Glucan, water dikinase activity stimulates breakdown of starch granules by plastidial beta-amylases

Glucan, water dikinase (GWD) penetrates the starch granule surface and introduces C6 phosphate in the vicinity of branching points

Starch phosphorylation mediated by α-glucan, water dikinase is an integral part of starch metabolism. So far however, it is not fully understood. For getting deeper insights, several in vitro assays and intensive mass spectrometry analyses were performed. Such analyses allowed us to determine the phosphorylation position within the amylopectin in detail. Thus, unique features of the starch structure and GWD action were correlated. Therefore, recombinant potato GWD (Solanum tuberosum L.; StGWD) was used for detailed analyses of the phosphorylation pattern of various starches. Additionally, oil palm (Elaeis guineensis Jacq.; EgGWD) GWD was cloned and characterized, representing the first characterization of GWD of a monocot species. The distribution patterns of single phosphorylated glucan chains catalyzed by both GWDs were compared. The phosphorylation distribution patterns of both GWDs varied for different starches. It was proven that GWD phosphorylates different positions within the amylopectin of native starch granules. GWD enters the starch granule surface and phosphorylates the glucosyl units in the proximity of branching points to convert the highly ordered glucan chains into a less ordered state and to render them accessible for the downstream acting hydrolases. This enables deciphering the GWD actions and the related structural properties of starch granules.

Assessing the impacts of genetic defects on starch metabolism in Arabidopsis plants using the carbon homeostasis model

Starch serves as an important carbon storage mechanism for many plant species, facilitating their adaptation to the cyclic variations in the light environment, including day-night cycles as well as seasonal changes in photoperiod. By dynamically adjusting starch accumulation and degradation rates, plants maintain carbon homeostasis, enabling continuous growth under fluctuating environmental conditions. To understand dynamic nature of starch metabolism at the molecular level, it is necessary to integrate empirical knowledge from genetic defects in specific regulatory pathways into the dynamical system of starch metabolism. To achieve this, we evaluated the impact of genetic defects in the circadian clock, sugar sensing and starch degradation pathways using the carbon homeostasis model that encompasses the interplay between these pathways. Through the collection of starch metabolism data from 10 Arabidopsis mutants, we effectively fitted the experimental data to the model. The system-level assessment revealed that genetic defects in both circadian clock components and sugar sensing pathway hindered the appropriate adjustment of the starch degradation rate, particularly under long-day conditions. These findings not only confirmed the previous empirical findings but also provide the novel insights into the role of each gene within the gene regulatory network on the emergence of carbon homeostasis.

TgLaforin, a glucan phosphatase, reveals the dynamic role of storage polysaccharides in Toxoplasma gondii tachyzoites and bradyzoites

The asexual stages of Toxoplasma gondii are defined by the rapidly growing tachyzoite during the acute infection and by the slow growing bradyzoite housed within tissue cysts during the chronic infection. These stages represent unique physiological states, each with distinct glucans reflecting differing metabolic needs. A defining feature of T. gondii bradyzoites is the presence of insoluble storage glucans known as amylopectin granules (AGs) that are believed to play a role in reactivation, but their functions during the chronic infection remain largely unexplored. More recently, the presence of storage glucans has been recognized in tachyzoites where their precise function and architecture have yet to be fully defined. Importantly, the T. gondii genome encodes activities needed for glucan turnover: a glucan phosphatase (TgLaforin; TGME49_205290) and a glucan kinase (TgGWD; TGME49_214260) that catalyze a cycle of reversible glucan phosphorylation required for glucan degradation by amylases. The expression of these enzymes in tachyzoites supports the existence of a storage glucan, evidence that is corroborated by specific labeling with the anti-glycogen antibody IV58B6. Disruption of reversible glucan phosphorylation via a CRISPR/Cas9 knockout (KO) of TgLaforin revealed no growth defects under nutrient-replete conditions in tachyzoites. However, the growth of TgLaforin-KO tachyzoites was severely stunted when starved of glutamine, even under glucose replete conditions. The loss of TgLaforin also resulted in the attenuation of acute virulence in mice accompanied by a lower cyst burden. Defective cyst formation due to profound changes in AG morphology was also observed in TgLaforin-KO parasites, both in vitro and in vivo. Together, these data demonstrate the importance of glucan turnover across the T. gondii asexual cycle. These findings, alongside our previously identified class of small molecules that inhibit TgLaforin, implicate reversible glucan phosphorylation as a legitimate target for the development of new drugs against chronic T. gondii infections.

