The cytoskeleton maintains organelle partitioning required for single-cell C4 photosynthesis in Chenopodiaceae species

Chloroplast Envelopes Play a Role in the Formation of Autophagy-Related Structures in Plants

Autophagy is a degradation process of cytoplasmic components that is conserved in eukaryotes. One of the hallmark features of autophagy is the formation of double-membrane structures known as autophagosomes, which enclose cytoplasmic content destined for degradation. Although the membrane source for the formation of autophagosomes remains to be determined, recent studies indicate the involvement of various organelles in autophagosome biogenesis. In this study, we examined the autophagy process in Bienertia sinuspersici: one of four terrestrial plants capable of performing C₄ photosynthesis in a single cell (single-cell C₄ species). We demonstrated that narrow tubules (stromule-like structures) 30-50 nm in diameter appear to extend from chloroplasts to form the membrane-bound structures (autophagosomes or autophagy-related structures) in chlorenchyma cells of B. sinuspersici during senescence and under oxidative stress. Immunoelectron microscopic analysis revealed the localization of stromal proteins to the stromule-like structures, sequestering portions of the cytoplasm in chlorenchyma cells of oxidative stress-treated leaves of B. sinuspersici and Arabidopsis thaliana. Moreover, the fluorescent marker for autophagosomes GFP-ATG8, colocalized with the autophagic vacuole maker neutral red in punctate structures in close proximity to the chloroplasts of cells under oxidative stress conditions. Together our results implicate a role for chloroplast envelopes in the autophagy process induced during senescence or under certain stress conditions in plants.

Development of C4 Biochemistry and Change in Expression of Markers for Photosystems I and II in the Single-Cell C4 Species, Bienertia sinuspersici

Bienertia sinuspersici is one of four identified terrestrial plants that perform C₄ photosynthesis within a single chlorenchyma cell via the compartmentation of organelles and photosynthetic enzymes. The patterns of accumulation of key photosynthetic enzymes and transcripts in developing leaves were examined using immunolocalization and in situ hybridization. The polypeptides of Rubisco large subunit (RbcL) and pyruvate Pi dikinase (PPDK) accumulated equally in all chloroplasts before the formation of two intracellular cytoplasmic compartments: the central (CCC) and peripheral (PCC) cytoplasmic compartments. The differential accumulation of these enzymes was not completed until the leaf had reached maturity, indicating that the transition from C₃ to C₄ photosynthesis occurred during leaf maturation. In mature chlorenchyma cells, RbcL accumulated 20-fold higher in the CCC than in the PCC, while PPDK exhibited a concentration gradient that was the lowest in the chloroplasts in the central region of the CCC and the highest in PCC chloroplasts. The pattern of rbcL transcript accumulation followed that of its polypeptides in developing leaves, suggesting that the expression of this gene was likely controlled by transcriptional and/or post-transcriptional processes. Immunocytochemical results examining the distribution of photosystems I and II in the chloroplasts of chlorenchyma cells from mature leaves showed that PSII is more abundant in chloroplasts of the central compartment, whereas PSI is higher in those of the peripheral compartment. The quantitative real-time PCR results of rbcL, psbA, and psaB transcripts from the isolated chloroplasts of each compartment further supported this observation. Our results suggest that multiple levels of regulation play a role in controlling the differential accumulation of photosynthetic gene expression in the dimorphic chloroplasts of single-cell C₄ species during leaf development.

Leaf microscopy applications in photosynthesis research: identifying the gaps

Leaf imaging via microscopy has provided critical insights into research on photosynthesis at multiple junctures, from the early understanding of the role of stomata, through elucidating C4 photosynthesis via Kranz anatomy and chloroplast arrangement in single cells, to detailed explorations of diffusion pathways and light utilization gradients within leaves. In recent decades, the original two-dimensional (2D) explorations have begun to be visualized in three-dimensional (3D) space, revising our understanding of structure-function relationships between internal leaf anatomy and photosynthesis. In particular, advancing new technologies and analyses are providing fresh insight into the relationship between leaf cellular components and improving the ability to model net carbon fixation, water use efficiency, and metabolite turnover rate in leaves. While ground-breaking developments in imaging tools and techniques have expanded our knowledge of leaf 3D structure via high-resolution 3D and time-series images, there is a growing need for more in vivo imaging as well as metabolite imaging. However, these advances necessitate further improvement in microscopy sciences to overcome the unique challenges a green leaf poses. In this review, we discuss the available tools, techniques, challenges, and gaps for efficient in vivo leaf 3D imaging, as well as innovations to overcome these difficulties.

Protoplast Isolation and Transfection in the Single-Cell C4 Species Bienertia sinuspersici

We have developed an optimized protocol for isolating protoplasts from chlorenchyma cells of the single-cell C₄ species Bienertia sinuspersici. The isolated protoplasts maintained the integrity of the unique single-cell C₄ intracellular compartmentation of organelles as observed in chlorenchyma cells after cell wall digestion. Approximately over 80% of isolated protoplasts expressed the fusion reporter gene following the polyethylene glycol-mediated transfection procedures. Overall, fluorescent protein fusion tagged with various intraorganellular sorting signals validated the potential use of the transient gene expression system in subcellular localization and organelle dynamics studies.

A Review of C4 Plants in Southwest Asia: An Ecological, Geographical and Taxonomical Analysis of a Region With High Diversity of C4 Eudicots

Southwest Asia is climatically and topographically a highly diverse region in the xeric belt of the Old World. Its diversity of arid habitats and climatic conditions acted as an important area for the evolution and diversification of up to 20 (of 38 known) independent Eudicot C4 origins. Some of these lineages present unique evolutionary strategies like single-cell functioning C4 and C3-C4 switching mechanisms. The high diversity of C4 taxa in Southwest (SW) Asia is also related to the presence of seven phytogeographic zones including the Irano-Turanian region as a center of diversification of many Caryophyllales lineages and the Somali-Masai region (Southern Oman and Yemen) as a center of diversification for C4 Monocots. Nevertheless, the C4 flora of SW Asia has not received detailed attention. This paper presents a comprehensive review of all known C4 species in the area based on a literature survey, own floristic observations, as well as taxonomic, phylogenetic and herbarium data, and δ13C-isotope ratio analysis. The resulting checklist includes a total number of 923 (861 native, of which 141 endemic, and 62 introduced) C4 species, composed of 350 Eudicots and 509 Monocots, most of which are therophytic and hemicryptophytic xerophytes with pluriregional and Irano-Turanian distribution. Two hundred thirty-nine new δ13C-isotope ratios of C4 and C3 plants, as well as some taxonomic changes are presented. An analysis of the distribution of the three main C4 plant families (Chenopodiaceae, Poaceae, and Cyperaceae) in the region in relation to climatic variables indicates that the increase of C4 species follows more or less a latitudinal gradient similar to global patterns, while separate taxonomic groups seem to depend on specific factors as continentality (Chenopodiaceae), average annual temperature (Cyperaceae), and the presence of summer precipitation (Poaceae). An increase of C4 Eudicots in W-E direction even in similar longitudinal belts is explained by a combination of edaphic and climatic conditions. The provided data should encourage a deeper interest in the evolution of C4 lineages in SW Asia and their adaptation to ecological and climatical conditions and awaken interest in the importance of local C4 crops, the conservation of threatened C4 taxa, and awareness of human impacts on the rapid environmental changes in the region.

