5 rescued the chloroplast division defect, but not the defect in far-red light signaling, of cpd45. fhy1, which is defective in far-red light signaling, exhibited normal chloroplast division. Constitutive overexpression of FHY1 rescued the far-red light signaling defect, but not the chloroplast division defect, of cpd45. Moreover, ARC5 and FHY1 expression were not affected in fhy1 and arc5-3, respectively. Based on these results, we propose that FHY3/CPD45 regulates far-red light signaling and chloroplast division in parallel by activating the expression of FHY1 and ARC5 independently. This work demonstrates how relationships between different pathways in a gene regulatory network can be explored.

FtsZ1/FtsZ2 Turnover in Chloroplasts and the Role of ARC3

Chloroplast division requires filamentation temperature-sensitive Z (FtsZ), a tubulin-like GTPase of cyanobacterial endosymbiotic origin. Plants and algae possess two distinct FtsZ protein families, FtsZ1 and FtsZ2 that co-assemble into a ring (Z-ring) at the division site. Z-ring assembly and disassembly and division site positioning is controlled by both positive and negative factors via their specific interactions with FtsZ1 and FtsZ2. Here we present the in planta analysis of Arabidopsis FtsZ1 and FtsZ2 turnover in the context of a native chloroplast division machinery. Fluorescence recovery after photobleaching analysis was conducted using fluorescently tagged FtsZ at wild-type (WT)-like levels. Rapid photobleaching, low signal-to-noise ratio, and phototropic movements of chloroplasts were overcome by (i) using progressive intervals in time-lapse imaging, (ii) analyzing epidermal rather than stromal chloroplasts, and (iii) employing image stack alignment during postprocessing. In plants of WT background, fluorescence recovery half-times averaged 117 and 325 s for FtsZ1 and FtsZ2, respectively. In plants lacking ARC3, the key negative regulator of FtsZ assembly, the turnover was threefold slower. The findings are discussed in the context of previous results conducted in a heterologous system.

The endosomal protein CHARGED MULTIVESICULAR BODY PROTEIN1 regulates the autophagic turnover of plastids in Arabidopsis

Endosomal Sorting Complex Required for Transport (ESCRT)-III proteins mediate membrane remodeling and the release of endosomal intraluminal vesicles into multivesicular bodies. Here, we show that the ESCRT-III subunit paralogs CHARGED MULTIVESICULAR BODY PROTEIN1 (CHMP1A) and CHMP1B are required for autophagic degradation of plastid proteins in Arabidopsis thaliana. Similar to autophagy mutants, chmp1a chmp1b (chmp1) plants hyperaccumulated plastid components, including proteins involved in plastid division. The autophagy machinery directed the release of bodies containing plastid material into the cytoplasm, whereas CHMP1A and B were required for delivery of these bodies to the vacuole. Autophagy was upregulated in chmp1 as indicated by an increase in vacuolar green fluorescent protein (GFP) cleavage from the autophagic reporter GFP-ATG8. However, autophagic degradation of the stromal cargo RECA-GFP was drastically reduced in the chmp1 plants upon starvation, suggesting that CHMP1 mediates the efficient delivery of autophagic plastid cargo to the vacuole. Consistent with the compromised degradation of plastid proteins, chmp1 plastids show severe morphological defects and aberrant division. We propose that CHMP1 plays a direct role in the autophagic turnover of plastid constituents.

Promotion of chloroplast proliferation upon enhanced post-mitotic cell expansion in leaves

BACKGROUND

Leaves are determinate organs; hence, precise control of cell proliferation and post-mitotic cell expansion is essential for their growth. A defect in cell proliferation often triggers enhanced post-mitotic cell expansion in leaves. This phenomenon is referred to as 'compensation'. Several lines of evidence from studies on compensation have shown that cell proliferation and post-mitotic cell expansion are coordinately regulated during leaf development. Therefore, compensation has attracted much attention to the mechanisms for leaf growth. However, our understanding of compensation at the subcellular level remains limited because studies of compensation have focused mainly on cellular-level phenotypes. Proper leaf growth requires quantitative control of subcellular components in association with cellular-level changes. To gain insight into the subcellular aspect of compensation, we investigated the well-known relationship between cell area and chloroplast number per cell in compensation-exhibiting lines, and asked whether chloroplast proliferation is modulated in response to the induction of compensation.