Mechanical wounds expedited starch degradation in the wound tissues of potato tubers

Starch degradation occurs rapidly in stressed plants, but it is unclear how starch degradation occurs in potato tubers after they incur mechanical wounding. In this study, we found that wounding significantly upregulated the expression levels of StGWD, StAMY, StBAM, and StISA, and decreased the starch content of potato tubers. Meanwhile, wounding markedly upregulated the expression levels of StSUS, StBG, and StINV genes, and increased the content of sucrose, glucose, and fructose. Furthermore, wounding reduced the proportion of small starch granules and increase that of large as well as medium starch granules, in this way enhancing the average size distribution of starch. Initially, the hard surface layer of starch granules was removed by wounding, but the internal channels and other structures were only slightly affected. Taken together, the results show that wounding can accelerate starch degradation by promoting the accumulation of sucrose, glucose, and fructose, and the hydrolysis of starch granules in potato tubers.

Physiological and Transcriptional Responses of Apocynum venetum to Salt Stress at the Seed Germination Stage

Apocynum venetum is a semi-shrubby perennial herb that not only prevents saline-alkaline land degradation but also produces leaves for medicinal uses. Although physiological changes during the seed germination of A. venetum in response to salt stress have been studied, the adaptive mechanism to salt conditions is still limited. Here, the physiological and transcriptional changes during seed germination under different NaCl treatments (0-300 mmol/L) were examined. The results showed that the seed germination rate was promoted at low NaCl concentrations (0-50 mmol/L) and inhibited with increased concentrations (100-300 mmol/L); the activity of antioxidant enzymes exhibited a significant increase from 0 (CK) to 150 mmol/L NaCl and a significant decrease from 150 to 300 mmol/L; and the content of osmolytes exhibited a significant increase with increased concentrations, while the protein content peaked at 100 mmol/L NaCl and then significantly decreased. A total of 1967 differentially expressed genes (DEGs) were generated during seed germination at 300 mmol/L NaCl versus (vs.) CK, with 1487 characterized genes (1293 up-regulated, UR; 194 down-regulated, DR) classified into 11 categories, including salt stress (29), stress response (146), primary metabolism (287), cell morphogenesis (156), transcription factor (TFs, 62), bio-signaling (173), transport (144), photosynthesis and energy (125), secondary metabolism (58), polynucleotide metabolism (21), and translation (286). The relative expression levels (RELs) of selected genes directly involved in salt stress and seed germination were observed to be consistent with the changes in antioxidant enzyme activities and osmolyte contents. These findings will provide useful references to improve seed germination and reveal the adaptive mechanism of A. venetum to saline-alkaline soils.

Uncovering the genetic basis for quality traits in the Mediterranean old wheat germplasm and phenotypic and genomic prediction assessment by cross-validation test

The release of new wheat varieties is based on two main characteristics, grain yield and quality, to meet the consumer's demand. Identifying the genetic architecture for yield and key quality traits has wide attention for genetic improvement to meet the global requirement. In this sense, the use of landraces represents an impressive source of natural allelic variation. In this study, a genome-wide association analysis (GWAS) with PCA and kinship matrix was performed to detect QTLs in bread wheat for fifteen quality and agronomic traits using 170 diverse landraces from 24 Mediterranean countries in two years of field trials. A total of 53 QTL hotspots containing 165 significant marker-trait associations (MTAs) were located across the genome for quality and agronomical traits except for chromosome 2D. The major specific QTL hotspots for quality traits were QTL_3B.3 (13 MTAs with a mean PVE of 8.2%) and QTL_4A.3 (15 MTAs, mean PVE of 11.0%), and for yield-related traits were QTL_2B.1 (8 MTAs, mean PVE of 7.4%) and QTL_4B.2 (5 MTAs, mean PVE of 10.0%). A search for candidate genes (CG) identified 807 gene models within the QTL hotspots. Ten of these CGs were expressed specifically in grain supporting the role of identified QTLs in Landraces, associated to bread wheat quality traits and grain formation. A cross-validation approach within the collection was performed to calculate the accuracies of genomic prediction for quality and agronomical traits, ranging from -0.03 to 0.64 for quality and 0.46 to 0.65 for agronomic traits. In addition, five prediction equations using the phenotypic data were developed to predict bread loaf volume in landraces. The prediction ability varied from 0.67 to 0.82 depending on the complexity of the traits considered to predict loaf volume.