Electron Microscopy Views of Dimorphic Chloroplasts in C4 Plants

C4 plants enhance photosynthesis efficiency by concentrating CO₂ to the site of Rubisco action. Chloroplasts in C4 plants exhibit structural dimorphism because thylakoid architectures vary depending on energy requirements. Advances in electron microscopy imaging capacity and sample preparation technologies allowed characterization of thylakoid structures and their macromolecular arrangements with unprecedented precision mostly in C3 plants. The thylakoid is assembled during chloroplast biogenesis through collaboration between the plastid and nuclear genomes. Recently, the membrane dynamics involved in the assembly process has been investigated with 3D electron microscopy, and molecular factors required for thylakoid construction have been characterized. The two classes of chloroplasts in C4 plants arise from common precursors, but little is known about how a single type of chloroplasts grow, divide, and differentiate to mature into distinct chloroplasts. Here, we outline the thylakoid structure and its assembly processes in C3 plants to discuss ultrastructural analyses of dimorphic chloroplast biogenesis in C4 plant species. Future directions for electron microscopy research of C4 photosynthetic systems are also proposed.

Exploring natural variation of photosynthesis in a site-specific manner: evolution, progress, and prospects

Site-specific changes of photosynthesis, a relatively new concept, can be used to improve the productivity of critical food crops to mitigate the foreseen food crisis. Global food security is threatened by an increasing population and the effects of climate change. Large yield improvements were achieved in major cereal crops between the 1950s and 1980s through the Green Revolution. However, we are currently experiencing a significant decline in yield progress. Of the many approaches to improved cereal yields, exploitation of the mode of photosynthesis has been intensely studied. Even though the C₄ pathway is considered the most efficient, mainly because of the carbon concentrating mechanisms around the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, which minimize photorespiration, much is still unknown about the specific gene regulation of this mode of photosynthesis. Most of the critical cereal crops, including wheat and rice, are categorized as C₃ plants based on the photosynthesis of major photosynthetic organs. However, recent findings raise the possibility of different modes of photosynthesis occurring at different sites in the same plant and/or in plants grown in different habitats. That is, it seems possible that efficient photosynthetic traits may be expressed in specific organs, even though the major photosynthetic pathway is C₃. Knowledge of site-specific differences in photosynthesis, coupled with site-specific regulation of gene expression, may therefore hold a potential to enhance the yields of economically important C₃ crops.

A draft genome assembly of halophyte Suaeda aralocaspica, a plant that performs C4 photosynthesis within individual cells

BACKGROUND

The halophyte Suaeda aralocaspica performs complete C4 photosynthesis within individual cells (SCC4), which is distinct from typical C4 plants, which require the collaboration of 2 types of photosynthetic cells. However, despite SCC4 plants having features that are valuable in engineering higher photosynthetic efficiencies in agriculturally important C3 species such as rice, there are no reported sequenced SCC4 plant genomes, limiting our understanding of the mechanisms involved in, and evolution of, SCC4 photosynthesis.

FINDINGS

Using Illumina and Pacific Biosciences sequencing platforms, we generated ∼202 Gb of clean genomic DNA sequences having a 433-fold coverage based on the 467 Mb estimated genome size of S. aralocaspica. The final genome assembly was 452 Mb, consisting of 4,033 scaffolds, with a scaffold N50 length of 1.83 Mb. We annotated 29,604 protein-coding genes using Evidence Modeler based on the gene information from ab initio predictions, homology levels with known genes, and RNA sequencing-based transcriptome evidence. We also annotated noncoding genes, including 1,651 long noncoding RNAs, 21 microRNAs, 382 transfer RNAs, 88 small nuclear RNAs, and 325 ribosomal RNAs. A complete (circular with no gaps) chloroplast genome of S. aralocaspica 146,654 bp in length was also assembled.

CONCLUSIONS

We have presented the genome sequence of the SCC4 plant S. aralocaspica. Knowledge of the genome of S. aralocaspica should increase our understanding of the evolution of SCC4 photosynthesis and contribute to the engineering of C4 photosynthesis into economically important C3 crops.

Agrobacterium-mediated transient transformation of Bienertia sinuspersici to assay recombinant protein distribution between dimorphic chloroplasts

Bienertia sinuspersici, a species capable of performing C4 photosynthesis within individual photosynthetic cells, can be transformed with Agrobacterium allowing for the analysis of subcellular protein distribution under preservation of the unique cellular morphology.

Subcellular Phenotyping: Using Proteomics to Quantitatively Link Subcellular Leaf Protein and Organelle Distribution Analyses of Pisum sativum Cultivars

Plant phenotyping to date typically comprises morphological and physiological profiling in a high-throughput manner. A powerful method that allows for subcellular characterization of organelle stoichiometric/functional characteristics is still missing. Organelle abundance and crosstalk in cell dynamics and signaling plays an important role for understanding crop growth and stress adaptations. However, microscopy cannot be considered a high-throughput technology. The aim of the present study was to develop an approach that enables the estimation of organelle functional stoichiometry and to determine differential subcellular dynamics within and across cultivars in a high-throughput manner. A combination of subcellular non-aqueous fractionation and liquid chromatography mass spectrometry was applied to assign membrane-marker proteins to cell compartmental abundances and functions of Pisum sativum leaves. Based on specific subcellular affiliation, proteotypic marker peptides of the chloroplast, mitochondria and vacuole membranes were selected and synthesized as heavy isotope labeled standards. The rapid and unbiased Mass Western approach for accurate stoichiometry and targeted absolute protein quantification allowed for a proportional organelle abundances measure linked to their functional properties. A 3D Confocal Laser Scanning Microscopy approach was developed to evaluate the Mass Western. Two P. sativum cultivars of varying morphology and physiology were compared. The Mass Western assay enabled a cultivar specific discrimination of the chloroplast to mitochondria to vacuole relations.

The role of alanine and aspartate aminotransferases in C4 photosynthesis

Alanine and aspartate are essential transfer metabolites for C₄ species of the NAD-malic enzyme and phosphoenolpyruvate carboxykinase subtype. To some degree both amino acids are also part of the metabolite shuttle in NADP-malic enzyme plants. In comparison with C₃ species, the majority of C₄ species are therefore characterised by enhanced expression and activity of alanine and aspartate aminotransferases (AT) in the photosynthetically active tissue. Both enzymes exist in multiple copies and have been found in different subcellular compartments. We tested whether different C₄ species show preferential recruitment of enzymes from specific lineages and subcellular compartments. Phylogenetic analysis of alanine and aspartate ATs from a variety of monocot and eudicot C₄ species and their C₃ relatives was combined with subcellular prediction tools and analysis of the subsequent transcript amounts in mature leaves. Recruitment of aspartate AT from a specific subcellular compartment was strongly connected to the biochemical subtype. Deviation from the main model was however observed in Gynandropsis gynandra. The configuration of alanine AT generally differed in monocot and eudicot species. C₄ monocots recruited an alanine AT from a specific cytosolic branch, but eudicots use alanine AT copies from a mitochondrial branch. Generally, plants display high plasticity in the setup of the C₄ pathway. Beside the common models for the different C₄ subtypes, individual solutions were found for plant groups or lineages.