RESULTS

We first established a convenient and reliable method for observation of chloroplasts in situ. Using this method, we analyzed Arabidopsis thaliana mutants fugu5 and angustifolia3 (an3), and a transgenic line KIP-RELATED PROTEIN2 overexpressor (KRP2 OE), which are known to exhibit typical features of compensation. We here showed that chloroplast number per cell increased in the subepidermal palisade tissue of these lines. We analyzed tetraploidized wild type, fugu5, an3 and KRP2 OE, and found that cell area itself, but not nuclear ploidy, is a key parameter that determines the activity of chloroplast proliferation. In particular, in the case of an3, we uncovered that promotion of chloroplast proliferation depends on the enhanced post-mitotic cell expansion. The expression levels of chloroplast proliferation-related genes are similar to or lower than that in the wild type during this process.

CONCLUSIONS

This study demonstrates that chloroplast proliferation is promoted in compensation-exhibiting lines. This promotion of chloroplast proliferation takes place in response to cell-area increase in post-mitotic phase in an3. The expression of chloroplast proliferation-related genes were not promoted in compensation-exhibiting lines including an3, arguing that an as-yet-unknown mechanism is responsible for modulation of chloroplast proliferation in these lines.

SUB1A-mediated submergence tolerance response in rice involves differential regulation of the brassinosteroid pathway

· Submergence 1A (SUB1A), is an ethylene response factor (ERF) that confers submergence tolerance in rice (Oryza sativa) via limiting shoot elongation during the inundation period. SUB1A has been proposed to restrict shoot growth by modulating gibberellic acid (GA) signaling. · Our transcriptome analysis indicated that SUB1A differentially regulates genes associated with brassinosteroid (BR) synthesis during submergence. Consistent with the gene expression data, the SUB1A genotype had higher brassinosteroid levels after submergence compared to the intolerant genotype. Tolerance to submergence can be activated in the intolerant genotype by pretreatment with exogenous brassinolide, which results in restricted shoot elongation during submergence. · BR induced a GA catabolic gene, resulting in lower GA levels in SUB1A plants. BR treatment also induced the DELLA protein SLR1, a known repressor of GA responses such as shoot elongation. We propose that BR limits GA levels during submergence in the SUB1A rice through a GA catabolic enzyme as part of an early response and may repress GA responses by inducing SLR1 after several days of submergence. · Our results suggest that BR biosynthesis is regulated in a SUB1A-dependent manner during submergence and is involved in modulating the GA signaling and homeostasis.

Single particle tracking analysis of the chloroplast division protein FtsZ anchoring to the inner envelope membrane

Replication of chloroplast in plant cells is an essential process that requires co-assembly of the tubulin-like plastid division proteins FtsZ1 and FtsZ2 at mid-chloroplast to form a ring structure called the Z-ring. The Z-ring is stabilized via its interaction with the transmembrane protein ARC6 on the inner envelope membrane of chloroplasts. Plants lacking ARC6 are defective in plastid division and contain only one or two enlarged chloroplasts per cell with abnormal localization of FtsZ: instead of a single Z-ring, many short FtsZ filaments are distributed throughout the chloroplast. ARC6 is thought to be the anchoring point for FtsZ assemblies. To investigate the role of ARC6 in FtsZ anchoring, the mobility of green fluorescent protein-tagged FtsZ assemblies was assessed by single particle tracking in mutant plants lacking the ARC6 protein. Mean square displacement analysis showed that the mobility of FtsZ assemblies is to a large extent characterized by anomalous diffusion behavior (indicative of intermittent binding) and restricted diffusion suggesting that besides ARC6-mediated anchoring, an additional FtsZ-anchoring mechanism is present in chloroplasts.