Dpe2/phs1 revealed unique starch metabolism with three distinct phases characterized by different starch granule numbers per chloroplast, allowing insights into the control mechanism of granule number regulation by gene co-regulation and metabolic profiling

An Arabidopsis mutant lacking both the cytosolic Disproportionating enzyme 2 (DPE2) and the plastidial glucan Phosphorylase 1 (PHS1) revealed a unique starch metabolism. Dpe2/phs1 has been reported to have only one starch granule number per chloroplast when grown under diurnal rhythm. For this study, we analyzed dpe2/phs1 in details following the mutant development, and found that it showed three distinct periods of granule numbers per chloroplast, while there was no obvious change observed in Col-0. In young plants, the starch granule number was similar to that in Col-0 at first, and then decreased significantly, down to one or no granule per chloroplast, followed by an increase in the granule number. Thus, in dpe2/phs1, control over the starch granule number is impaired, but it is not defective in starch granule initiation. The data also indicate that the granule number is not fixed, and is regulated throughout plant growth. Furthermore, the chloroplasts revealed alterations during these three periods, with a partially strong aberrant morphology in the middle phase. Interestingly, the unique metabolism was perpetuated when starch degradation was further impaired through an additional lack of Isoamylase 3 (ISA3) or Starch excess 4 (SEX4). Transcriptomic studies and metabolic profiling revealed the co-regulation of starch metabolism-related genes and a clear metabolic separation between the periods. Most senescence-induced genes were found to be up-regulated more than twice in the starch-less mature leaves. Thus, dpe2/phs1 is a unique plant material source, with which we may study starch granule number regulation to obtain a more detailed understanding.

The Toxoplasma glucan phosphatase TgLaforin utilizes a distinct functional mechanism that can be exploited by therapeutic inhibitors

Toxoplasma gondii is an intracellular parasite that generates amylopectin granules (AGs), a polysaccharide associated with bradyzoites that define chronic T. gondii infection. AGs are postulated to act as an essential energy storage molecule that enable bradyzoite persistence, transmission, and reactivation. Importantly, reactivation can result in the life-threatening symptoms of toxoplasmosis. T. gondii encodes glucan dikinase and glucan phosphatase enzymes that are homologous to the plant and animal enzymes involved in reversible glucan phosphorylation and which are required for efficient polysaccharide degradation and utilization. However, the structural determinants that regulate reversible glucan phosphorylation in T. gondii are unclear. Herein, we define key functional aspects of the T. gondii glucan phosphatase TgLaforin (TGME49_205290). We demonstrate that TgLaforin possesses an atypical split carbohydrate-binding-module domain. AlphaFold2 modeling combined with hydrogen-deuterium exchange mass spectrometry and differential scanning fluorimetry also demonstrate the unique structural dynamics of TgLaforin with regard to glucan binding. Moreover, we show that TgLaforin forms a dual specificity phosphatase domain-mediated dimer. Finally, the distinct properties of the glucan phosphatase catalytic domain were exploited to identify a small molecule inhibitor of TgLaforin catalytic activity. Together, these studies define a distinct mechanism of TgLaforin activity, opening up a new avenue of T. gondii bradyzoite biology as a therapeutic target.

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.

Transcription Factor MdWRKY32 Participates in Starch-Sugar Metabolism by Binding to the MdBam5 Promoter in Apples During Postharvest Storage

Starch degradation with fruit ripening is closely related to the aging process and flavor formation in apples. In this study, ethylene, 1-methylcyclopropene (1-MCP), and apples treated at different temperatures were used to determine the key genes of starch-sugar metabolism during storage. Compared with 4 °C storage, 20 °C storage promoted starch degradation and sugar accumulation in apples. In addition, ethylene treatment promoted starch degradation and sugar accumulation in apples, while 1-MCP treatment showed the opposite effects. The expression of MdBams indicated the crucial role of MdBam5 in starch-sugar conversion. Transient overexpression of MdBam5 significantly reduced the starch content in apples. Furthermore, MdWRKY32 directly combined the MdBam5 promoter and activated the MdBam5 expression, which may promote the starch degradation in apples. Therefore, it was concluded that MdWRKY32 may be involved in the regulation of starch-sugar metabolism in postharvest apples by activating the MdBam5 expression.