Roles of actin cytoskeleton for regulation of chloroplast anchoring

Chloroplasts are known to maintain specific intracellular distribution patterns under specific environmental conditions, enabling the optimal performance of photosynthesis. To this end, chloroplasts are anchored in the cortical cytoplasm. In leaf epidermal cells of aquatic monocot Vallisneria, we recently demonstrated that the anchored chloroplasts are rapidly de-anchored upon irradiation with high-intensity blue light and that the process is probably mediated by the blue-light receptor phototropins. Chloroplast de-anchoring is a necessary step rendering the previously anchored chloroplasts mobile to allow their migration. In this article, based on the results obtained in Vallisneria together with those in other plant species, we briefly discussed possible modes of regulation of chloroplast anchoring and de-anchoring by actin cytoskeleton. The topics include roles of photoreceptor systems, actin-filament-dependent and -independent chloroplast anchoring, and independence of chloroplast de-anchoring from actomyosin and microtubule systems.

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

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

Expression of Arabidopsis SHN1 in Indian Mulberry (Morus indica L.) Increases Leaf Surface Wax Content and Reduces Post-harvest Water Loss

Mulberry (Morus species) leaf is the sole food for monophagous silkworms, Bombyx mori L. Abiotic stresses such as drought, salinity, and high temperature, significantly decrease mulberry productivity and post-harvest water loss from leaves influence silkworm growth and cocoon yield. Leaf surface properties regulate direct water loss through the cuticular layer. Leaf surface waxes, contribute for cuticular resistance and protect mesophyll cells from desiccation. In this study we attempted to overexpress AtSHN1, a transcription factor associated with epicuticular wax biosynthesis to increase leaf surface wax load in mulberry. Agrobacterium mediated in vitro transformation was carried out using hypocotyl and cotyledonary explants of Indian mulberry (cv. M5). Mulberry transgenic plants expressing AtSHN1 displayed dark green shiny appearance with increased leaf surface wax content. Scanning electron microscopy (SEM) and gas chromatograph-mass spectrometry (GC-MS) analysis showed change in pattern of surface wax deposition and significant change in wax composition in AtSHN1 overexpressors. Increased wax content altered leaf surface properties as there was significant difference in water droplet contact angle and diameter between transgenic and wild type plants. The transgenic plants showed significant improvement in leaf moisture retention capacity even 5 h after harvest and there was slow degradation of total buffer soluble protein in detached leaves compared to wild type. Silkworm bioassay did not indicate any undesirable effects on larval growth and cocoon yield. This study demonstrated that expression of AtSHN1, can increase surface wax load and reduce the post-harvest water loss in mulberry.

Transit peptide elements mediate selective protein targeting to two different types of chloroplasts in the single-cell C4 species Bienertia sinuspersici

Bienertia sinuspersici is a terrestrial plant that performs C4 photosynthesis within individual cells through operating a carbon concentrating mechanism between different subcellular domains including two types of chloroplasts. It is currently unknown how differentiation of two highly specialized chloroplasts within the same cell occurs as no similar cases have been reported. Here we show that this differentiation in photosynthetic cells of B. sinuspersici is enabled by a transit peptide (TP) mediated selective protein targeting mechanism. Mutations in the TPs cause loss of selectivity but not general loss of chloroplast import, indicating the mechanism operates by specifically blocking protein accumulation in one chloroplast type. Hybrid studies indicate that this selectivity is transferable to transit peptides of plants which perform C4 by cooperative function of chloroplasts between two photosynthetic cells. Codon swap experiments as well as introducing an artificial bait mRNA show that RNA affects are not crucial for the sorting process. In summary, our analysis shows how the mechanism of subcellular targeting to form two types of chloroplast within the same cell can be achieved. This information is not only crucial for understanding single-cell C4 photosynthesis; it provides new insights in control of subcellular protein targeting in cell biology.

Engineering C4 photosynthesis into C3 chassis in the synthetic biology age

C4 photosynthetic plants outperform C3 plants in hot and arid climates. By concentrating carbon dioxide around Rubisco C4 plants drastically reduce photorespiration. The frequency with which plants evolved C4 photosynthesis independently challenges researchers to unravel the genetic mechanisms underlying this convergent evolutionary switch. The conversion of C3 crops, such as rice, towards C4 photosynthesis is a long-standing goal. Nevertheless, at the present time, in the age of synthetic biology, this still remains a monumental task, partially because the C4 carbon-concentrating biochemical cycle spans two cell types and thus requires specialized anatomy. Here we review the advances in understanding the molecular basis and the evolution of the C4 trait, advances in the last decades that were driven by systems biology methods. In this review we emphasise essential genetic engineering tools needed to translate our theoretical knowledge into engineering approaches. With our current molecular understanding of the biochemical C4 pathway, we propose a simplified rational engineering model exclusively built with known C4 metabolic components. Moreover, we discuss an alternative approach to the progressing international engineering attempts that would combine targeted mutagenesis and directed evolution.

Development, subcellular positioning and selective protein accumulation in the dimorphic chloroplasts of single-cell C4 species

C4 photosynthesis is typically associated with a carbon concentrating mechanism based on close collaboration between two photosynthetic cell types (Kranz C4). Surprisingly, four species in the family Chenopodiaceae have been described, which perform all required steps for a functional and effective C4 cycle within individual photosynthetic cells. These single-cell C4 species utilize a unique subcellular compartmentation and two functionally different chloroplast types that mimic the functions of the two cell types of the Kranz C4 system. In this review, we will summarize and discuss studies on chloroplast development, positioning and selective accumulation of nuclear encoded proteins, which ultimately allow the operation of a C4 carbon concentrating mechanism within individual cells.

The unique structural and biochemical development of single cell C4 photosynthesis along longitudinal leaf gradients in Bienertia sinuspersici and Suaeda aralocaspica (Chenopodiaceae)

Temporal and spatial patterns of photosynthetic enzyme expression and structural maturation of chlorenchyma cells along longitudinal developmental gradients were characterized in young leaves of two single cell C4 species, Bienertia sinuspersici and Suaeda aralocaspica Both species partition photosynthetic functions between distinct intracellular domains. In the C4-C domain, C4 acids are formed in the C4 cycle during capture of atmospheric CO2 by phosphoenolpyruvate carboxylase. In the C4-D domain, CO2 released in the C4 cycle via mitochondrial NAD-malic enzyme is refixed by Rubisco. Despite striking differences in origin and intracellular positioning of domains, these species show strong convergence in C4 developmental patterns. Both progress through a gradual developmental transition towards full C4 photosynthesis, with an associated increase in levels of photosynthetic enzymes. Analysis of longitudinal sections showed undeveloped domains at the leaf base, with Rubisco rbcL mRNA and protein contained within all chloroplasts. The two domains were first distinguishable in chlorenchyma cells at the leaf mid-regions, but still contained structurally similar chloroplasts with equivalent amounts of rbcL mRNA and protein; while mitochondria had become confined to just one domain (proto-C4-D). The C4 state was fully formed towards the leaf tips, Rubisco transcripts and protein were compartmentalized specifically to structurally distinct chloroplasts in the C4-D domains indicating selective regulation of Rubisco expression may occur by control of transcription or stability of rbcL mRNA. Determination of CO2 compensation points showed young leaves were not functionally C4, consistent with cytological observations of the developmental progression from C3 default to intermediate to C4 photosynthesis.

Metabolic flux ratio analysis and cell staining suggest the existence of C4 photosynthesis in Phaeodactylum tricornutum

AIMS

Mechanisms for carbon fixation via photosynthesis in the diatom Phaeodactylum tricornutum Bohlin were studied recently but there remains a long-standing debate concerning the occurrence of C4 photosynthesis in this species. A thorough investigation of carbon metabolism and the evidence for C4 photosynthesis based on organelle partitioning was needed.

