Starch synthase 4 is located in the thylakoid membrane and interacts with plastoglobule-associated proteins in Arabidopsis

A point mutation in PsbW disrupts thylakoid membrane organization and causes aberrant starch granule formation

Chloroplast thylakoid membranes are the sites of the light reactions of photosynthesis. They are also thought to influence starch granule biogenesis via the thylakoid anchored protein MAR-BINDING FILAMENT-LIKE PROTEIN 1 (MFP1), but mechanistic understanding is scarce. Here we report an Arabidopsis mutant affected in PsbW, an integral thylakoid membrane protein associated with photosynthetic complexes of PSII. This mutant (psbw-2) was identified in a large-scale mutant screen designed to find proteins that regulate starch granule shape and size because it produces an excessive number of small, irregularly shaped starch granules. The mutation in psbw-2 causes a glycine-to-arginine substitution in PsbW's transmembrane helix. The resulting PsbWG107R protein remains membrane-associated but has lost its ability to stabilize PSII supercomplexes. In addition, the transgenic expression of this mutated version results in abnormal thylakoid membranes that have drastically enlarged luminal spaces and no longer form distinct grana stacks, leading to reduced plant growth and impaired photosynthesis. These effects increase with PsbWG107R expression levels but are not observed in the psbw knockout mutant, suggesting that PsbWG107R has acquired an aberrant function. We analyzed psbw-2 mutants also lacking either MFP1 or STARCH SYNTHASE 4, a key factor involved in granule initiation and growth. These data suggest that thylakoid distortion is caused by the membrane insertion of PsbWG107R, which in turn affects the initiation and growth of starch granules. Our results reaffirm the link between thylakoid membrane system and starch formation and highlight the importance of proper thylakoid architecture for plant fitness.

Functional Characterization of the MeSSIII-1 Gene and Its Promoter from Cassava

Soluble starch synthases (SSs) play important roles in the synthesis of cassava starch. However, the expression characteristics of the cassava SSs genes have not been elucidated. In this study, the MeSSIII-1 gene and its promoter, from SC8 cassava cultivars, were respectively isolated by PCR amplification. MeSSIII-1 protein was localized to the chloroplasts. qRT-PCR analysis revealed that the MeSSIII-1 gene was expressed in almost all tissues tested, and the expression in mature leaves was 18.9 times more than that in tuber roots. MeSSIII-1 expression was induced by methyljasmonate (MeJA), abscisic acid (ABA), and ethylene (ET) hormones in cassava. MeSSIII-1 expression patterns were further confirmed in proMeSSIII-1 transgenic cassava. The promoter deletion analysis showed that the -264 bp to -1 bp MeSSIII-1 promoter has basal activity. The range from -1228 bp to -987 bp and -488 bp to -264 bp significantly enhance promoter activity. The regions from -987 bp to -747 bp and -747 bp to -488 bp have repressive activity. These findings will provide an important reference for research on the potential function and transcriptional regulation mechanisms of the MeSSIII-1 gene and for further in-depth exploration of the regulatory network of its internal functional elements.

Arabidopsis FIBRILLIN6 influences carotenoid biosynthesis by directly promoting phytoene synthase activity

Carotenoids are health-promoting plastidial isoprenoids with essential functions in plants as photoprotectants and photosynthetic pigments in chloroplasts. They also accumulate in specialized plastids named chromoplasts, providing color to non-photosynthetic tissues such as flower petals and ripe fruit. Carotenoid accumulation in chromoplasts requires specialized structures and proteins such as fibrillins (FBNs). The FBN family includes structural components of carotenoid sequestering structures in chromoplasts and members with metabolic roles in chloroplasts and other plastid types. However, the association of FBNs with carotenoids in plastids other than chromoplasts has remained unexplored. Here, we show that Arabidopsis (Arabidopsis thaliana) FBN6 interacts with phytoene synthase (PSY), the first enzyme of the carotenoid pathway. FBN6, but not FBN4 (a FBN that does not interact with PSY), enhances the activity of plant PSY (but not of the bacterial PSY crtB) in Escherichia coli cells. Overexpression of FBN6 in Nicotiana benthamiana leaves results in a higher production of phytoene, the product of PSY activity, whereas loss of FBN6 activity in Arabidopsis mutants dramatically reduces the production of carotenoids during seedling de-etiolation and after exposure to high light. Our work hence demonstrates that FBNs promote not only the accumulation of carotenoids in chromoplasts but also their biosynthesis in chloroplasts.

Mutations in starch biosynthesis genes affect chloroplast development in wheat pericarp

Starch bioengineering in cereals has produced a plethora of genotypes with new nutritional and technological functionalities. Modulation of amylose content from 0 to 100% was inversely correlated with starch digestibility and promoted a lower glycemic index in food products. In wheat, starch mutants have been reported to exhibit various side effects, mainly related to the seed phenotype. However, little is known about the impact of altered amylose content and starch structure on plant metabolism. Here, three bread wheat starch mutant lines with extreme phenotypes in starch branching and amylose content were used to study plant responses to starch structural changes. Omics profiling of gene expression and metabolic patterns supported changes, confirmed by ultrastructural analysis in the chloroplast of the immature seeds. In detail, the identification of differentially expressed genes belonging to functional categories related to photosynthesis, chloroplast and thylakoid (e.g. CURT1), the alteration in the accumulation of photosynthesis-related compounds, and the chloroplast alterations (aberrant shape, grana stacking alteration, and increased number of plastoglobules) suggested that the modification of starch structure greatly affects starch turnover in the chloroplast, triggering oxidative stress (ROS accumulation) and premature tissue senescence. In conclusion, this study highlighted a correlation between starch structure and chloroplast functionality in the wheat kernel.

