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

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.

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.

Physical and Enzymatic Hydrolysis Modifications of Potato Starch Granules

In this work, a valorization of the starch stemming from downgraded potatoes was approached through the preparation of starch nanoparticles using different physical methods, namely liquid and supercritical carbon dioxide, high energy ball milling (HEBM), and ultrasonication on the one hand and enzymatic hydrolysis on the other hand. Starch nanoparticles are beneficial as a reinforcement in food packaging technology as they enhance the mechanical and water vapor resistance of polymers. Also, starch nanoparticles are appropriate for medical applications as carriers for the delivery of bioactive or therapeutic agents. The obtained materials were characterized using X-ray diffraction as well as scanning and transmission electron microscopies (SEM and TEM), whereas the hydrolysates were analyzed using size exclusion chromatography coupled with pulsed amperometric detection (SEC-PAD). The acquired results revealed that the physical modification methods led to moderate alterations of the potato starch granules’ size and crystallinity. However, enzymatic hydrolysis conducted using Pullulanase enzyme followed by nanoprecipitation of the hydrolysates allowed us to obtain very tiny starch nanoparticles sized between 20 and 50 nm, much smaller than the native starch granules, which have an average size of 10 μm. The effects of enzyme concentration, temperature, and reaction medium pH on the extent of hydrolysis in terms of the polymer carbohydrates’ fractions were investigated. The most promising results were obtained with a Pullulanase enzyme concentration of 160 npun/g of starch, at a temperature of 60 °C in a pH 4 phosphate buffer solution resulting in the production of hydrolysates containing starch polymers with low molecular weights corresponding mainly to P-10, P-5, and fractions with molecular weights lower than P-5 Pullulan standards.

Starch structure and nutritional functionality - Past revelations and future prospects

Starch exists naturally as insoluble semi-crystalline granules assembled by amylose and amylopectin. Acknowledging the pioneers, we have reviewed the major accomplishments in the area of starch structure from the early 18th century and further established the relation of starch structure to nutritional functionality. Although a huge array of work is reported in the area, the review identified that some features of starch are still not fully understood and needs further elucidation. With the rise of diet-related diseases, it has never been more important to understand starch structure and use that knowledge to improve the nutritional value of the world's principal energy source.

Starch granule size: Does it matter?

Nature has developed starch granules varying in size from less than 1 μm to more than 100 μm. The granule size is an important factor affecting the functional properties and the applicability of starch for food and non-food applications. Within the same botanical species, the range of starch granule size can be up to sevenfold. This review critically evaluated the biological and environmental factors affecting the size of starch granules, the methods for the separation of starch granules and the measurement of size distribution. Further, the structure at different length scales and properties of starch-based on the granule size is elucidated by specifying the typical applications of granules with varying sizes. An amylopectin cluster model showing the arrangement of amylopectin from inside toward the granule surface is proposed with the hypothesis that the steric hindrance for the growth of lamellar structure may limit the size of starch granules.

Understanding Starch Metabolism in Pea Seeds towards Tailoring Functionality for Value-Added Utilization

Starch is the most abundant storage carbohydrate and a major component in pea seeds, accounting for about 50% of dry seed weight. As a by-product of pea protein processing, current uses for pea starch are limited to low-value, commodity markets. The globally growing demand for pea protein poses a great challenge for the pea fractionation industry to develop new markets for starch valorization. However, there exist gaps in our understanding of the genetic mechanism underlying starch metabolism, and its relationship with physicochemical and functional properties, which is a prerequisite for targeted tailoring functionality and innovative applications of starch. This review outlines the understanding of starch metabolism with a particular focus on peas and highlights the knowledge of pea starch granule structure and its relationship with functional properties, and industrial applications. Using the currently available pea genetics and genomics knowledge and breakthroughs in omics technologies, we discuss the perspectives and possible avenues to advance our understanding of starch metabolism in peas at an unprecedented level, to ultimately enable the molecular design of multi-functional native pea starch and to create value-added utilization.