Chloroplast division protein ARC3 regulates chloroplast FtsZ-ring assembly and positioning in arabidopsis through interaction with FtsZ2

Chloroplast division is initiated by assembly of a mid-chloroplast FtsZ (Z) ring comprising two cytoskeletal proteins, FtsZ1 and FtsZ2. The division-site regulators ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3), MinD1, and MinE1 restrict division to the mid-plastid, but their roles are poorly understood. Using genetic analyses in Arabidopsis thaliana, we show that ARC3 mediates division-site placement by inhibiting Z-ring assembly, and MinD1 and MinE1 function through ARC3. ftsZ1 null mutants exhibited some mid-plastid FtsZ2 rings and constrictions, whereas neither constrictions nor FtsZ1 rings were observed in mutants lacking FtsZ2, suggesting FtsZ2 is the primary determinant of Z-ring assembly in vivo. arc3 ftsZ1 double mutants exhibited multiple parallel but no mid-plastid FtsZ2 rings, resembling the Z-ring phenotype in arc3 single mutants and showing that ARC3 affects positioning of FtsZ2 rings as well as Z rings. ARC3 overexpression in the wild type and ftsZ1 inhibited Z-ring and FtsZ2-ring assembly, respectively. Consistent with its effects in vivo, ARC3 interacted with FtsZ2 in two-hybrid assays and inhibited FtsZ2 assembly in a heterologous system. Our studies are consistent with a model wherein ARC3 directly inhibits Z-ring assembly in vivo primarily through interaction with FtsZ2 in heteropolymers and suggest that ARC3 activity is spatially regulated by MinD1 and MinE1 to permit Z-ring assembly at the mid-plastid.

Rice cytokinin GATA transcription Factor1 regulates chloroplast development and plant architecture

Chloroplast biogenesis has been well documented in higher plants, yet the complex methods used to regulate chloroplast activity under fluctuating environmental conditions are not well understood. In rice (Oryza sativa), the CYTOKININ-RESPONSIVE GATA TRANSCRIPTION FACTOR1 (Cga1) shows increased expression following light, nitrogen, and cytokinin treatments, while darkness and gibberellin reduce expression. Strong overexpression of Cga1 produces dark green, semidwarf plants with reduced tillering, whereas RNA interference knockdown results in reduced chlorophyll and increased tillering. Coexpression, microarray, and real-time expression analyses demonstrate a correlation between Cga1 expression and the expression of important nucleus-encoded, chloroplast-localized genes. Constitutive Cga1 overexpression increases both chloroplast biogenesis and starch production but also results in delayed senescence and reduced grain filling. Growing the transgenic lines under different nitrogen regimes indicates potential agricultural applications for Cga1, including manipulation of biomass, chlorophyll/chloroplast content, and harvest index. These results indicate a conserved mechanism by which Cga1 regulates chloroplast development in higher plants.

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.

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.

ROLE OF MULTIPLE FTSZ RINGS IN CHLOROPLAST DIVISION UNDER OLIGOTROPHIC AND EUTROPHIC CONDITIONS IN THE UNICELLULAR GREEN ALGA NANNOCHLORIS BACILLARIS (CHLOROPHYTA, TREBOUXIOPHYCEAE)(1)