METHODS AND RESULTS

In this study, we identified the flux ratios between C3 and C4 compounds in P. tricornutum using (13)C-labelling metabolic flux ratio analysis, and stained cells with various cell-permeant fluorescent probes to investigate the likely organelle partitioning required for single-cell C4 photosynthesis. Metabolic flux ratio analysis indicated the C3/C4 exchange ratios were high. Cell staining indicated organelle partitioning required for single-cell C4 photosynthesis might exist in P. tricornutum.

CONCLUSION

The results of (13)C-labelling metabolic flux ratio analysis and cell staining suggest single-cell C4 photosynthesis exists in P. tricornutum.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides insights into photosynthesis patterns of P. tricornutum and the evidence for C4 photosynthesis based on (13)C-labelling metabolic flux ratio analysis and organelle partitioning.

Developmental and Subcellular Organization of Single-Cell C₄ Photosynthesis in Bienertia sinuspersici Determined by Large-Scale Proteomics and cDNA Assembly from 454 DNA Sequencing

Kranz C4 species strictly depend on separation of primary and secondary carbon fixation reactions in different cell types. In contrast, the single-cell C4 (SCC4) species Bienertia sinuspersici utilizes intracellular compartmentation including two physiologically and biochemically different chloroplast types; however, information on identity, localization, and induction of proteins required for this SCC4 system is currently very limited. In this study, we determined the distribution of photosynthesis-related proteins and the induction of the C4 system during development by label-free proteomics of subcellular fractions and leaves of different developmental stages. This was enabled by inferring a protein sequence database from 454 sequencing of Bienertia cDNAs. Large-scale proteome rearrangements were observed as C4 photosynthesis developed during leaf maturation. The proteomes of the two chloroplasts are different with differential accumulation of linear and cyclic electron transport components, primary and secondary carbon fixation reactions, and a triose-phosphate shuttle that is shared between the two chloroplast types. This differential protein distribution pattern suggests the presence of a mRNA or protein-sorting mechanism for nuclear-encoded, chloroplast-targeted proteins in SCC4 species. The combined information was used to provide a comprehensive model for NAD-ME type carbon fixation in SCC4 species.

The C-terminus of Bienertia sinuspersici Toc159 contains essential elements for its targeting and anchorage to the chloroplast outer membrane

Most nucleus-encoded chloroplast proteins rely on an N-terminal transit peptide (TP) as a post-translational sorting signal for directing them to the organelle. Although Toc159 is known to be a receptor for specific preprotein TPs at the chloroplast surface, the mechanism for its own targeting and integration into the chloroplast outer membrane is not completely understood. In a previous study, we identified a novel TP-like sorting signal at the C-terminus (CT) of a Toc159 homolog from the single-cell C4 species, Bienertia sinuspersici. In the current study, we have extended our understanding of the sorting signal using transient expression of fluorescently-tagged fusion proteins of variable-length, and with truncated and swapped versions of the CT. As was shown in the earlier study, the 56 residues of the CT contain crucial sorting information for reversible interaction of the receptor with the chloroplast envelope. Extension of this region to 100 residues in the current study stabilized the interaction via membrane integration, as demonstrated by more prominent plastid-associated signals and resistance of the fusion protein to alkaline extraction. Despite a high degree of sequence similarity, the plastid localization signals of the equivalent CT regions of Arabidopsis thaliana Toc159 homologs were not as strong as that of the B. sinuspersici counterparts. Together with computational and circular dichroism analyses of the CT domain structures, our data provide insights into the critical elements of the CT for the efficient targeting and anchorage of Toc159 receptors to the dimorphic chloroplasts in the single-cell C4 species.

Mesophyll cells of C4 plants have fewer chloroplasts than those of closely related C3 plants

The evolution of C(4) photosynthesis from C(3) ancestors eliminates ribulose bisphosphate carboxylation in the mesophyll (M) cell chloroplast while activating phosphoenolpyruvate (PEP) carboxylation in the cytosol. These changes may lead to fewer chloroplasts and different chloroplast positioning within M cells. To evaluate these possibilities, we compared chloroplast number, size and position in M cells of closely related C(3), C(3) -C(4) intermediate and C(4) species from 12 lineages of C(4) evolution. All C(3) species had more chloroplasts per M cell area than their C(4) relatives in high-light growth conditions. C(3) species also had higher chloroplast coverage of the M cell periphery than C(4) species, particularly opposite intercellular air spaces. In M cells from 10 of the 12 C(4) lineages, a greater fraction of the chloroplast envelope was pulled away from the plasmalemma in the C(4) species than their C(3) relatives. C(3) -C(4) intermediate species generally exhibited similar patterns as their C(3) relatives. We interpret these results to reflect adaptive shifts that facilitate efficient C(4) function by enhancing diffusive access to the site of primary carbon fixation in the cytosol. Fewer chloroplasts in C(4) M cells would also reduce shading of the bundle sheath chloroplasts, which also generate energy required by C(4) photosynthesis.

Endoreduplication is not involved in bundle-sheath formation in the C4 species Cleome gynandra

There is currently significant interest in engineering the two-celled C4 photosynthesis pathway into crops such as rice in order to increase yield. This will require alterations to the biochemistry of photosynthesis in both mesophyll (M) and bundle-sheath (BS) cells, but also alterations to leaf anatomy. For example, the BS of C4 species is enlarged compared with that in C3 species. Because cell and nucleus size are often correlated, this study investigated whether nuclear endoreduplication is associated with increased differentiation and expansion of BS cells. Nuclei in the BS of C4 Cleome gynandra were tagged with green fluorescent protein. Confocal laser-scanning microscopy and flow cytometry of isolated nuclei were used to quantify size and DNA content in BS cells. The results showed a significant endoreduplication in BS cells of C. gynandra but not in additional C4 lineages from both the monocotyledonous and dicotyledenous plants. Furthermore, in the C3 species Arabidopsis thaliana, BS cells undergo endoreduplication. Due to this significant endoreduplication in the small BS cells of C3 A. thaliana, it was concluded that endoreduplication of BS nuclei in C4 plants is not linked to expansion and differentiation of BS cells, and therefore that alternative strategies to increase this compartment need to be sought in order to engineer C4 traits into C3 crops such as rice.

One decade after the discovery of single-cell C4 species in terrestrial plants: what did we learn about the minimal requirements of C4 photosynthesis?

Until about 10 years ago the general accepted textbook knowledge was that terrestrial C4 photosynthesis requires separation of photosynthetic functions into two specialized cell types, the mesophyll and bundle sheath cells forming the distinctive Kranz anatomy typical for C4 plants. This paradigm has been broken with the discovery of Suaeda aralocaspica, a chenopod from central Asia, performing C4 photosynthesis within individual chlorenchyma cells. Since then, three more single-cell C4 (SCC4) species have been discovered in the genus Bienertia. They are interesting not only because of their unusual mode of photosynthesis but also present a puzzle for cell biologists. In these species, two morphological and biochemical specialized types of chloroplasts develop within individual chlorenchyma cells, a situation that has never been observed in plants before. Here we review recent literature concerning the biochemistry, physiology, and molecular biology of SCC4 photosynthesis. Particularly, we focus on what has been learned in relation to the following questions: How does the specialized morphology required for the operation of SCC4 develop and is there a C3 intermediate type of photosynthesis during development? What is the degree of specialization between the two chloroplast types and how does this compare to the chloroplasts of Kranz C4 species? How do nucleus-encoded proteins that are targeted to chloroplasts accumulate differentially in the two chloroplast types and how efficient is the CO2 concentrating mechanism in SCC4 species compared to the Kranz C4 forms?