ENLARGED STARCH GRAIN1 affects amyloplast development and starch biosynthesis in rice endosperm

Cereal crops accumulate large amounts of starch which is synthesized and stored in amyloplasts in the form of starch grains (SGs). Despite significant progress in deciphering starch biosynthesis, our understanding of amyloplast development in rice (Oryza sativa) endosperm remains largely unknown. Here, we report a novel rice floury mutant named enlarged starch grain1 (esg1). The mutant has decreased starch content, altered starch physicochemical properties, slower grain-filling rate and reduced 1000-grain weight. A distinctive feature in esg1 endosperm is that SGs are much larger, mainly due to an increased number of starch granules per SG. Spherical and loosely assembled granules, together with those weakly stained SGs may account for decreased starch content in esg1. Map-based cloning revealed that ESG1 encodes a putative permease subunit of a bacterial-type ABC (ATP-binding cassette) lipid transporter. ESG1 is constitutively expressed in various tissues. It encodes a protein localized to the chloroplast and amyloplast membranes. Mutation of ESG1 causes defective galactolipid synthesis. The overall study indicates that ESG1 is a newly identified protein affecting SG development and subsequent starch biosynthesis, which provides novel insights into amyloplast development in rice.

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.

A Review of Starch Biosynthesis in Relation to the Building Block-Backbone Model

Starch is a water-insoluble polymer of glucose synthesized as discrete granules inside the stroma of plastids in plant cells. Starch reserves provide a source of carbohydrate for immediate growth and development, and act as long term carbon stores in endosperms and seed tissues for growth of the next generation, making starch of huge agricultural importance. The starch granule has a highly complex hierarchical structure arising from the combined actions of a large array of enzymes as well as physicochemical self-assembly mechanisms. Understanding the precise nature of granule architecture, and how both biological and abiotic factors determine this structure is of both fundamental and practical importance. This review outlines current knowledge of granule architecture and the starch biosynthesis pathway in relation to the building block-backbone model of starch structure. We highlight the gaps in our knowledge in relation to our understanding of the structure and synthesis of starch, and argue that the building block-backbone model takes accurate account of both structural and biochemical data.

Organelle DNA contents and starch accumulation in potato tubers

Starch contents were found to be positively correlated with organelle/nuclear DNA ratios, suggesting that these ratios are involved in starch accumulation and may serve as a target trait in genetic engineering and a biomarker in breeding for improving the dry matter and starch production in potato. Starch is the main dry matter component of various staple food crops, including potato. Starch synthesis and accumulation is in plastids, uses sugar, consumes cellular energy, and requires active expression of starch synthesis genes. We hypothesized that the plastid/nuclear DNA ratios and mitochondrial/nuclear DNA ratios are involved in this accumulation. We analyzed the dry mater, starch, plastid DNA, mitochondrial DNA, and nuclear DNA in tuber stem ends and tuber bud ends in two potato cultivars and verified the results using whole tubers in nine potato cultivars. Dry matter contents (DMC) and organelle/nuclear DNA ratios increased rapidly during tuber bulking. DMC and starch contents were greater at the tuber stem ends than at the tuber bud ends. Both the comparisons between tuber ends and among whole tubers indicated that DMC and starch contents were positively correlated with both plastid/nuclear DNA ratios and mitochondrial/nuclear DNA ratios. The results suggest that pt/nuc and mt/nuc DNA ratios are important and may serve as a biomarker in selection, genetic engineering, and cytoplasm manipulation, for dry matter and starch accumulation in potato.

Observation of wet specimens sensitive to evaporation using scanning electron microscopy

Wet specimens are notoriously difficult to image in scanning electron microscopes (SEM) owing to evaporation from the required vacuum of the specimen chamber. Traditionally, this issue has been addressed by increasing the specimen chamber pressure. Unfortunately, observation under high specimen chamber pressure cannot prevent the initial evaporation effects. The wet cover method, where the original surface water is retained (and, therefore, considered wet), provides a way to introduce and subsequently image specimens that are sensitive to evaporation within a SEM, while preventing evaporation-related damage, and to observe interesting specimen-water interactions.

Starch formation inside plastids of higher plants

Starch is a water-insoluble polyglucan synthesized inside the plastid stroma within plant cells, serving a crucial role in the carbon budget of the whole plant by acting as a short-term and long-term store of energy. The highly complex, hierarchical structure of the starch granule arises from the actions of a large suite of enzyme activities, in addition to physicochemical self-assembly mechanisms. This review outlines current knowledge of the starch biosynthetic pathway operating in plant cells in relation to the micro- and macro-structures of the starch granule. We highlight the gaps in our knowledge, in particular, the relationship between enzyme function and operation at the molecular level and the formation of the final, macroscopic architecture of the granule.