Carotenoid sequestration protein FIBRILLIN participates in CmOR-regulated β-carotene accumulation in melon

Chromoplasts are plant organelles with a unique ability to sequester and store massive carotenoids. Chromoplasts have been hypothesized to enable high levels of carotenoid accumulation due to enhanced sequestration ability or sequestration substructure formation. However, the regulators that control the substructure component accumulation and substructure formation in chromoplasts remain unknown. In melon (Cucumis melo) fruit, β-carotene accumulation in chromoplasts is governed by ORANGE (OR), a key regulator for carotenoid accumulation in chromoplasts. By using comparative proteomic analysis of a high β-carotene melon variety and its isogenic line low-β mutant that is defective in CmOr with impaired chromoplast formation, we identified carotenoid sequestration protein FIBRILLIN1 (CmFBN1) as differentially expressed. CmFBN1 expresses highly in melon fruit tissue. Overexpression of CmFBN1 in transgenic Arabidopsis (Arabidopsis thaliana) containing ORHis that genetically mimics CmOr significantly enhances carotenoid accumulation, demonstrating its involvement in CmOR-induced carotenoid accumulation. Both in vitro and in vivo evidence showed that CmOR physically interacts with CmFBN1. Such an interaction occurs in plastoglobules and results in promoting CmFBN1 accumulation. CmOR greatly stabilizes CmFBN1, which stimulates plastoglobule proliferation and subsequently carotenoid accumulation in chromoplasts. Our findings show that CmOR directly regulates CmFBN1 protein levels and suggest a fundamental role of CmFBN1 in facilitating plastoglobule proliferation for carotenoid sequestration. This study also reveals an important genetic tool to further enhance OR-induced carotenoid accumulation in chromoplasts in crops.

Further insight into the involvement of PII1 in starch granule initiation in Arabidopsis leaf chloroplasts

The control of starch granule initiation in plant leaves is a complex process that requires active enzymes like Starch Synthase 4 and 3 (SS4 or SS3) and several noncatalytic proteins such as Protein Involved in starch Initiation 1 (PII1). In Arabidopsis leaves, SS4 is the main enzyme that control starch granule initiation, but in its absence, SS3 partly fulfills this function. How these proteins collectively act to control the initiation of starch granules remains elusive. PII1 and SS4 physically interact, and PII1 is required for SS4 to be fully active. However, Arabidopsis mutants lacking SS4 or PII1 still accumulate starch granules. Combining pii1 KO mutation with either ss3 or ss4 KO mutations provide new insights of how the remaining starch granules are synthesized. The ss3 pii1 line still accumulates starch, while the phenotype of ss4 pii1 is stronger than that of ss4. Our results indicate first that SS4 initiates starch granule synthesis in the absence of PII1 albeit being limited to one large lenticular granule per plastid. Second, that if in the absence of SS4, SS3 is able to initiate starch granules with low efficiency, this ability is further reduced with the additional absence of PII1.

OsFBN7-OsKAS I module promotes formation of plastoglobules clusters in rice chloroplasts

Plastoglobules (PGs) contiguous with the outer leaflets of thylakoid membranes regulate lipid metabolism, plastid developmental transitions, and responses to environmental stimuli. However, the function of OsFBN7, a PG-core fibrillin gene in rice, has not been elucidated. Using molecular genetics and physiobiochemical approaches, we observed that OsFBN7 overexpression promoted PG clustering in rice chloroplasts. OsFBN7 interacted with two KAS I enzymes, namely OsKAS Ia and OsKAS Ib, in rice chloroplasts. Lipidomic analysis of chloroplast subcompartments, including PGs in the OsFBN7 overexpression lines, confirmed that levels of diacylglycerol (DAG), a chloroplast lipid precursor and monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), the main chloroplast membrane lipids, were increased in PGs and chloroplasts. Furthermore, OsFBN7 enhanced the abundances of OsKAS Ia/Ib in planta and their stability under oxidative and heat stresses. In addition, RNA sequencing and real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analyses showed that the expression of the DAG synthetase gene PAP1 and MGDG synthase gene MDG2 was upregulated by OsFBN7. In conclusion, this study proposes a new model in which OsFBN7 binds to OsKAS Ia/Ib in chloroplast and enhances their abundance and stability, thereby regulating the chloroplast and PG membrane lipids involved in the formation of PG clusters.

The mysterious role of fibrillin in plastid metabolism: current advances in understanding

Fibrillins (FBNs) are a family of genes in cyanobacteria, algae, and plants. The proteins they encode possess a lipid-binding motif, exist in various types of plastids, and are associated with lipid bodies called plastoglobules, implicating them in lipid metabolism. FBNs present in the thylakoid and stroma are involved in the storage, transport, and synthesis of lipid molecules for photoprotective functions against high-light stress. In this review, the diversity of subplastid locations in the evolution of FBNs, regulation of FBNs expression by various stresses, and the role of FBNs in plastid lipid metabolism are comprehensively summarized and directions for future research are discussed.