Chloroplasts of the unicellular green alga Nannochloris bacillaris Naumann cultured under nutrient-enriched conditions have multiple rings of FtsZ, a prokaryote-derived chloroplast division protein. We previously reported that synthesis of excess chloroplast DNA and formation of multiple FtsZ rings occur simultaneously. To clarify the role of multiple FtsZ rings in chloroplast division, we investigated chloroplast DNA synthesis and ring formation in cells cultured under various culture conditions. Cells transferred from a nutrient-enriched medium to an inorganic medium in the light showed a drop in cell division rate, a reduction in chloroplast DNA content, and changes in the shape of chloroplast nucleoids as cells divided. We then examined DNA synthesis by immunodetecting BrdU incorporated into DNA strands using the anti-BrdU antibody. BrdU-labeled nuclei were clearly observed in cells 48 h after transfer into the inorganic medium, while only weak punctate signals were visible in the chloroplasts. In parallel, the number of FtsZ rings decreased from 6 to only 1. When the cells were transferred from an inorganic medium to a nutrient-enriched medium, the number of cells increased only slightly in the first 12 h after transfer; after this time, however, they started to divide more quickly and increased exponentially. Chloroplast nucleoids changed from punctate to rod-like structures, and active chloroplast DNA synthesis and FtsZ ring formation were observed. On the basis of our results, we conclude that multiple FtsZ ring assembly and chloroplast DNA duplication under nutrient-rich conditions facilitate chloroplast division after transfer to oligotrophic conditions without further duplication of chloroplast DNA and formation of new FtsZ rings.

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.

Giant chloroplast development in ethylene response1-1 is caused by a second mutation in ACCUMULATION AND REPLICATION OF CHLOROPLAST3 in Arabidopsis

The higher plants of today array a large number of small chloroplasts in their photosynthetic cells. This array of small chloroplasts results from organelle division via prokaryotic binary fission in a eukaryotic plant cell environment. Functional abnormalities of the tightly coordinated biochemical event of chloroplast division lead to abnormal chloroplast development in plants. Here, we described an abnormal chloroplast phenotype in an ethylene insensitive ethylene response1-1 (etr1-1) of Arabidopsis thaliana. Extensive transgenic and genetic analyses revealed that this organelle abnormality was not linked to etr1-1 or ethylene signaling, but linked to a second mutation in ACCUMULATION AND REPLICATION3 (ARC3), which was further verified by genetic complementation analysis. Despite the normal expression of other plastid division-related genes, the loss of ARC3 caused the enlargement of chloroplasts as well as the diminution of a photosynthetic protein Rubisco in etr1-1. Our study has suggested that the increased size of the abnormal chloroplasts may not be able to fully compensate for the loss of a greater array of small chloroplasts in higher plants.

Host-dependent suppression of RNA silencing mediated by the viral suppressor p19 in potato

p19 protein encoded by tomato bushy stunt virus (TBSV) is known as a suppressor of RNA silencing via inhibition of small RNA-guided cleavage in plants. In this study, we generated TBSVp19-expressing patatin-RNAi transgenic potatoes to identify the inhibitory mechanisms of RNA silencing mediated by TBSVp19. In TBSVp19-expressing patatin-RNAi lines, reduction of patatin-derived siRNA accumulation and complementation of patatin transcripts were detected in comparison with the non-TBSVp19-expressing patatin-RNAi line, suggesting that TBSVp19 suppresses the siRNA-mediated silencing pathway. Interestingly, no apparent effect on the accumulation of miRNA168 and other miRNAs was detected in TBSVp19-expressing lines; previous studies reported that p19 induced the accumulation of both miRNA168 and its target Argonaute 1 (AGO1) mRNA, but suppressed AGO1 translation via up-regulation of miRNA168 in Arabidopsis. In addition, the expression of Argonaute 1 (AGO1-1 and AGO1-2) and Dicer-like 1 (DCL1) was not significantly altered in p19-expressing lines. Interestingly, no translational inhibition of AGO1 mediated by p19 was detected. These results suggest that p19 suppresses siRNA-mediated silencing in potato, but may not affect miRNA-mediated silencing, possibly due to the host-dependent manner of p19 activity.

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.