Starch Granule Size and Morphology of Arabidopsis thaliana Starch-Related Mutants Analyzed during Diurnal Rhythm and Development

Transitory starch plays a central role in the life cycle of plants. Many aspects of this important metabolism remain unknown; however, starch granules provide insight into this persistent metabolic process. Therefore, monitoring alterations in starch granules with high temporal resolution provides one significant avenue to improve understanding. Here, a previously established method that combines LCSM and safranin-O staining for in vivo imaging of transitory starch granules in leaves of Arabidopsis thaliana was employed to demonstrate, for the first time, the alterations in starch granule size and morphology that occur both throughout the day and during leaf aging. Several starch-related mutants were included, which revealed differences among the generated granules. In ptst2 and sex1-8, the starch granules in old leaves were much larger than those in young leaves; however, the typical flattened discoid morphology was maintained. In ss4 and dpe2/phs1/ss4, the morphology of starch granules in young leaves was altered, with a more rounded shape observed. With leaf development, the starch granules became spherical exclusively in dpe2/phs1/ss4. Thus, the presented data provide new insights to contribute to the understanding of starch granule morphogenesis.

Transcriptional Controls for Early Bolting and Flowering in Angelica sinensis

The root of the perennial herb Angelica sinensis is a widely used source for traditional Chinese medicines. While the plant thrives in cool-moist regions of western China, early bolting and flowering (EBF) for young plants significantly reduces root quality and yield. Approaches to inhibit EBF by changes in physiology during the vernalization process have been investigated; however, the mechanism for activating EBF is still limited. Here, transcript profiles for bolted and unbolted plants (BP and UBP, respectively) were compared by transcriptomic analysis, expression levels of candidate genes were validated by qRT-PCR, and the accumulations of gibberellins (GA₁, GA₄, GA₈, GA₉ and GA₂₀) were also monitored by HPLC-MS/MS. A total of over 72,000 unigenes were detected with ca. 2600 differentially expressed genes (DEGs) observed in the BP compared with UBP. While various signaling pathways participate in flower induction, it is genes associated with floral development and the sucrose pathway that are observed to be coordinated in EBF plants, coherently up- and down-regulating flowering genes that activate and inhibit flowering, respectively. The signature transcripts pattern for the developmental pathways that drive flowering provides insight into the molecular signals that activate plant EBF.

Engineering Properties of Sweet Potato Starch for Industrial Applications by Biotechnological Techniques including Genome Editing

Sweet potato (Ipomoea batatas) is one of the largest food crops in the world. Due to its abundance of starch, sweet potato is a valuable ingredient in food derivatives, dietary supplements, and industrial raw materials. In addition, due to its ability to adapt to a wide range of harsh climate and soil conditions, sweet potato is a crop that copes well with the environmental stresses caused by climate change. However, due to the complexity of the sweet potato genome and the long breeding cycle, our ability to modify sweet potato starch is limited. In this review, we cover the recent development in sweet potato breeding, understanding of starch properties, and the progress in sweet potato genomics. We describe the applicational values of sweet potato starch in food, industrial products, and biofuel, in addition to the effects of starch properties in different industrial applications. We also explore the possibility of manipulating starch properties through biotechnological means, such as the CRISPR/Cas-based genome editing. The ability to target the genome with precision provides new opportunities for reducing breeding time, increasing yield, and optimizing the starch properties of sweet potatoes.

Cooperative Kinetics of the Glucan Phosphatase Starch Excess4

Glucan phosphatases are members of a functionally diverse family of dual-specificity phosphatase (DSP) enzymes. The plant glucan phosphatase Starch Excess4 (SEX4) binds and dephosphorylates glucans, contributing to processive starch degradation in the chloroplast at night. Little is known about the complex kinetics of SEX4 when acting on its complex physiologically relevant glucan substrate. Therefore, we explored the kinetics of SEX4 against both insoluble starch and soluble amylopectin glucan substrates. SEX4 displays robust activity and a unique sigmoidal kinetic response to amylopectin, characterized by a Hill coefficient of 2.77 ± 0.63, a signature feature of cooperativity. We investigated the basis for this positive kinetic cooperativity and determined that the SEX4 carbohydrate-binding module (CBM) dramatically influences the binding cooperativity and substrate transformation rates. These findings provide insights into a previously unknown but important regulatory role for SEX4 in reversible starch phosphorylation and further advances our understanding of atypical kinetic mechanisms.