Arabidopsis fibrillin 1-2 subfamily members exert their functions via specific protein-protein interactions

Fibrillins (FBNs) are plastidial proteins found in photosynthetic organisms from cyanobacteria to higher plants. The function of most FBNs remains unknown. Here, we focused on members of the FBN subgroup comprising FBN1a, FBN1b, and FBN2. We show that these three polypeptides interact between each other, potentially forming a network around the plastoglobule surface. Both FBN2 and FBN1s interact with allene oxide synthase, and the elimination of any of these FBNs results in a delay in jasmonate-mediated anthocyanin accumulation in response to a combination of moderate high light and low temperature. Mutations in the genes encoding FBN1s or FBN2 also affect the protection of PSII under the combination of these stresses. Fully developed leaves of these mutants have lower maximum quantum efficiency of PSII (Fv/Fm) and higher oxidative stress than wild-type plants. These effects are additive, and the fbn1a-1b-2 triple mutant shows a stronger decrease in Fv/Fm and a greater increase in oxidative stress than fbn1a-1b or fbn2 mutants. Co-immunoprecipitation analysis indicated that FBN2 also interacts with other proteins involved in different metabolic processes. We propose that these fibrillins facilitate accurate positioning of different proteins involved in distinct metabolic processes, and that their elimination leads to dysfunction of those proteins.

Turning the Knobs: The Impact of Post-translational Modifications on Carbon Metabolism

Plants rely on the carbon fixed by photosynthesis into sugars to grow and reproduce. However, plants often face non-ideal conditions caused by biotic and abiotic stresses. These constraints impose challenges to managing sugars, the most valuable plant asset. Hence, the precise management of sugars is crucial to avoid starvation under adverse conditions and sustain growth. This review explores the role of post-translational modifications (PTMs) in the modulation of carbon metabolism. PTMs consist of chemical modifications of proteins that change protein properties, including protein-protein interaction preferences, enzymatic activity, stability, and subcellular localization. We provide a holistic view of how PTMs tune resource distribution among different physiological processes to optimize plant fitness.

Coalescence and directed anisotropic growth of starch granule initials in subdomains of Arabidopsis thaliana chloroplasts

Living cells orchestrate enzyme activities to produce myriads of biopolymers but cell-biological understanding of such processes is scarce. Starch, a plant biopolymer forming discrete, semi-crystalline granules within plastids, plays a central role in glucose storage, which is fundamental to life. Combining complementary imaging techniques and Arabidopsis genetics we reveal that, in chloroplasts, multiple starch granules initiate in stromal pockets between thylakoid membranes. These initials coalesce, then grow anisotropically to form lenticular granules. The major starch polymer, amylopectin, is synthesized at the granule surface, while the minor amylose component is deposited internally. The non-enzymatic domain of STARCH SYNTHASE 4, which controls the protein's localization, is required for anisotropic growth. These results present us with a conceptual framework for understanding the biosynthesis of this key nutrient.

Starch granule initiation in Arabidopsis thaliana chloroplasts

The initiation of starch granule formation and the mechanism controlling the number of granules per plastid have been some of the most elusive aspects of starch metabolism. This review covers the advances made in the study of these processes. The analyses presented herein depict a scenario in which starch synthase isoform 4 (SS4) provides the elongating activity necessary for the initiation of starch granule formation. However, this protein does not act alone; other polypeptides are required for the initiation of an appropriate number of starch granules per chloroplast. The functions of this group of polypeptides include providing suitable substrates (maltooligosaccharides) to SS4, the localization of the starch initiation machinery to the thylakoid membranes, and facilitating the correct folding of SS4. The number of starch granules per chloroplast is tightly regulated and depends on the developmental stage of the leaves and their metabolic status. Plastidial phosphorylase (PHS1) and other enzymes play an essential role in this process since they are necessary for the synthesis of the substrates used by the initiation machinery. The mechanism of starch granule formation initiation in Arabidopsis seems to be generalizable to other plants and also to the synthesis of long-term storage starch. The latter, however, shows specific features due to the presence of more isoforms, the absence of constantly recurring starch synthesis and degradation, and the metabolic characteristics of the storage sink organs.

Tissue-type specific accumulation of the plastoglobular proteome, transcriptional networks, and plastoglobular functions

Plastoglobules are dynamic protein-lipid microcompartments in plastids enriched for isoprenoid-derived metabolites. Chloroplast plastoglobules support formation, remodeling, and controlled dismantling of thylakoids during developmental transitions and environmental responses. However, the specific molecular functions of most plastoglobule proteins are still poorly understood. This review harnesses recent co-mRNA expression data from combined microarray and RNA-seq information in ATTED-II on an updated inventory of 34 PG proteins, as well as proteomics data across 30 Arabidopsis tissue types from ATHENA. Hierarchical clustering based on relative abundance for the plastoglobule proteins across non-photosynthetic and photosynthetic tissue types showed their coordinated protein accumulation across Arabidopsis parts, tissue types, development, and senescence. Evaluation of mRNA-based forced networks at different coefficient thresholds identified a central hub with seven plastoglobule proteins and four peripheral modules. Enrichment of specific nuclear transcription factors (e.g. Golden2-like) and support for crosstalk between plastoglobules and the plastid gene expression was observed, and specific ABC1 kinases appear part of a light signaling network. Examples of other specific findings are that FBN7b is involved with upstream steps of tetrapyrrole biosynthesis and that ABC1K9 is involved in starch metabolism. This review provides new insights into the functions of plastoglobule proteins and an improved framework for experimental studies.