Exploring mechanisms linked to differentiation and function of dimorphic chloroplasts in the single cell C4 species Bienertia sinuspersici

BACKGROUND

In the model single-cell C4 plant Bienertia sinuspersici, chloroplast- and nuclear-encoded photosynthetic enzymes, characteristically confined to either bundle sheath or mesophyll cells in Kranz-type C4 leaves, all occur together within individual leaf chlorenchyma cells. Intracellular separation of dimorphic chloroplasts and key enzymes within central and peripheral compartments allow for C4 carbon fixation analogous to NAD-malic enzyme (NAD-ME) Kranz type species. Several methods were used to investigate dimorphic chloroplast differentiation in B. sinuspersici.

RESULTS

Confocal analysis revealed that Rubisco-containing chloroplasts in the central compartment chloroplasts (CCC) contained more photosystem II proteins than the peripheral compartment chloroplasts (PCC) which contain pyruvate,Pi dikinase (PPDK), a pattern analogous to the cell type-specific chloroplasts of many Kranz type NAD-ME species. Transient expression analysis using GFP fusion constructs containing various lengths of a B. sinuspersici Rubisco small subunit (RbcS) gene and the transit peptide of PPDK revealed that their import was not specific to either chloroplast type. Immunolocalization showed the rbcL-specific mRNA binding protein RLSB to be selectively localized to the CCC in B. sinuspersici, and to Rubisco-containing BS chloroplasts in the closely related Kranz species Suaeda taxifolia. Comparative fluorescence analyses were made using redox-sensitive and insensitive GFP forms, as well comparative staining using the peroxidase indicator 3,3-diaminobenzidine (DAB), which demonstrated differences in stromal redox potential, with the CCC having a more negative potential than the PCC.

CONCLUSIONS

Both CCC RLSB localization and the differential chloroplast redox state are suggested to have a role in post-transcriptional rbcL expression.

Improvement of photosynthesis in rice (Oryza sativa L.) by inserting the C4 pathway

To boost food production for a rapidly growing global population, crop yields must significantly increase. One of the avenues being recently explored is the improvement of photosynthetic capacity by installing the C4 photosynthetic pathway into C3 crops like rice to drastically increase their yield. Crops with an enhanced photosynthetic mechanism would better utilize the solar radiation that can be translated into yield. This subsequently will help in producing more grain yield, reduce water loss and increase nitrogen use efficiency especially in hot and dry environments. This review provides a summary of the factors that need to be modified in rice so that the C4 pathway can be introduced successfully. It also discusses the differences between the C3 and C4 photosynthetic pathways in terms of anatomy, biochemistry and genetics.

Interaction of the microtubule-associated host protein HIP2 with viral helper component proteinase is important in infection with potato virus A

Microtubules (MT) outline and maintain the overall shape of cells and can reorganize cellular membranes to serve as sites of RNA virus replication. Here, we provide data on involvement of an MT-associated protein in infection of plants with a potyvirus, Potato virus A (PVA), representing the largest family of plant-infecting RNA viruses. Our results showed that helper-component proteinase (HCpro)-interacting protein 2 (HIP2) of potato (Solanum tuberosum) is an MT-associated protein similar to Arabidopsis SPR2. Virus-induced silencing of HIP2 in Nicotiana benthamiana resulted in a spiral-like growth phenotype, similar to the Arabidopsis spr2 mutant, and the spr2 phenotype in Arabidopsis was complemented with potato HIP2. HCpro of PVA interacted with HIP2 of potato and tobacco (Nicotiana tabacum). The interaction was detected by bimolecular fluorescence complementation in PVA-infected leaves on MT and MT intersections at the cell cortex. HIP2-HCpro interaction was determined by the C-proximal α-helix-rich domain of HIP2, whereas the N-proximal putative TOG domain and the central coiled-coil domain of HIP2 controlled HIP2 dimerization and binding to MT. Accumulation of PVA was significantly reduced in the HIP2-silenced leaves of N. benthamiana, which indicates that HIP2-HCpro interactions are important for virus infection.

Bundling actin filaments from membranes: some novel players

Progress in live-cell imaging of the cytoskeleton has significantly extended our knowledge about the organization and dynamics of actin filaments near the plasma membrane of plant cells. Noticeably, two populations of filamentous structures can be distinguished. On the one hand, fine actin filaments which exhibit an extremely dynamic behavior basically characterized by fast polymerization and prolific severing events, a process referred to as actin stochastic dynamics. On the other hand, thick actin bundles which are composed of several filaments and which are comparatively more stable although they constantly remodel as well. There is evidence that the actin cytoskeleton plays critical roles in trafficking and signaling at both the cell cortex and organelle periphery but the exact contribution of actin bundles remains unclear. A common view is that actin bundles provide the long-distance tracks used by myosin motors to deliver their cargo to growing regions and accordingly play a particularly important role in cell polarization. However, several studies support that actin bundles are more than simple passive highways and display multiple and dynamic roles in the regulation of many processes, such as cell elongation, polar auxin transport, stomatal and chloroplast movement, and defense against pathogens. The list of identified plant actin-bundling proteins is ever expanding, supporting that plant cells shape structurally and functionally different actin bundles. Here I review the most recently characterized actin-bundling proteins, with a particular focus on those potentially relevant to membrane trafficking and/or signaling.

Probing and tracking organelles in living plant cells

Intracellular organelle movements and positioning play pivotal roles in enabling plants to proliferate life efficiently and to survive diverse environmental stresses. The elaborate dissection of organelle dynamics and their underlying mechanisms (e.g., the role of the cytoskeleton in organelle movements) largely depends on the advancement and efficiency of organelle tracking systems. Here, we provide an overview of some recently developed tools for labeling and tracking organelle dynamics in living plant cells.

A basal carbon concentrating mechanism in plants?

Many photosynthetic organisms have developed inorganic carbon (Ci) concentrating mechanisms (CCMs) that increase the CO₂ concentration within the vicinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO). Several CCMs, such as four carbon (C4) and crassulacean acid metabolism (CAM), bicarbonate accumulation systems and capsular structures around RubisCO have been described in great detail. These systems are believed to have evolved several times as mechanisms that acclimate organisms to unfavourable growth conditions. Based on recent experimental evidence we propose the occurrence of another more general CCM system present in all plants. This basal CCM (bCCM) is supposed to be composed of mitochondrial carbonic anhydrases (a β-type carbonic anhydrase and the γ-type carbonic anhydrase domain of the mitochondrial NADH dehydrogenase complex) and probably further unknown components. The bCCM is proposed to reduce leakage of CO₂ from plant cells and allow efficient recycling of mitochondrial CO₂ for carbon fixation in chloroplasts.

Molecular evolution of genes recruited into C₄ photosynthesis

The C₄ pathway is found in 62 lineages of land plants. We assess evidence for parallel versus convergent evolution of C₄ photosynthesis from three approaches: (i) studies of specific genes and cis-elements controlling their expression; (ii) phylogenetic analyses of mRNAs and inferred amino acid sequences; and (iii) analysis of C₃ and C₄ genomes and transcriptomes. Evidence suggests that although convergent evolution is common, parallel evolution can underlie both changes to gene expression and amino acid sequence. cis-elements that direct cell specificity in C₄ leaves are present in C₃ orthologues of genes recruited into C₄, probably facilitating this parallel evolution. From this, and genomic data, we propose that gene duplication followed by neofunctionalisation is not necessarily important in the evolution of C₄ biochemistry.