Posttranslational Modification of Waxy to Genetically Improve Starch Quality in Rice Grain

The waxy (Wx) gene, encoding the granule-bound starch synthase (GBSS), is responsible for amylose biosynthesis and plays a crucial role in defining eating and cooking quality. The waxy locus controls both the non-waxy and waxy rice phenotypes. Rice starch can be altered into various forms by either reducing or increasing the amylose content, depending on consumer preference and region. Low-amylose rice is preferred by consumers because of its softness and sticky appearance. A better way of improving crops other than downregulation and overexpression of a gene or genes may be achieved through the posttranslational modification of sites or regulatory enzymes that regulate them because of their significance. The impact of posttranslational GBSSI modifications on extra-long unit chains (ELCs) remains largely unknown. Numerous studies have been reported on different crops, such as wheat, maize, and barley, but the rice starch granule proteome remains largely unknown. There is a need to improve the yield of low-amylose rice by employing posttranslational modification of Wx, since the market demand is increasing every day in order to meet the market demand for low-amylose rice in the regional area that prefers low-amylose rice, particularly in China. In this review, we have conducted an in-depth review of waxy rice, starch properties, starch biosynthesis, and posttranslational modification of waxy protein to genetically improve starch quality in rice grains.

Diurnal variation of transitory starch metabolism is regulated by plastid proteins WXR1/WXR3 in Arabidopsis young seedlings

Transitory starch is the portion of starch that is synthesized during the day in the chloroplast and usually used for plant growth overnight. Here, we report altered metabolism of transitory starch in the wxr1/wxr3 (weak auxin response 1/3) mutants of Arabidopsis. WXR1/WXR3 were previously reported to regulate root growth of young seedlings and affect the auxin response mediated by auxin polar transport in Arabidopsis. In this study the wxr1/wxr3 mutants accumulated transitory starch in cotyledon, young leaf, and hypocotyl at the end of night. WXR1/WXR3 expression showed diurnal variation. Grafting experiments indicated that the WXRs in root were necessary for proper starch metabolism and plant growth. We also found that photosynthesis was inhibited and the transcription level of DIN1/DIN6 (Dark-Inducible 1/6) was reduced in wxr1/wxr3. The mutants also showed a defect in the ionic equilibrium of Na+ and K+, consistent with our bioinformatics data that genes related to ionic equilibrium were misregulated in wxr1. Loss of function of WXR1 also resulted in abnormal trafficking of membrane lipids and proteins. This study reveals that the plastid proteins WXR1/WXR3 play important roles in promoting transitory starch degradation for plant growth over night, possibly through regulating ionic equilibrium in the root.

ELONGATED HYPOCOTYL 5 mediates blue light-induced starch degradation in tomato

Starch is the major storage carbohydrate in plants, and its metabolism in chloroplasts depends mainly on light. However, the mechanism through which photoreceptors regulate starch metabolism in chloroplasts is unclear. In this study, we found that the cryptochrome 1a (CRY1a)-mediated blue light signal is critical for regulating starch accumulation by inducing starch degradation through the transcription factor HY5 in chloroplasts in tomato. cry1a mutants and HY5-RNAi plants accumulated more starch and presented lower transcript levels of starch degradation-related genes in their leaves than wild-type plants. Blue light significantly induced the transcription of starch degradation-related genes in wild-type and CRY1a- or HY5-overexpressing plants but had little effect in cry1a and HY5-RNAi plants. Dual-luciferase assays, electrophoretic mobility shift assays, and chromatin immunoprecipitation-qPCR revealed that HY5 could activate the starch degradation-related genes PWD, BAM1, BAM3, BAM8, MEX1, and DPE1 by directly binding to their promoters. Silencing of HY5 and these starch degradation-related genes in CRY1a-overexpressing plants led to increased accumulation of starch and decreased accumulation of soluble sugars. The findings presented here not only deepen our understanding of how light controls starch degradation and sugar accumulation but also allow us to explore potential targets for improving crop quality.