Interplay Between the N-Terminal Domains of Arabidopsis Starch Synthase 3 Determines the Interaction of the Enzyme With the Starch Granule

The elongation of the linear chains of starch is undertaken by starch synthases. class 3 of starch synthase (SS3) has a specific feature: a long N-terminal region containing starch binding domains (SBDs). In this work, we analyze in vivo the contribution of these domains to the localization pattern of the enzyme. For this purpose, we divided the N-terminal region of Arabidopsis SS3 in three domains: D1, D2, and D3 (each of which contains an SBD and a coiled-coil site). Our analyses indicate that the N-terminal region is sufficient to determine the same localization pattern observed with the full-length protein. D2 binds tightly the polypeptide to the polymer and it is necessary the contribution of D1 and D3 to avoid the polypeptide to be trapped in the growing polymer. The localization pattern of Arabidopsis SS3 appears to be the result of the counterbalanced action of the different domains present in its N-terminal region.

Amylose in starch: towards an understanding of biosynthesis, structure and function

Starch granules are composed of two distinct glucose polymers - amylose and amylopectin. Amylose constitutes 5-35% of most natural starches and has a major influence over starch properties in foods. Its synthesis and storage occurs within the semicrystalline amylopectin matrix of starch granules, this poses a great challenge for biochemical and structural analyses. However, the last two decades have seen vast progress in understanding amylose synthesis, including new insights into the action of GRANULE BOUND STARCH SYNTHASE (GBSS), the major glucosyltransferase that synthesises amylose, and the discovery of PROTEIN TARGETING TO STARCH1 (PTST1) that targets GBSS to starch granules. Advances in analytical techniques have resolved the fine structure of amylose, raising new questions on how structure is determined during biosynthesis. Furthermore, the discovery of wild plants that do not produce amylose revives a long-standing question of why starch granules contain amylose, rather than amylopectin alone. Overall, these findings contribute towards a full understanding of amylose biosynthesis, structure and function that will be essential for future approaches to improve starch quality in crops.

Evolutionary innovations in starch metabolism

The traditional view of starch metabolism has focused on the multiplicity of enzymes and enzyme isoforms contributing to the production of the constituent polymers, amylopectin and amylose. However, knowledge of these enzymes has not provided a full insight into many aspects of starch biosynthesis. This enzyme-centered view has recently been augmented by the discovery and characterization of novel proteins with proposed regulatory, scaffolding, and interactive roles. This begins to reveal an unprecedented level of complexity beyond mere glucan biosynthesis, enabling us to envisage how starch granules are initiated and grow into specific forms, allowing it to serve biological roles beyond just carbohydrate storage. This review focuses on very recent findings in this vibrant field, highlighting the evolutionary novelty.

Dissecting the Regulatory Network of Leaf Premature Senescence in Maize (Zea mays L.) Using Transcriptome Analysis of ZmELS5 Mutant

Leaf premature senescence largely determines maize (Zea mays L.) grain yield and quality. A natural recessive premature-senescence mutant was selected from the breeding population, and near-isogenic lines were constructed using Jing24 as the recurrent parent. In the near-isogenic lines, the dominant homozygous material was wild-type (WT), and the recessive material of early leaf senescence was the premature-senescence-type ZmELS5. To identify major genes and regulatory mechanisms involved in leaf senescence, a transcriptome analysis of the ZmELS5 and WT near-isogenic lines (NILs) was performed. A total of 8796 differentially expressed transcripts were identified between ZmELS5 and WT, including 3811 up-regulated and 4985 down-regulated transcripts. By combining gene ontology, Kyoto Encyclopedia of Genes and Genomes, gene set, and transcription factor enrichment analyses, key differentially expressed genes were screened. The senescence regulatory network was predicted based on these key differentially expressed genes, which indicated that the senescence process is mainly regulated by bHLH, WRKY, and AP2/EREBP family transcription factors, leading to the accumulations of jasmonic acid and ethylene. This causes stress responses and reductions in the chlorophyll a/b-binding protein activity level. Then, decreased ATP synthase activity leads to increased photosystem II photodamage, ultimately leading to leaf senescence.

Whole-Genome Re-Sequencing of Corylus heterophylla Blank-Nut Mutants Reveals Sequence Variations in Genes Associated With Embryo Abortion

Yield loss in the economically important hazelnut (Corylus spp.) occurs through the frequent formation of blank nuts. Although the condition is associated with embryo abortion, we have not yet identified the regulatory genes involved. Therefore, this study aimed to determine the genes related to embryo abortion in hazel. We performed whole-genome re-sequencing and single-nucleotide polymorphism (SNP) analysis on four mutant hazelnut trees (Empty1 to Empty4, C. heterophylla) bearing blank nuts and four wild-type trees (Full1 to Full4, C. heterophylla). A paired comparison of Empty1 vs. Full1, Empty2 vs. Full2, Empty3 vs. Full3, and Empty4 vs. Full4, along with the intersection of Empty1 to Empty4, revealed 3 081 common SNPs in the four blank-nut mutants. Of these, 215 synonymous SNPs in exonic regions were distributed across 178 candidate genes. Heterozygosity analysis showed that average homozygous and heterozygous SNP ratios were respectively 0.409 and 0.591 in the samples. According to Gene Ontology classification, candidate genes were enriched in the categories of binding, catalysis, molecular transducer, transporter, and molecular function regulator. Among these, 18 of 178 genes had homozygous SNPs in Empty1-4. Cis elements in the promoter region of starch synthase 4 (SS4) contain the RY-element, implying seed-specific expression. Starch granules were absent from Empty1-4 cotyledon cells, but abundantly present in Full1-Full4 cotyledon cells. The blank-nut phenotype has heavier nut shells. Overall, we conclude that single-nucleotide variants of Acetyl-CoA carboxylase 1 (ACC1), intracellular sodium/hydrogen exchanger 2 (NHX2), UDP-glycosyltransferase 74E2 (UGT74E2), DEFECTIVE IN MERISTEM SILENCING 3 (DMS3), DETOXIFICATION 43 (FRD3), and SS4 may induce embryo abortion, leading to blank-nut formation. Our results will benefit future research on how the gain or loss of candidate genes influences seed development. Moreover, our study provides novel prospects for seedless cultivar development.