A transit peptide-like sorting signal at the C terminus directs the Bienertia sinuspersici preprotein receptor Toc159 to the chloroplast outer membrane

Although Toc159 is known to be one of the key GTPase receptors for selective recognition of chloroplast preproteins, the mechanism for its targeting to the chloroplast surface remains unclear. To compare the targeting of these GTPase receptors, we identified two Toc159 isoforms and a Toc34 from Bienertia sinuspersici, a single-cell C₄ species with dimorphic chloroplasts in individual chlorenchyma cells. Fluorescent protein tagging and immunogold studies revealed that the localization patterns of Toc159 were distinctive from those of Toc34, suggesting different targeting pathways. Bioinformatics analyses indicated that the C-terminal tails (CTs) of Toc159 possess physicochemical and structural properties of chloroplast transit peptides (cTPs). These results were further confirmed by fluorescent protein tagging, which showed the targeting of CT fusion proteins to the chloroplast surface. The CT of Bs Toc159 in reverse orientation functioned as a cleavable cTP that guided the fluorescent protein to the stroma. Moreover, a Bs Toc34 mutant protein was retargeted to the chloroplast envelope using the CTs of Toc159 or reverse sequences of other cTPs, suggesting their conserved functions. Together, our data show that the C terminus and the central GTPase domain represent a novel dual domain-mediated sorting mechanism that might account for the partitioning of Toc159 between the cytosol and the chloroplast envelope for preprotein recognition.

Isolation of dimorphic chloroplasts from the single-cell C4 species Bienertia sinuspersici

Three terrestrial plants are known to perform C4 photosynthesis without the dual-cell system by partitioning two distinct types of chloroplasts in separate cytoplasmic compartments. We report herein a protocol for isolating the dimorphic chloroplasts from Bienertia sinuspersici. Hypo-osmotically lysed protoplasts under our defined conditions released intact compartments containing the central chloroplasts and intact vacuoles with adhering peripheral chloroplasts. Following Percoll step gradient purification both chloroplast preparations demonstrated high homogeneities as evaluated from the relative abundance of respective protein markers. This protocol will open novel research directions toward understanding the mechanism of single-cell C4 photosynthesis.

Actin-dependent chloroplast anchoring is regulated by Ca(2+)-calmodulin in spinach mesophyll cells

Chloroplasts are actively anchored at the appropriate intracellular regions to maintain advantageous distribution patterns under specific environmental conditions. Redistribution of chloroplasts is accompanied by their de-anchoring and re-anchoring, respectively, from and to the cortical cytoplasm. In spinach mesophyll cells, high-intensity blue light and Ca(2+) treatment induced the disappearance of the meshwork-like array of actin filaments surrounding chloroplasts, which was suppressed by a calmodulin antagonist. Regulatory mechanisms of chloroplast anchoring were investigated using plasma membrane (PM) ghosts, on which the cortical cytoplasm underlying the PM was exposed. Addition of an actin-depolymerizing reagent or > 1 µM Ca(2+) induced detachment of a substantial number of chloroplasts from the PM ghosts concomitant with disordered actin organization. Calmodulin antagonists and anti-calmodulin antibodies negated the effects of Ca(2+). In addition, Ca(2+)-induced detachment of chloroplasts was no longer evident on the calmodulin-depleted PM ghosts. We propose that chloroplasts are anchored onto the cortical cytoplasm through interaction with the actin cytoskeleton, and that Ca(2+)-calmodulin-sensitized de-anchoring of chloroplasts is a critical early step in chloroplast redistribution induced by environmental stimuli.

C4 cycles: past, present, and future research on C4 photosynthesis

In the late 1960s, a vibrant new research field was ignited by the discovery that instead of fixing CO(2) into a C(3) compound, some plants initially fix CO(2) into a four-carbon (C(4)) compound. The term C(4) photosynthesis was born. In the 20 years that followed, physiologists, biochemists, and molecular and developmental biologists grappled to understand how the C(4) photosynthetic pathway was partitioned between two morphologically distinct cell types in the leaf. By the early 1990s, much was known about C(4) biochemistry, the types of leaf anatomy that facilitated the pathway, and the patterns of gene expression that underpinned the biochemistry. However, virtually nothing was known about how the pathway was regulated. It should have been an exciting time, but many of the original researchers were approaching retirement, C(4) plants were proving recalcitrant to genetic manipulation, and whole-genome sequences were not even a dream. In combination, these factors led to reduced funding and the failure to attract young people into the field; the endgame seemed to be underway. But over the last 5 years, there has been a resurgence of interest and funding, not least because of ambitious multinational projects that aim to increase crop yields by introducing C(4) traits into C(3) plants. Combined with new technologies, this renewed interest has resulted in the development of more sophisticated approaches toward understanding how the C(4) pathway evolved, how it is regulated, and how it might be manipulated. The extent of this resurgence is manifest by the publication in 2011 of more than 650 pages of reviews on different aspects of C(4). Here, I provide an overview of our current understanding, the questions that are being addressed, and the issues that lie ahead.

A chloroplast envelope-bound PHD transcription factor mediates chloroplast signals to the nucleus

Chloroplast development, maintenance and function depend on the coordinated expression of chloroplast and nuclear genes. The retrograde chloroplast signals are essential in coordinating nuclear gene expression. Although the sources of signals in chloroplasts have been identified and the associated transcription factors in the nucleus extensively studied, the molecular mechanism that relays chloroplast signals to the nucleus remains a mystery. Here we show that PTM, a chloroplast envelope-bound plant homeodomain (PHD) transcription factor with transmembrane domains, functions in multiple retrograde signal pathways. The proteolytic cleavage of PTM occurs in response to retrograde signals and amino-terminal PTM accumulates in the nucleus, where it activates ABI4 transcription in a PHD-dependent manner associated with histone modifications. These results provide a molecular basis for the critical function of PTM in retrograde chloroplast signaling and shed new light on the mechanism whereby chloroplast signals are transmitted to the nucleus through the cytosol.

In vitro cultures and regeneration of Bienertia sinuspersici (Chenopodiaceae) under increasing concentrations of sodium chloride and carbon dioxide

To study the developmental transition of chloroplasts from C(3) to C(4) photosynthesis in the terrestrial single-cell C(4) species Bienertia sinuspersici, a regeneration protocol was developed. Stem explant material developed callus either with or without red nodular structures (RNS) when cultured on Murashige-Skoog (MS) salts and vitamins, supplemented with 5 mM phosphate, plus 1 mg L(-1) dichloropenoxy-acetic acid (2,4-D), and 87 mM sucrose (Stage 1 media). Only calli having RNS were able to regenerate plantlets. MS media plus phosphate was used throughout regeneration, with the Stage 2 media containing 2 mg L(-1) 6-benzylaminopurine, 43 mM sucrose and 1.5% soluble starch. Stage 3 media had no hormones or organic sources of carbon, and cultures were grown under ambient (~400 ppm) versus CO(2) enrichment (1.2% CO(2)). When calli without RNS were cultured under Stage 3 conditions with 1.2% CO(2), there was an increase in growth, protein content, and photosystem II yield, while structural and biochemical analyses indicated the cells in the calli had C(3) type photosynthesis. CO(2) enrichment during growth of RNS during Stage 3 had a large effect on regeneration success, increasing efficiency of shoot and root development, size of plantlets, leaf soluble protein, and chlorophyll concentration. Anatomical analysis of plantlets, which developed under 1.2% CO(2), showed leaves developed C(4) type chlorenchyma cells, including expression of key C(4) biochemical enzymes. Increasing salinity in the media, from 0 to 200 mM NaCl, increased tissue osmolality, average plantlet area and regeneration success, but did not affect protein or chlorophyll content.