Variations of Structural and Functional Traits of Azolla pinnata R. Br. in Response to Crude Oil Pollution in Arid Regions

In oil-producing countries, water pollution by crude petroleum oil frequently occurs and causes many environmental problems. This study aims to investigate the effect of crude petroleum oil on the growth and functional trails of the economically important freshwater plant Azolla pinnata R. Br. and to report on the plant’s resistance to this abiotic stress. Plants were raised in an open greenhouse experiment under different levels of crude oil pollution ranging from 0.5 to 2.0 g/L. Plant functional traits were monitored over a three-week period. Plant cover of A. pinnata was decreased with the increased levels of oil pollution. The total chlorophyll content decreased from 0.76 mg/g fresh weight under 2 g/L oil treatment after 21 days of growth. The chlorophyll a/b ratio exceeded the unity at crude oil treatments above 1 g/L, with values reaching 2.78 after seven days, while after 21 days, the ratio ranged from 1.14 to 1.31. The carotenoid content ranged from 0.17 mg/g in the control to 0.11 mg/g in the 2 g/L oil treatment. The carotenoid content varied over time in relation to DNA% damage, which increased from 3.63% in the control to 11.36% in the highest oil treatment level of 2 g/L. The crude oil stress caused severe damage in the frond tissues and chloroplast structure of A. pinnata, including a less compacted palisade, the malformation of the epidermis, the disintegration of parenchyma tissue, and the lysis and malformation of the chloroplasts. Since A. pinnata cannot withstand high concentrations of crude oil pollution, it is for use in the remediation of slightly polluted freshwaters up to 0.5 g/L.

Transcriptome skimming of lentil (Lens culinaris Medikus) cultivars with contrast reaction to salt stress

Extensive transcriptomic skimming was conducted to decipher molecular, morphological, physiological, and biochemical responses in salt-tolerant (PDL-1) and salt-sensitive (L-4076) cultivars under control (0 mM NaCl) and salinity stress (120 mM NaCl) conditions at seedling stage. Morphological, physiological, and biochemical studies revealed that PDL-1 exhibited no salt injury and had higher K⁺/Na⁺ ratio, relative water content (RWC), chlorophyll, glycine betaine, and soluble sugars in leaves while lower H₂O₂ induced fluorescence signals in roots as compared to L-4076. Transcriptomic profile revealed a total of 17,433 significant differentially expressed genes (DEGs) under different treatments and cultivar combinations that include 2557 upregulated and 1533 downregulated transcripts between contrasting cultivars under salt stress. Accuracy of transcriptomic analysis was validated through quantification of 10 DEGs via quantitative real-time polymerase chain reaction (qRT-PCR). DEGs were functionally characterized by Gene Ontology (GO) analysis and assigned to various metabolic pathways using MapMan. DEGs were found to be significantly associated with phytohormone-mediated signal transduction, cellular redox homoeostasis, secondary metabolism, nitrogen metabolism, and cellular stress signaling. The present study revealed putative molecular mechanism of salinity tolerance in lentil together with identification of 5643 simple sequence repeats (SSRs) and 176,433 single nucleotide polymorphisms (SNPs) which can be utilized to enhance linkage maps density along with detection of quantitative trait loci (QTLs) associated with traits of interests. Stress-related pathways identified in this study divulged plant functioning that can be targeted to improve salinity stress tolerance in crop species.

Molecular and physiological mechanisms underlying magnesium-deficiency-induced enlargement, cracking and lignification of Citrus sinensis leaf veins