Starch granule initiation and morphogenesis-progress in Arabidopsis and cereals

Starch, the major storage carbohydrate in plants, is synthesized in plastids as semi-crystalline, insoluble granules. Many organs and cell types accumulate starch at some point during their development and maturation. The biosynthesis of the starch polymers, amylopectin and amylose, is relatively well understood and mostly conserved between organs and species. However, we are only beginning to understand the mechanism by which starch granules are initiated, and the factors that control the number of granules per plastid and the size/shape of granules. Here, we review recent progress in understanding starch granule initiation and morphogenesis. In Arabidopsis, granule initiation requires several newly discovered proteins with specific locations within the chloroplast, and also on the availability of maltooligosaccharides which act as primers for initiation. We also describe progress in understanding granule biogenesis in the endosperm of cereal grains-within which there is large interspecies variation in granule initiation patterns and morphology. Investigating whether this diversity results from differences between species in the functions of known proteins, and/or from the presence of novel, unidentified proteins, is a promising area of future research. Expanding our knowledge in these areas will lead to new strategies for improving the quality of cereal crops by modifying starch granule size and shape in vivo.

PII1: a protein involved in starch initiation that determines granule number and size in Arabidopsis chloroplast

The initiation of starch granule formation is still poorly understood. However, the soluble starch synthase 4 (SS4) appears to be a major component of this process since it is required to synthesize the correct number of starch granules in the chloroplasts of Arabidopsis thaliana plants. A yeast two-hybrid screen allowed the identification of several putative SS4 interacting partners. We identified the product of At4g32190 locus as a chloroplast-targeted PROTEIN INVOLVED IN STARCH INITIATION (named PII1). Arabidopsis mutants devoid of PII1 display an alteration of the starch initiation process and accumulate, on average, one starch granule per plastid instead of the five to seven granules found in plastids of wild-type plants. These granules are larger than in wild-type, and they remain flat and lenticular. pii1 mutants display wild-type growth rates and accumulate standard starch amounts. Moreover, starch characteristics, such as amylopectin chain length distribution, remain unchanged. Our results reveal the involvement of PII1 in the starch priming process in Arabidopsis leaves through interaction with SS4.

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.

Bioinformatic and in vitro Analyses of Arabidopsis Starch Synthase 2 Reveal Post-translational Regulatory Mechanisms

Starch synthase 2 (SS2) is an important enzyme in leaf starch synthesis, elongating intermediate-length glucan chains. Loss of SS2 results in a distorted starch granule phenotype and altered physiochemical properties, highlighting its importance in starch biosynthesis, however, the post-translational regulation of SS2 is poorly understood. In this study, a combination of bioinformatic and in vitro analysis of recombinant SS2 was used to identify and characterize SS2 post-translational regulatory mechanisms. The SS2 N-terminal region, comprising the first 185 amino acids of the mature protein sequence, was shown to be highly variable between species, and was predicted to be intrinsically disordered. Intrinsic disorder in proteins is often correlated with protein phosphorylation and protein-protein interactions. Recombinant Arabidopsis thaliana SS2 formed homodimers that required the N-terminal region, but N-terminal peptides could not form stable homodimers alone. Recombinant SS2 was shown to be phosphorylated by chloroplast protein kinases and recombinant casein kinase II at two N-terminal serine residues (S63, S65), but mutation of these phosphorylation sites (Ser>Ala) revealed that they are not required for homo-dimerization. Heteromeric enzyme complex (HEC) formation between SS2 and SBE2.2 was shown to be ATP-dependent. However, SS2 homo-dimerization and protein phosphorylation are not required for its interaction with SBE2.2, as truncation of the SS2 N-terminus did not disrupt ATP-dependent HEC assembly. SS2 phosphorylation had no affect on its catalytic activity. Intriguingly, the removal of the N-terminal region of SS2 resulted in a 47-fold increase in its activity. As N-terminal truncation disrupted dimerization, this suggests that SS2 is more active when monomeric, and that transitions between oligomeric state may be a mechanism for SS2 regulation.

Functional Role of Fibrillin5 in Acclimation to Photooxidative Stress

The functional role of a lipid-associated soluble protein, fibrillin5 (FBN5), was determined with the Arabidopsis thaliana homozygous fbn5-knockout mutant line (SALK_064597) that carries a T-DNA insertion within the FBN5 gene. The fbn5 mutant remained alive, displaying a slow growth and a severe dwarf phenotype. The mutant grown even under growth light conditions at 80 µmol m-2 s-1 showed a drastic decrease in electron transfer activities around PSII, with little change in electron transfer activities around PSI, a phenomenon which was exaggerated under high light stress. The accumulation of plastoquinone-9 (PQ-9) was suppressed in the mutant, and >90% of the PQ-9 pool was reduced under growth light conditions. Non-photochemical quenching (NPQ) in the mutant functioned less efficiently, resulting from little contribution by energy-dependent quenching (qE). The ultrastructure of thylakoids in the mutant revealed that their grana were unstacked and transformed into loose and disordered structures. Light-harvesting complex (LHC)-containing large photosystem complexes and photosystem core complexes in the mutant were less abundant than those in wild-type plants. These results suggest that the lack of FBN5 causes a decrease in PQ-9 and imbalance of the redox state of PQ-9, resulting in misconducting both short-term and long-term control of the input of light energy to photosynthetic reaction centers. Furthermore, in the fbn5 mutant, the expression of genes involved in jasmonic acid biosynthesis was suppressed to ≤10% of that in the wild type under both growth-light and high-light conditions, suggesting that FBN5 functions as a transmitter of 1O2 in the stroma.