Protoplast isolation and transient gene expression in the single-cell C4 species, Bienertia sinuspersici

Although transient gene expression using reporters such as green fluorescent protein is a versatile tool for examining gene functions and intracellular protein trafficking, the establishment of a highly efficient gene manipulation method remains a challenge in many plant species. A reliable transformation protocol has not yet been established for the three single-cell C(4) species, despite their potential of serving as model systems for their extraordinary C(4) photosynthetic metabolism. We report the first protocol optimized for isolating a large-scale and homogenous population of protoplasts from chlorenchyma cells of the single-cell C(4) species Bienertia sinuspersici. Cytochemical staining confirmed the preservation of the unusual subcellular compartmentation of organelles in chlorenchyma cells after cell wall digestion. Approximately 84% of isolated protoplasts expressed the reporter fluorescent protein following our optimized polyethylene glycol-mediated transfection procedures. Fluorescent fusion protein tagged with various intracellular sorting signals demonstrated potential use of the transient gene expression system in subcellular protein localization and organelle dynamics studies. Further applications of the current protoplast isolation and transfection techniques in understanding the novel single-cell C(4) photosynthetic mechanism are discussed.

Resolving the compartmentation and function of C4 photosynthesis in the single-cell C4 species Bienertia sinuspersici

Bienertia sinuspersici is a land plant known to perform C(4) photosynthesis through the location of dimorphic chloroplasts in separate cytoplasmic domains within a single photosynthetic cell. A protocol was developed with isolated protoplasts to obtain peripheral chloroplasts (P-CP), a central compartment (CC), and chloroplasts from the CC (C-CP) to study the subcellular localization of photosynthetic functions. Analyses of these preparations established intracellular compartmentation of processes to support a NAD-malic enzyme (ME)-type C(4) cycle. Western-blot analyses indicated that the CC has Rubisco from the C(3) cycle, the C(4) decarboxylase NAD-ME, a mitochondrial isoform of aspartate aminotransferase, and photorespiratory markers, while the C-CP and P-CP have high levels of Rubisco and pyruvate, Pidikinase, respectively. Other enzymes for supporting a NAD-ME cycle via an aspartate-alanine shuttle, carbonic anhydrase, phosophoenolpyruvate carboxylase, alanine, and an isoform of aspartate aminotransferase are localized in the cytosol. Functional characterization by photosynthetic oxygen evolution revealed that only the C-CP have a fully operational C(3) cycle, while both chloroplast types have the capacity to photoreduce 3-phosphoglycerate. The P-CP were enriched in a putative pyruvate transporter and showed light-dependent conversion of pyruvate to phosphoenolpyruvate. There is a larger investment in chloroplasts in the central domain than in the peripheral domain (6-fold more chloroplasts and 4-fold more chlorophyll). The implications of this uneven distribution for the energetics of the C(4) and C(3) cycles are discussed. The results indicate that peripheral and central compartment chloroplasts in the single-cell C(4) species B. sinuspersici function analogous to mesophyll and bundle sheath chloroplasts of Kranz-type C(4) species.

RICE MORPHOLOGY DETERMINANT encodes the type II formin FH5 and regulates rice morphogenesis

Multicellular organisms contain a large number of formins; however, their physiological roles in plants remain poorly understood. Here, we reveal that formin homology 5 (FH5), a type II formin mutated in rice morphology determinant (rmd), plays a crucial role in determining rice (Oryza sativa) morphology. FH5/RMD encodes a formin-like protein consisting of an N-terminal phosphatase tensin (PTEN)-like domain, an FH1 domain, and an FH2 domain. The rmd mutants display a bending growth pattern in seedlings, are stunted as adult plants, and have aberrant inflorescence (panicle) and seed shape. Cytological analysis showed that rmd mutants have severe cell elongation defects and abnormal microtubule and microfilament arrays. FH5/RMD is ubiquitously expressed in rice tissues, and its protein localization to the chloroplast surface is mediated by the PTEN domain. Biochemical assays demonstrated that recombinant FH5 protein can nucleate actin polymerization from monomeric G-actin or actin/profilin complexes, cap the barbed end of actin filaments, and bundle actin filaments in vitro. Moreover, FH5 can directly bind to and bundle microtubules through its FH2 domain in vitro. Our findings suggest that the rice formin protein FH5 plays a critical role in determining plant morphology by regulating actin dynamics and proper spatial organization of microtubules and microfilaments.

Photosynthetic features of non-Kranz type C4 versus Kranz type C4 and C3 species in subfamily Suaedoideae (Chenopodiaceae)

The objective of this study was to characterise photosynthesis in terrestrial non-Kranz (NK) C₄ species, Bienertia sinuspersici Akhani and Suaeda aralocaspica (Bunge) Freitag & Schütze (formerly Borszczowia aralocaspica), compared with closely related Kranz type C₄ Suaeda eltonica Iljin and Suaeda taxifolia Standley, and C₃ species Suaeda heterophylla Bunge and Suaeda maritima Dumort in subfamily Suaedoideae (Chenopodiaceae). Traditional Kranz type C₄ photosynthesis has several advantages over C₃ photosynthesis under certain environmental conditions by suppressing photorespiration. The different photosynthetic types were evaluated under varying levels of CO₂ and light at 25°C. Both NK and Kranz type species had C₄ type CO₂ compensation points (corrected for dark-type respiration) and half maximum saturation of photosynthesis at similar levels of atmospheric CO₂ (average of 145 µbar for the C₄ species v. 330 µbar CO₂ for C₃ species) characteristic of C₄ photosynthesis. CO₂ saturated rates of photosynthesis per unit chlorophyll was higher in the C₃ (at ~2.5 current ambient CO₂ levels) than the C₄ species, which is likely related to their higher Rubisco content. The amount of Rubisco as a percentage of total protein was similar in NK and Kranz type species (mean 10.2%), but much lower than in the C₃ species (35%). Light saturated rates of CO₂ fixation per unit leaf area at 25°C and 340 µbar CO₂ were higher in the Kranz species and the NK C₄ S. aralocaspica than in the C₃ species. In response to light at 340 µbar CO₂, there was a difference in rates of photosynthesis per unit Rubisco with NK > Kranz > C₃ species. There were no significant differences between the three photosynthetic types in maximum quantum yields, convexity of light response curves, and light compensation points at 25°C. The water use efficiency (CO₂ fixed per water transpired) at 340 µbar CO₂, 25°C and 1000 µmol quanta m^-2 s^-1 was on average 3-fold higher in the C₄ (NK and Kranz) compared with the C₃ species. The results show that the NK species have several C₄ traits like the Kranz type species in subfamily Suaedoideae.

Chloroplast anchoring: its implications for the regulation of intracellular chloroplast distribution

The intracellular distribution of organelles plays a pivotal role in the maintenance and adaptation of a wide spectrum of cellular activities in plants. Chloroplasts are a special type of organelle able to photosynthesize, capturing light energy to fix atmospheric CO2. Consequently, the intracellular positioning of chloroplasts is crucial for plant growth and development. Knowledge of the photoreceptors and cellular apparatus responsible for chloroplast movement has gradually accumulated over time, yet recent advances have allowed improved understanding. In this article, several aspects of research progress into the mechanisms for maintaining the specific intracellular distribution patterns of chloroplasts, namely, chloroplast anchoring, are summarized, together with a brief consideration of the future prospects of this subject. Our discussion covers developmental, physiological, ecophysiological, and recent cell biological research areas.