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.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

The starch-rich endosperms of the Poaceae, which includes wild grasses and their domesticated descendents the cereals, have provided humankind and their livestock with the bulk of their daily calories since the dawn of civilization up to the present day. There are currently unprecedented pressures on global food supplies, largely resulting from population growth, loss of agricultural land that is linked to increased urbanization, and climate change. Since cereal yields essentially underpin world food and feed supply, it is critical that we understand the biological factors contributing to crop yields. In particular, it is important to understand the biochemical pathway that is involved in starch biosynthesis, since this pathway is the major yield determinant in the seeds of six out of the top seven crops grown worldwide. This review outlines the critical stages of growth and development of the endosperm tissue in the Poaceae, including discussion of carbon provision to the growing sink tissue. The main body of the review presents a current view of our understanding of storage starch biosynthesis, which occurs inside the amyloplasts of developing endosperms.

Expression of an amylosucrase gene in potato results in larger starch granules with novel properties

Expression of amylosucrase in potato resulted in larger starch granules with rough surfaces and novel physico-chemical properties, including improved freeze-thaw stability, higher end viscosity, and better enzymatic digestibility. Starch is a very important carbohydrate in many food and non-food applications. In planta modification of starch by genetic engineering has significant economic and environmental benefits as it makes the chemical or physical post-harvest modification obsolete. An amylosucrase from Neisseria polysaccharea fused to a starch-binding domain (SBD) was introduced in two potato genetic backgrounds to synthesize starch granules with altered composition, and thereby to broaden starch applications. Expression of SBD-amylosucrase fusion protein in the amylose-containing potato resulted in starch granules with a rough surface, a twofold increase in median granule size, and altered physico-chemical properties including improved freeze-thaw stability, higher end viscosity, and better enzymatic digestibility. These effects are possibly a result of the physical interaction between amylosucrase and starch granules. The modified larger starches not only have great benefit to the potato starch industry by reducing losses during starch isolation, but also have an advantage in many food applications such as frozen food due to its extremely high freeze-thaw stability.

Poly(ADP-ribose)polymerases are involved in microhomology mediated back-up non-homologous end joining in Arabidopsis thaliana

Besides the KU-dependent classical non-homologous end-joining (C-NHEJ) pathway, an alternative NHEJ pathway first identified in mammalian systems, which is often called the back-up NHEJ (B-NHEJ) pathway, was also found in plants. In mammalian systems PARP was found to be one of the essential components in B-NHEJ. Here we investigated whether PARP1 and PARP2 were also involved in B-NHEJ in Arabidopsis. To this end Arabidopsis parp1, parp2 and parp1parp2 (p1p2) mutants were isolated and functionally characterized. The p1p2 double mutant was crossed with the C-NHEJ ku80 mutant resulting in the parp1parp2ku80 (p1p2k80) triple mutant. As expected, because of their role in single strand break repair (SSBR) and base excision repair (BER), the p1p2 and p1p2k80 mutants were shown to be sensitive to treatment with the DNA damaging agent MMS. End-joining assays in cell-free leaf protein extracts of the different mutants using linear DNA substrates with different ends reflecting a variety of double strand breaks were performed. The results showed that compatible 5'-overhangs were accurately joined in all mutants, that KU80 protected the ends preventing the formation of large deletions and that PARP proteins were involved in microhomology mediated end joining (MMEJ), one of the characteristics of B-NHEJ.

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.

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.

Control of starch granule numbers in Arabidopsis chloroplasts

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves. Lack of quantitative information on the extent of genetic, temporal, developmental, and environmental variation in granule numbers is an important limitation in understanding control of starch degradation and the mechanism of granule initiation. Two methods were developed for reliable estimation of numbers of granules per chloroplast. First, direct measurements were made on large series of consecutive sections of mesophyll tissue obtained by focused ion beam-scanning electron microscopy. Second, average numbers were calculated from the starch contents of leaves and chloroplasts and estimates of granule mass based on granule dimensions. Examination of wild-type plants and accumulation and regulation of chloroplast (arc) mutants with few, large chloroplasts provided the following new insights. There is wide variation in chloroplast volumes in cells of wild-type leaves. Granule numbers per chloroplast are correlated with chloroplast volume, i.e. large chloroplasts have more granules than small chloroplasts. Mature leaves of wild-type plants and arc mutants have approximately the same number of granules per unit volume of stroma, regardless of the size and number of chloroplasts per cell. Granule numbers per unit volume of stroma are also relatively constant in immature leaves but are greater than in mature leaves. Granule initiation occurs as chloroplasts divide in immature leaves, but relatively little initiation occurs in mature leaves. Changes in leaf starch content over the diurnal cycle are largely brought about by changes in the volume of a fixed number of granules.