Plastid Envelope-Localized Proteins Exhibit a Stochastic Spatiotemporal Relationship to Stromules

Plastids in the viridiplantae sporadically form thin tubules called stromules that increase the interactive surface between the plastid and the surrounding cytoplasm. Several recent publications that report observations of certain proteins localizing to the extensions have then used the observations to suggest stromule-specific functions. The mechanisms by which specific localizations on these transient and sporadically formed extensions might occur remain unclear. Previous studies have yet to address the spatiotemporal relationship between a particular protein localization pattern and its distribution on an extended stromules and/or the plastid body. Here, we have used discrete protein patches found in several transgenic plants as fiducial markers to investigate this relationship. While we consider the inner plastid envelope-membrane localized protein patches of the GLUCOSE 6-PHOSPHATE/PHOSPHATE TRANSLOCATOR1 and the TRIOSE-PHOSPHATE/ PHOSPHATE TRANSLOCATOR 1 as artifacts of fluorescent fusion protein over-expression, stromule formation is not compromised in the respective stable transgenic lines that maintain normal growth and development. Our analysis of chloroplasts in the transgenic lines in the Arabidopsis Columbia background, and in the arc6 mutant, under stromule-inducing conditions shows that the possibility of finding a particular protein-enriched domain on an extended stromule or on a region of the main plastid body is stochastic. Our observations provide insights on the behavior of chloroplasts, the relationship between stromules and the plastid-body and strongly challenge claims of stromule-specific functions based solely upon protein localization to plastid extensions.

ONE SENTENCE SUMMARY

Observations of the spatiotemporal relationship between plastid envelope localized fluorescent protein fusions of two sugar-phosphate transporters and stromules suggest a stochastic rather than specific localization pattern that questions the idea of independent functions for stromules.

Parameters of Starch Granule Genesis in Chloroplasts of Arabidopsis thaliana

Starch is the primary storage carbohydrate in most photosynthetic organisms and allows the accumulation of carbon and energy in form of an insoluble and semi-crystalline particle. In the last decades large progress, especially in the model plant Arabidopsis thaliana, was made in understanding the structure and metabolism of starch and its conjunction. The process underlying the initiation of starch granules remains obscure, although this is a fundamental process and seems to be strongly regulated, as in Arabidopsis leaves the starch granule number per chloroplast is fixed with 5-7. Several single, double, and triple mutants were reported in the last years that showed massively alterations in the starch granule number per chloroplast and allowed further insights in this important process. This mini review provides an overview of the current knowledge of processes involved in the initiation and formation of starch granules. We discuss the central role of starch synthase 4 and further proteins for starch genesis and affecting metabolic factors.

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.

Starch as a source, starch as a sink: the bifunctional role of starch in carbon allocation

Starch commands a central role in the carbon budget of the majority of plants on earth, and its biological role changes during development and in response to the environment. Throughout the life of a plant, starch plays a dual role in carbon allocation, acting as both a source, releasing carbon reserves in leaves for growth and development, and as a sink, either as a dedicated starch store in its own right (in seeds and tubers), or as a temporary reserve of carbon contributing to sink strength, in organs such as flowers, fruits, and developing non-starchy seeds. The presence of starch in tissues and organs thus has a profound impact on the physiology of the growing plant as its synthesis and degradation governs the availability of free sugars, which in turn control various growth and developmental processes. This review attempts to summarize the large body of information currently available on starch metabolism and its relationship to wider aspects of carbon metabolism and plant nutrition. It highlights gaps in our knowledge and points to research areas that show promise for bioengineering and manipulation of starch metabolism in order to achieve more desirable phenotypes such as increased yield or plant biomass.

A soluble starch synthase I gene, IbSSI, alters the content, composition, granule size and structure of starch in transgenic sweet potato

Soluble starch synthase I (SSI) is a key enzyme in the biosynthesis of plant amylopectin. In this study, the gene named IbSSI, was cloned from sweet potato, an important starch crop. A high expression level of IbSSI was detected in the leaves and storage roots of the sweet potato. Its overexpression significantly increased the content and granule size of starch and the proportion of amylopectin by up-regulating starch biosynthetic genes in the transgenic plants compared with wild-type plants (WT) and RNA interference plants. The frequency of chains with degree of polymerization (DP) 5-8 decreased in the amylopectin fraction of starch, whereas the proportion of chains with DP 9-25 increased in the IbSSI-overexpressing plants compared with WT plants. Further analysis demonstrated that IbSSI was responsible for the synthesis of chains with DP ranging from 9 to 17, which represents a different chain length spectrum in vivo from its counterparts in rice and wheat. These findings suggest that the IbSSI gene plays important roles in determining the content, composition, granule size and structure of starch in sweet potato. This gene may be utilized to improve the content and quality of starch in sweet potato and other plants.