Little is known about the physiological and molecular mechanisms underlying magnesium (Mg)-deficiency-induced enlargement, cracking and lignification of midribs and main lateral veins of Citrus leaves. Citrus sinensis (L.) Osbeck seedlings were irrigated with nutrient solution at a concentration of 0 (Mg-deficiency) or 2 (Mg-sufficiency) mM Mg(NO3)2 for 16 weeks. Enlargement, cracking and lignification of veins occurred only in lower leaves, but not in upper leaves. Total soluble sugars (glucose + fructose + sucrose), starch and cellulose concentrations were less in Mg-deficiency veins of lower leaves (MDVLL) than those in Mg-sufficiency veins of lower leaves (MSVLL), but lignin concentration was higher in MDVLL than that in MSVLL. However, all four parameters were similar between Mg-deficiency veins of upper leaves (MDVUL) and Mg-sufficiency veins of upper leaves (MSVUL). Using label-free, liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis, we identified 1229 and 492 differentially abundant proteins (DAPs) in MDVLL vs MSVLL and MDVUL vs MSVUL, respectively. Magnesium-deficiency-induced alterations of Mg, nonstructural carbohydrates, cell wall components, and protein profiles were greater in veins of lower leaves than those in veins of upper leaves. The increased concentration of lignin in MDVLL vs MSVLL might be caused by the following factors: (i) repression of cellulose and starch accumulation promoted lignin biosynthesis; (ii) abundances of proteins involved in phenylpropanoid biosynthesis pathway, hormone biosynthesis and glutathione metabolism were increased; and (iii) the abundances of the other DAPs [viz., copper/zinc-superoxide dismutase, ascorbate oxidase (AO) and ABC transporters] involved in lignin biosynthesis were elevated. Also, the abundances of several proteins involved in cell wall metabolism (viz., expansins, Rho GTPase-activating protein gacA, AO, monocopper oxidase-like protein and xyloglucan endotransglucosylase/hydrolase) were increased in MDVLL vs MSVLL, which might be responsible for the enlargement and cracking of leaf veins.

Starch and Glycogen Analyses: Methods and Techniques

For complex carbohydrates, such as glycogen and starch, various analytical methods and techniques exist allowing the detailed characterization of these storage carbohydrates. In this article, we give a brief overview of the most frequently used methods, techniques, and results. Furthermore, we give insights in the isolation, purification, and fragmentation of both starch and glycogen. An overview of the different structural levels of the glucans is given and the corresponding analytical techniques are discussed. Moreover, future perspectives of the analytical needs and the challenges of the currently developing scientific questions are included.

Starch phosphorylation and the in vivo regulation of starch metabolism and characteristics

Starch is the most significant carbon and energy reserve in plants and is also a sustainable feedstock for many industrial applications. Substantial research effort has been devoted to enhancing the yield and quality of starch. Over the past century, starch phosphorylation has aroused increasing interest as the only naturally occurring covalent modification in starch. Many studies have investigated the role of phosphorylation in starch metabolism and its impact on the starch granule. In this review, the two key enzymes involved in starch phosphorylation and their catalytic mechanisms are described at the molecular level; the vital roles of phosphorylation in starch degradation and biosynthesis are illuminated in detail; and the multiple influences of phosphorylation on starch composition, granule structure and physicochemical properties are discussed. This review systematically summarizes the importance of phosphorylation in starch metabolism, and describes the advanced methods used to precisely measure phosphate and increase the level of starch phosphorylation.

Starch: A Flexible, Adaptable Carbon Store Coupled to Plant Growth

Research in the past decade has uncovered new and surprising information about the pathways of starch synthesis and degradation. This includes the discovery of previously unsuspected protein families required both for processes and for the long-sought mechanism of initiation of starch granules. There is also growing recognition of the central role of leaf starch turnover in making carbon available for growth across the day-night cycle. Sophisticated systems-level control mechanisms involving the circadian clock set rates of nighttime starch mobilization that maintain a steady supply of carbon until dawn and modulate partitioning of photosynthate into starch in the light, optimizing the fraction of assimilated carbon that can be used for growth. These discoveries also uncover complexities: Results from experiments with Arabidopsis leaves in conventional controlled environments are not necessarily applicable to other organs or species or to growth in natural, fluctuating environments.

Mutations in Glucan, Water Dikinase Affect Starch Degradation and Gametophore Development in the Moss Physcomitrella patens

The role of starch degradation in non-vascular plants is poorly understood. To expand our knowledge of this area, we have studied this process in Physcomitrella patens. This has been achieved through examination of the step known to initiate starch degradation in angiosperms, glucan phosphorylation, catalysed by glucan, water dikinase (GWD) enzymes. Phylogenetic analysis indicates that GWD isoforms can be divided into two clades, one of which contains GWD1/GWD2 and the other GWD3 isoforms. These clades split at a very early stage within plant evolution, as distinct sequences that cluster within each were identified in all major plant lineages. Of the five genes we identified within the Physcomitrella genome that encode GWD-like enzymes, two group within the GWD1/GWD2 clade and the others within the GWD3 clade. Proteins encoded by both loci in the GWD1/GWD2 clade, named PpGWDa and PpGWDb, are localised in plastids. Mutations of either PpGWDa or PpGWDb reduce starch phosphate abundance, however, a mutation at the PpGWDa locus had a much greater influence than one at PpGWDb. Only mutations affecting PpGWDa inhibited starch degradation. Mutants lacking this enzyme also failed to develop gametophores, a phenotype that could be chemically complemented using glucose supplementation within the growth medium.