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

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

Association genetics in Solanum tuberosum provides new insights into potato tuber bruising and enzymatic tissue discoloration

BACKGROUND

Most agronomic plant traits result from complex molecular networks involving multiple genes and from environmental factors. One such trait is the enzymatic discoloration of fruit and tuber tissues initiated by mechanical impact (bruising). Tuber susceptibility to bruising is a complex trait of the cultivated potato (Solanum tuberosum) that is crucial for crop quality. As phenotypic evaluation of bruising is cumbersome, the application of diagnostic molecular markers would empower the selection of low bruising potato varieties. The genetic factors and molecular networks underlying enzymatic tissue discoloration are sparsely known. Hitherto there is no association study dealing with tuber bruising and diagnostic markers for enzymatic discoloration are rare.

RESULTS

The natural genetic diversity for bruising susceptibility was evaluated in elite middle European potato germplasm in order to elucidate its molecular basis. Association genetics using a candidate gene approach identified allelic variants in genes that function in tuber bruising and enzymatic browning. Two hundred and five tetraploid potato varieties and breeding clones related by descent were evaluated for two years in six environments for tuber bruising susceptibility, specific gravity, yield, shape and plant maturity. Correlations were found between different traits. In total 362 polymorphic DNA fragments, derived from 33 candidate genes and 29 SSR loci, were scored in the population and tested for association with the traits using a mixed model approach, which takes into account population structure and kinship. Twenty one highly significant (p < 0.001) and robust marker-trait associations were identified.

CONCLUSIONS

The observed trait correlations and associated marker fragments provide new insight in the molecular basis of bruising susceptibility and its natural variation. The markers diagnostic for increased or decreased bruising susceptibility will facilitate the combination of superior alleles in breeding programs. In addition, this study presents novel candidates that might control enzymatic tissue discoloration and tuber bruising. Their validation and characterization will increase the knowledge about the underlying biological processes.

The priming of storage glucan synthesis from bacteria to plants: current knowledge and new developments

Starch is the main polymer in which carbon and energy are stored in land plants, algae and some cyanobacteria. It plays a crucial role in the physiology of these organisms and also represents an important polymer for humans, in terms of both diet and nonfood industry uses. Recent efforts have elucidated most of the steps involved in the synthesis of starch. However, the process that initiates the synthesis of the starch granule remains unclear. Here, we outline the similarities between the synthesis of starch and the synthesis of glycogen, the other widespread and abundant glucose-based polymer in living cells. We place special emphasis on the mechanisms of initiation of the glycogen granule and current knowledge concerning the initiation of the starch granule. We also discuss recent discoveries regarding the function of starch synthases in the priming of the starch granule and possible interactions with other elements of the starch synthesis machinery.

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.

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.

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.

Excess copper induces structural changes in cultured photosynthetic soybean cells

Soybean [Glycine max (L.) Merr.] cell suspensions have the capacity to develop tolerance to excess copper, constituting a convenient system for studies on the mechanisms of copper tolerance. The functional cell organisation changes observed in these cell cultures after both short-term (stressed cells) and long-term (acclimated cells) exposure to 10 μm CuSO₄ are reported from structural, cytochemical and microanalytical approaches. Cells grown in the presence of 10 μm CuSO₄ shared some structural features with untreated cells, such as: (i) a large cytoplasmic vacuole, (ii) chloroplasts along the thin layer of cytoplasm, (iii) nucleus in a peripheral location exhibiting circular-shaped nucleolus and a decondensed chromatin pattern, and (iv) presence of Cajal bodies in the cell nuclei. In addition, cells exposed to 10 μm CuSO₄ exhibited important differences compared with untreated cells: (i) chloroplasts displayed rounded shape and smaller size with denser-structured internal membranes, especially in copper-acclimated cells; (ii) no starch granules were found within chloroplasts; (iii) the cytoplasmic vacuole was larger, especially after long-term copper exposure; (iv) the levels of citrate and malate increased. Extracellular dark-coloured deposits with high copper content attached at the outer surface of the cell wall were observed only in cells exposed to a short-term copper stress. Structural cell modifications, mainly affecting chloroplasts, accompanied the short-term copper-induced response and were maintained as stable characters during the period of adaptation to excess copper. Vacuolar changes accompanied the long-term copper response. The results indicate that the first response of soybean cells to excess copper prevents its entry into the cell by immobilising it in the cell wall, and after an adaptive period, acclimation to excess copper may be mainly due to vacuolar sequestration.