Plastoglobuli: Plastid Microcompartments with Integrated Functions in Metabolism, Plastid Developmental Transitions, and Environmental Adaptation

Plastoglobuli (PGs) are plastid lipoprotein particles surrounded by a membrane lipid monolayer. PGs contain small specialized proteomes and metabolomes. They are present in different plastid types (e.g., chloroplasts, chromoplasts, and elaioplasts) and are dynamic in size and shape in response to abiotic stress or developmental transitions. PGs in chromoplasts are highly enriched in carotenoid esters and enzymes involved in carotenoid metabolism. PGs in chloroplasts are associated with thylakoids and contain ∼30 core proteins (including six ABC1 kinases) as well as additional proteins recruited under specific conditions. Systems analysis has suggested that chloroplast PGs function in metabolism of prenyl lipids (e.g., tocopherols, plastoquinone, and phylloquinone); redox and photosynthetic regulation; plastid biogenesis; and senescence, including recycling of phytol, remobilization of thylakoid lipids, and metabolism of jasmonate. These functionalities contribute to chloroplast PGs' role in responses to stresses such as high light and nitrogen starvation. PGs are thus lipid microcompartments with multiple functions integrated into plastid metabolism, developmental transitions, and environmental adaptation. This review provides an in-depth overview of PG experimental observations, summarizes the present understanding of PG features and functions, and provides a conceptual framework for PG research and the realization of opportunities for crop improvement.

Soluble Starch Synthase III-1 in Amylopectin Metabolism of Banana Fruit: Characterization, Expression, Enzyme Activity, and Functional Analyses

Soluble starch synthase (SS) is one of the key enzymes involved in amylopectin biosynthesis in plants. However, no information is currently available about this gene family in the important fruit crop banana. Herein, we characterized the function of MaSSIII-1 in amylopectin metabolism of banana fruit and described the putative role of the other MaSS family members. Firstly, starch granules, starch and amylopectin content were found to increase during banana fruit development, but decline during storage. The SS activity started to increase later than amylopectin and starch content. Secondly, four putative SS genes were cloned and characterized from banana fruit. Among them, MaSSIII-1 showed the highest expression in banana pulp during fruit development at transcriptional levels. Further Western blot analysis suggested that the protein was gradually increased during banana fruit development, but drastically reduced during storage. This expression pattern was highly consistent with changes in starch granules, amylopectin content, and SS activity at the late phase of banana fruit development. Lastly, overexpression of MaSSIII-1 in tomato plants distinctly changed the morphology of starch granules and significantly increased the total starch accumulation, amylopectin content, and SS activity at mature-green stage in comparison to wild-type. The findings demonstrated that MaSSIII-1 is a key gene expressed in banana fruit and responsible for the active amylopectin biosynthesis, this is the first report in a fresh fruit species. Such a finding may enable the development of molecular markers for banana breeding and genetic improvement of nutritional value and functional properties of banana fruit.

Surveying the Oligomeric State of Arabidopsis thaliana Chloroplasts

Blue native-PAGE (BN-PAGE) resolves protein complexes in their native state. When combined with immunoblotting, it can be used to identify the presence of high molecular weight complexes at high resolution for any protein, given a suitable antibody. To identify proteins in high molecular weight complexes on a large scale and to bypass the requirement for specific antibodies, we applied a tandem mass spectrometry (MS/MS) approach to BN-PAGE-resolved chloroplasts. Fractionation of the gel into six bands allowed identification and label-free quantification of 1000 chloroplast proteins with native molecular weight separation. Significantly, this approach achieves a depth of identification comparable with traditional shotgun proteomic analyses of chloroplasts, indicating much of the known chloroplast proteome is amenable to MS/MS identification under our fractionation scheme. By limiting the number of fractionation bands to six, we facilitate scaled-up comparative analyses, as we demonstrate with the reticulata chloroplast mutant displaying a reticulated leaf phenotype. Our comparative proteomics approach identified a candidate interacting protein of RETICULATA as well as effects on lipid remodeling proteins, amino acid metabolic enzymes, and plastid division machinery. We additionally highlight selected proteins from each sub-compartment of the chloroplast that provide novel insight on known or hypothesized protein complexes to further illustrate the utility of this approach. Our results demonstrate the high sensitivity and reproducibility of this technique, which is anticipated to be widely adaptable to other sub-cellular compartments.

Degradation of Glucan Primers in the Absence of Starch Synthase 4 Disrupts Starch Granule Initiation in Arabidopsis

Arabidopsis leaf chloroplasts typically contain five to seven semicrystalline starch granules. It is not understood how the synthesis of each granule is initiated or how starch granule number is determined within each chloroplast. An Arabidopsis mutant lacking the glucosyl-transferase, STARCH SYNTHASE 4 (SS4) is impaired in its ability to initiate starch granules; its chloroplasts rarely contain more than one large granule, and the plants have a pale appearance and reduced growth. Here we report that the chloroplastic α-amylase AMY3, a starch-degrading enzyme, interferes with granule initiation in the ss4 mutant background. The amy3 single mutant is similar in phenotype to the wild type under normal growth conditions, with comparable numbers of starch granules per chloroplast. Interestingly, the ss4 mutant displays a pleiotropic reduction in the activity of AMY3. Remarkably, complete abolition of AMY3 (in the amy3 ss4 double mutant) increases the number of starch granules produced in each chloroplast, suppresses the pale phenotype of ss4, and nearly restores normal growth. The amy3 mutation also restores starch synthesis in the ss3 ss4 double mutant, which lacks STARCH SYNTHASE 3 (SS3) in addition to SS4. The ss3 ss4 line is unable to initiate any starch granules and is thus starchless. We suggest that SS4 plays a key role in granule initiation, allowing it to proceed in a way that avoids premature degradation of primers by starch hydrolases, such as AMY3.