Structural changes in the vacuole and cytoskeleton are key to development of the two cytoplasmic domains supporting single-cell C(4) photosynthesis in Bienertia sinuspersici

Bienertia sinuspersici Akhani has an unusual mechanism of C4 photosynthesis which occurs within individual chlorenchyma cells. To perform C4, the mature cells have two cytoplasmic compartments consisting of a central (CCC) and a peripheral (PCC) domain containing dimorphic chloroplasts which are interconnected by cytoplasmic channels. Based on leaf development studies, young chlorenchyma cells have not developed the two cytoplasmic compartments and dimorphic chloroplasts. Fluorescent dyes which are targeted to membranes or to specific organelles were used to follow changes in cell structure and organelle distribution during formation of C4-type chlorenchyma. Chlorenchyma cell development was divided into four stages: 1-the nucleus and chloroplasts occupy much of the cytoplasmic space and only small vacuoles are formed; 2-development of larger vacuoles, formation of a pre-CCC with some scattered chloroplasts; 3-the vacuole expands, cells have directional growth; 4-mature stage, cells have become elongated, with a distinctive CCC and PCC joined by interconnecting cytoplasmic channels. By staining vacuoles with a fluorescent dye and constructing 3D images of chloroplasts, and by microinjecting a fluorescence dye into the vacuole of living cells, it was demonstrated that the mature cell has only one vacuole, which is traversed by cytoplasmic channels connecting the CCC with the PCC. Immunofluorescent studies on isolated chlorenchyma cells treated with cytoskeleton disrupting drugs suspended in different levels of osmoticum showed that both microtubules and actin filaments are important in maintaining the cytoplasmic domains. With prolonged exposure of plants to dim light, the cytoskeleton undergoes changes and there is a dramatic shift of the CCC from the center toward the distal end of the cell.

Does Bienertia cycloptera with the single-cell system of C(4) photosynthesis exhibit a seasonal pattern of delta (13)C values in nature similar to co-existing C (4) Chenopodiaceae having the dual-cell (Kranz) system?

Family Chenopodiaceae is an intriguing lineage, having the largest number of C(4) species among dicots, including a number of anatomical variants of Kranz anatomy and three single-cell C(4) functioning species. In some previous studies, during the culture of Bienertia cycloptera Bunge ex Boiss., carbon isotope values (delta(13)C values) of leaves deviated from C(4) to C(3)-C(4) intermediate type, raising questions as to its mode of photosynthesis during growth in natural environments. This species usually co-occurs with several Kranz type C(4) annuals. The development of B. cycloptera morphologically and delta(13)C values derived from plant samples (cotyledons, leaves, bracts, shoots) were analyzed over a complete growing season in a salt flat in north central Iran, along with eight Kranz type C(4) species and one C(3) species. For a number of species, plants were greenhouse-grown from seeds collected from the site, in order to examine leaf anatomy and C(4) biochemical subtype. Among the nine C(4) species, the cotyledons of B. cycloptera, and of the Suaeda spp. have the same respective forms of C(4) anatomy occurring in leaves, while cotyledons of members of tribe Caroxyloneae lack Kranz anatomy, which is reflected in the delta(13)C values found in plants grown in the natural habitat. The nine C(4) species had average seasonal delta(13)C values of -13.9 per thousand (with a range between species from -11.3 to -15.9 per thousand). The measurements of delta(13)C values over a complete growing season show that B. cycloptera performs C(4) photosynthesis during its life cycle in nature, similar to Kranz type species, with a seasonal average delta(13)C value of -15.2 per thousand.

Differential positioning of C(4) mesophyll and bundle sheath chloroplasts: recovery of chloroplast positioning requires the actomyosin system

In C(4) plants, bundle sheath (BS) chloroplasts are arranged in the centripetal position or in the centrifugal position, although mesophyll (M) chloroplasts are evenly distributed along cell membranes. To examine the molecular mechanism for the intracellular disposition of these chloroplasts, we observed the distribution of actin filaments in BS and M cells of the C(4) plants finger millet (Eleusine coracana) and maize (Zea mays) using immunofluorescence. Fine actin filaments encircled chloroplasts in both cell types, and an actin network was observed adjacent to plasma membranes. The intracellular disposition of both chloroplasts in finger millet was disrupted by centrifugal force but recovered within 2 h in the dark. Actin filaments remained associated with chloroplasts during recovery. We also examined the effects of inhibitors on the rearrangement of chloroplasts. Inhibitors of actin polymerization, myosin-based activities and cytosolic protein synthesis blocked migration of chloroplasts. In contrast, a microtubule-depolymerizing drug had no effect. These results show that C(4) plants possess a mechanism for keeping chloroplasts in the home position which is dependent on the actomyosin system and cytosolic protein synthesis but not tubulin or light.

Leaf development in the single-cell C4 system in Bienertia sinuspersici: expression of genes and peptide levels for C4 metabolism in relation to chlorenchyma structure under different light conditions

Bienertia sinuspersici performs C(4) photosynthesis in individual chlorenchyma cells by the development of two cytoplasmic domains (peripheral and central) with dimorphic chloroplasts, an arrangement that spatially separates the fixation of atmospheric CO(2) into C(4) acids and the donation of CO(2) from C(4) acids to Rubisco in the C(3) cycle. In association with the formation of these cytoplasmic domains during leaf maturation, developmental stages were analyzed for the expression of a number of photosynthetic genes, including Rubisco small and large subunits and key enzymes of the C(4) cycle. Early in development, Rubisco subunits and Gly decarboxylase and Ser hydroxymethyltransferase of the glycolate pathway accumulated more rapidly than enzymes associated with the C(4) cycle. The levels of pyruvate,Pi dikinase and phosphoenolpyruvate carboxylase were especially low until spatial cytoplasmic domains developed and leaves reached maturity, indicating a developmental transition toward C(4) photosynthesis. In most cases, there was a correlation between the accumulation of mRNA transcripts and the respective peptides, indicating at least partial control of the development of photosynthesis at the transcriptional level. During growth under moderate light, when branches containing mature leaves were enclosed in darkness for 1 month, spatial domains were maintained and there was high retention of a number of photosynthetic peptides, including Rubisco subunits and pyruvate,Pi dikinase, despite a reduction in transcript levels. When plants were transferred from moderate to low light conditions for 1 month, there was a striking shift of the central cytoplasmic compartment toward the periphery of chlorenchyma cells; the mature leaves showed strong acclimation with a shade-type photosynthetic response to light while retaining C(4) features indicative of low photorespiration. These results indicate a progressive development of C(4) photosynthesis with differences in the control mechanisms for the expression of photosynthetic genes and peptide synthesis during leaf maturation and in response to light conditions.

Actin microfilament dynamics and actin side-binding proteins in plants

Actin microfilaments are highly organized and essential intracellular components of organelle movement and cell morphogenesis in plants. The organization of these microfilaments undergoes dynamic changes during cell division, elongation, and differentiation. Recent live-cell imaging of plant actin microfilaments has revealed their native organization and remarkable dynamics. In addition, characterization of plant actin side-binding proteins has progressed rapidly by genetic, biochemical, and bioinformatic approaches. The gathering and integration of microscopy-based information from actin microfilament dynamics and the molecular identification of actin side-binding proteins have provided considerable insights into actin microfilament-dependent events and actin microfilament organization in plants.