Visualisation of stromules on Arabidopsis plastids

Stromules are thin stroma-filled tubules that extend from all plastid types in all multicellular plants examined. They are most easily visualised by epifluorescence or confocal microscopy of plastids containing green fluorescent protein (GFP) or other fluorescent proteins. Transient expression of gene constructs encoding plastid-targeted GFP following bombardment of whole plants or organs of Arabidopsis with gold or tungsten particles coated with plasmid DNA is a relatively rapid and simple means of producing material for observation of stromules.

Arabidopsis β-ketoacyl-[acyl carrier protein] synthase i is crucial for fatty acid synthesis and plays a role in chloroplast division and embryo development

Lipid metabolism plays a pivotal role in cell structure and in multiple plant developmental processes. β-Ketoacyl-[acyl carrier protein] synthase I (KASI) catalyzes the elongation of de novo fatty acid (FA) synthesis. Here, we report the functional characterization of KASI in the regulation of chloroplast division and embryo development. Phenotypic observation of an Arabidopsis thaliana T-DNA insertion mutant, kasI, revealed multiple morphological defects, including chlorotic (in netted patches) and curly leaves, reduced fertility, and semidwarfism. There are only one to five enlarged chloroplasts in the mesophyll cells of chlorotic sectors of young kasI rosette leaves, indicating suppressed chloroplast division under KASI deficiency. KASI deficiency results in a significant change in the polar lipid composition, which causes the suppressed expression of FtsZ and Min system genes, disordered Z-ring placement in the oversized chloroplast, and inhibited polymerization of FtsZ protein at mid-site of the chloroplast in kasI. In addition, KASI deficiency results in disrupted embryo development before the globular stage and dramatically reduces FA levels (~33.6% of the wild type) in seeds. These results demonstrate that de novo FA synthesis is crucial and has pleiotropic effects on plant growth. The polar lipid supply is important for chloroplast division and development, revealing a key function of FA synthesis in plastid development.

Septum formation in amyloplasts produces compound granules in the rice endosperm and is regulated by plastid division proteins

Storage tissues such as seed endosperm and tubers store starch in the form of granules in the amyloplast. In the rice (Oryza sativa) endosperm, each amyloplast produces compound granules consisting of several dozen polyhedral, sharp-edged and easily separable granules; whereas in other cereals, including wheat (Triticum aestivum), barley (Hordeum vulgare) and maize (Zea mays), each amyloplast synthesizes one granule. Despite extensive studies on mutants of starch synthesis in cereals, the molecular mechanisms involved in compound granule synthesis in rice have remained elusive. In this study, we expressed green fluorescent protein (GFP) fused to rice Brittle1 (BT1), an inner envelope membrane protein, to characterize dividing amyloplasts in the rice endosperm. Confocal microscopic analyses revealed that a septum-like structure, or cross-wall, containing BT1-GFP divides granules in the amyloplast. Plastid division proteins including FtsZ, Min and PDV2 play significant roles not only in amyloplast division, but also in septum synthesis, suggesting that amyloplast division and septum synthesis are related processes that share common factors. We propose that successive septum syntheses which create sections inside the amyloplast and de novo granule synthesis in each section are primarily responsible for the synthesis of compound granules.

Dynamic morphologies of pollen plastids visualised by vegetative-specific FtsZ1-GFP in Arabidopsis thaliana

The behaviour and multiplication of pollen plastids have remained elusive despite their crucial involvement in cytoplasmic inheritance. Here, we present live images of plastids in pollen grains and growing tubes from transgenic Arabidopsis thaliana lines expressing stroma-localised FtsZ1-green-fluorescent protein fusion in a vegetative cell-specific manner. Vegetative cells in mature pollen contained a morphologically heterogeneous population of round to ellipsoidal plastids, whilst those in late-developing (maturing) pollen included plastids that could have one or two constriction sites. Furthermore, plastids in pollen tubes exhibited remarkable tubulation, stromule (stroma-filled tubule) extension, and back-and-forth movement along the direction of tube growth. Plastid division, which involves the FtsZ1 ring, was rarely observed in mature pollen grains.