Plastoglobules in algae: A comprehensive comparative study of the presence of major structural and functional components in complex plastids

Plastoglobules (PG) are lipophilic droplets attached to thylakoid membranes in higher plants and green algae and are implicated in prenyl lipid biosynthesis. They might also represent a central hub for integration of plastid signals under stress and therefore the adaptation of the thylakoid membrane under such conditions. In Arabidopsis thaliana, PG contain around 30 specific proteins of which Fibrillins (FBN) and Activity of bc1 complex kinases (ABC1K) represent the majority with respect to both number and protein mass. However, nothing is known about the presence of PG in most algal species, which are responsible for about 50% of global primary production. Therefore, we searched the genomes of publicly available algal genomes for components of PG and the associated functional network in order to predict their presence and potential evolutionary conservation of physiological functions. We could identify homologous sequences for core components of PG, like FBN and ABC1K, in most investigated algal species. Furthermore, proteins at central and interesting positions within the PG functional coexpression network were identified. Phylogenetic sequence analysis revealed diversity within FBN and ABC1K sequences among algal species with complex plastids of the red lineage and large differences compared with green lineage species. Two types of FBN were detected that differ in their isoelectric point which seems to correlate with subcellular localization. Subgroups of FBN were shared between many investigated species and modeling of their 3D-structure implied a conserved structure. FBN and ABC1K are essential structural and functional components of PG. Their occurrence in investigated algal species suggests presence of PG therein and functions in prenyl lipid metabolism and adaptation of the thylakoid membrane that are conserved during evolution.

Formation of starch in plant cells

Starch-rich crops form the basis of our nutrition, but plants have still to yield all their secrets as to how they make this vital substance. Great progress has been made by studying both crop and model systems, and we approach the point of knowing the enzymatic machinery responsible for creating the massive, insoluble starch granules found in plant tissues. Here, we summarize our current understanding of these biosynthetic enzymes, highlighting recent progress in elucidating their specific functions. Yet, in many ways we have only scratched the surface: much uncertainty remains about how these components function together and are controlled. We flag-up recent observations suggesting a significant degree of flexibility during the synthesis of starch and that previously unsuspected non-enzymatic proteins may have a role. We conclude that starch research is not yet a mature subject and that novel experimental and theoretical approaches will be important to advance the field.

Phosphorylation of plastoglobular proteins in Arabidopsis thaliana

Plastoglobules (PGs) are plastid lipid-protein particles with a small specialized proteome and metabolome. Among the 30 core PG proteins are six proteins of the ancient ABC1 atypical kinase (ABC1K) family and their locations in an Arabidopsis mRNA-based co-expression network suggested central regulatory roles. To identify candidate ABC1K targets and a possible ABC1K hierarchical phosphorylation network within the chloroplast PG proteome, we searched Arabidopsis phosphoproteomics data from publicly available sources. Evaluation of underlying spectra and/or associated information was challenging for a variety of reasons, but supported pSer sites and a few pThr sites in nine PG proteins, including five FIBRILLINS. PG phosphorylation motifs are discussed in the context of possible responsible kinases. The challenges of collection and evaluation of published Arabidopsis phosphorylation data are discussed, illustrating the importance of deposition of all mass spectrometry data in well-organized repositories such as PRIDE and ProteomeXchange. This study provides a starting point for experimental testing of phosho-sites in PG proteins and also suggests that phosphoproteomics studies specifically designed toward the PG proteome and its ABC1K are needed to understand phosphorylation networks in these specialized particles.

The N-terminal Part of Arabidopsis thaliana Starch Synthase 4 Determines the Localization and Activity of the Enzyme

Starch synthase 4 (SS4) plays a specific role in starch synthesis because it controls the number of starch granules synthesized in the chloroplast and is involved in the initiation of the starch granule. We showed previously that SS4 interacts with fibrillins 1 and is associated with plastoglobules, suborganelle compartments physically attached to the thylakoid membrane in chloroplasts. Both SS4 localization and its interaction with fibrillins 1 were mediated by the N-terminal part of SS4. Here we show that the coiled-coil region within the N-terminal portion of SS4 is involved in both processes. Elimination of this region prevents SS4 from binding to fibrillins 1 and alters SS4 localization in the chloroplast. We also show that SS4 forms dimers, which depends on a region located between the coiled-coil region and the glycosyltransferase domain of SS4. This region is highly conserved between all SS4 enzymes sequenced to date. We show that the dimerization seems to be necessary for the activity of the enzyme. Both dimerization and the functionality of the coiled-coil region are conserved among SS4 proteins from phylogenetically distant species, such as Arabidopsis and Brachypodium This finding suggests that the mechanism of action of SS4 is conserved among different plant species.

Fluorescent Protein Aided Insights on Plastids and their Extensions: A Critical Appraisal

Multi-colored fluorescent proteins targeted to plastids have provided new insights on the dynamic behavior of these organelles and their interactions with other cytoplasmic components and compartments. Sub-plastidic components such as thylakoids, stroma, the inner and outer membranes of the plastid envelope, nucleoids, plastoglobuli, and starch grains have been efficiently highlighted in living plant cells. In addition, stroma filled membrane extensions called stromules have drawn attention to the dynamic nature of the plastid and its interactions with the rest of the cell. Use of dual and triple fluorescent protein combinations has begun to reveal plastid interactions with mitochondria, the nucleus, the endoplasmic reticulum and F-actin and suggests integral roles of plastids in retrograde signaling, cell to cell communication as well as plant-pathogen interactions. While the rapid advances and insights achieved through fluorescent protein based research on plastids are commendable it is necessary to endorse meaningful observations but subject others to closer scrutiny. Here, in order to develop a better and more comprehensive understanding of plastids and their extensions we provide a critical appraisal of recent information that has been acquired using targeted fluorescent protein probes.