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.

Diversification in the genetic architecture of gene expression and transcriptional networks in organ differentiation of Populus

A fundamental goal of systems biology is to identify genetic elements that contribute to complex phenotypes and to understand how they interact in networks predictive of system response to genetic variation. Few studies in plants have developed such networks, and none have examined their conservation among functionally specialized organs. Here we used genetical genomics in an interspecific hybrid population of the model hardwood plant Populus to uncover transcriptional networks in xylem, leaves, and roots. Pleiotropic eQTL hotspots were detected and used to construct coexpression networks a posteriori, for which regulators were predicted based on cis-acting expression regulation. Networks were shown to be enriched for groups of genes that function in biologically coherent processes and for cis-acting promoter motifs with known roles in regulating common groups of genes. When contrasted among xylem, leaves, and roots, transcriptional networks were frequently conserved in composition, but almost invariably regulated by different loci. Similarly, the genetic architecture of gene expression regulation is highly diversified among plant organs, with less than one-third of genes with eQTL detected in two organs being regulated by the same locus. However, colocalization in eQTL position increases to 50% when they are detected in all three organs, suggesting conservation in the genetic regulation is a function of ubiquitous expression. Genes conserved in their genetic regulation among all organs are primarily cis regulated (approximately 92%), whereas genes with eQTL in only one organ are largely trans regulated. Trans-acting regulation may therefore be the primary driver of differentiation in function between plant organs.

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.

The PLASTID DIVISION1 and 2 components of the chloroplast division machinery determine the rate of chloroplast division in land plant cell differentiation

In most algae, the chloroplast division rate is held constant to maintain the proper number of chloroplasts per cell. By contrast, land plants evolved cell and chloroplast differentiation systems in which the size and number of chloroplasts change along with their respective cellular function by regulation of the division rate. Here, we show that PLASTID DIVISION (PDV) proteins, land plant-specific components of the division apparatus, determine the rate of chloroplast division. Overexpression of PDV proteins in the angiosperm Arabidopsis thaliana and the moss Physcomitrella patens increased the number but decreased the size of chloroplasts; reduction of PDV levels resulted in the opposite effect. The level of PDV proteins, but not other division components, decreased during leaf development, during which the chloroplast division rate also decreased. Exogenous cytokinins or overexpression of the cytokinin-responsive transcription factor CYTOKININ RESPONSE FACTOR2 increased the chloroplast division rate, where PDV proteins, but not other components of the division apparatus, were upregulated. These results suggest that the integration of PDV proteins into the division machinery enabled land plant cells to change chloroplast size and number in accord with the fate of cell differentiation.

A plant MinD homologue rescues Escherichia coli HL1 mutant (DeltaMinDE) in the absence of MinE

Background

In E. coli, the Min operon (MinCDE) plays a key role in determining the site of cell division. MinE oscillates from the middle to one pole or another to drive the MinCD complex to the end of the cell. The MinCD complex prevents FtsZ ring formation and the subsequent cell division at cell ends. In Arabidopsis thaliana, a homologue of MinD has been shown to be involved in the positioning of chloroplast division site.

Results

To learn whether the MinD homologue in plants is functional in bacteria, AtMinD was expressed in E. coli. Surprisingly, AtMinD can rescue the minicell phenotype of E. coli HL1 mutant (ΔMinDE) in the absence of EcMinE. This rescue requires EcMinC. AtMinD was localized to puncta at the poles of E. coli cells and puncta in chloroplasts without oscillation. AtMinD expressed in the HL1 mutant can cause a punctate localization pattern of GFP-EcMinC at cell ends. Yeast two hybrid and BiFC analysis showed that AtMinD can interact with EcMinC.

Conclusion

Similar to the MinD in Bacillus subtilis, AtMinD is localized to the polar region in E. coli and interacts with EcMinC to confine EcFtsZ polymerization and cell division at the midpoint of the cell.

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.