LIKE SEX4 1 Acts as a β-Amylase-Binding Scaffold on Starch Granules during Starch Degradation

In Arabidopsis (Arabidopsis thaliana) leaves, starch is synthesized during the day and degraded at night to fuel growth and metabolism. Starch is degraded primarily by β-amylases, liberating maltose, but this activity is preceded by glucan phosphorylation and is accompanied by dephosphorylation. A glucan phosphatase family member, LIKE SEX4 1 (LSF1), binds starch and is required for normal starch degradation, but its exact role is unclear. Here, we show that LSF1 does not dephosphorylate glucans. The recombinant dual specificity phosphatase (DSP) domain of LSF1 had no detectable phosphatase activity. Furthermore, a variant of LSF1 mutated in the catalytic cysteine of the DSP domain complemented the starch-excess phenotype of the lsf1 mutant. By contrast, a variant of LSF1 with mutations in the carbohydrate binding module did not complement lsf1 Thus, glucan binding, but not phosphatase activity, is required for the function of LSF1 in starch degradation. LSF1 interacts with the β-amylases BAM1 and BAM3, and the BAM1-LSF1 complex shows amylolytic but not glucan phosphatase activity. Nighttime maltose levels are reduced in lsf1, and genetic analysis indicated that the starch-excess phenotype of lsf1 is dependent on bam1 and bam3 We propose that LSF1 binds β-amylases at the starch granule surface, thereby promoting starch degradation.

The Starch Is (Not) Just Another Brick in the Wall: The Primary Metabolism of Sugars During Banana Ripening

The monocot banana fruit is one of the most important crops worldwide. As a typical climacteric fruit, the harvest of commercial bananas usually occurs when the fruit is physiologically mature but unripe. The universal treatment of green bananas with ethylene or ethylene-releasing compounds in order to accelerate and standardize the ripening of a bunch of bananas mimics natural maturation after increasing the exogenous production of ethylene. The trigger of autocatalytic ethylene production regulated by a dual positive feedback loop circuit derived from a NAC gene and three MADS genes results in metabolic processes that induce changes in the primary metabolism of bananas. These changes include pulp softening and sweetening which are sensorial attributes that determine banana postharvest quality. During fruit development, bananas accumulate large amounts of starch (between 15 and 35% w/w of their fresh weight, depending on the cultivar). Pulp softening and sweetening during banana ripening are attributed not only to changes in the activities of cell wall hydrolases but also to starch-to-sugar metabolism. Therefore, starch granule erosion and disassembling are key events that lead bananas to reach their optimal postharvest quality. The knowledge of the mechanisms that regulate sugar primary metabolism during banana ripening is fundamental to reduce postharvest losses and improve final product quality, though. Recent studies have shown that ethylene-mediated regulation of starch-degrading enzymes at transcriptional and translational levels is crucial for sugar metabolism in banana ripening. Furthermore, the crosstalk between ethylene and other hormones including indole-3-acetic acid and abscisic acid also influences primary sugar metabolism. In this review, we will describe the state-of-the-art sugar primary metabolism in bananas and discuss the recent findings that shed light on the understanding of the molecular mechanisms involved in the regulation of this metabolism during fruit ripening.

TaZFP1, a C2H2 type-ZFP gene of T. aestivum, mediates salt stress tolerance of plants by modulating diverse stress-defensive physiological processes

Salt stress suppresses plant growth, development, and crop productivity. In this study, we characterized the role of TaZFP1, a C2H2 type-zinc finger protein family member of T. aestivum, in salt stress tolerance. TaZFP1 possesses a conserved C2H2 motif (CX2-4CX12HX3-5H) shared by plant ZFP proteins, translocates to the nucleus after endoplasmic reticulum (ER) assortment, and displays a ZF 3-D structure similar to its eukaryote homologs. The transcripts of TaZFP1 were upregulated during salt stress condition and this effect was restored under normal conditions. Compared to wild type (WT), the transgenic lines of TaZFP1 overexpression or knockdown displayed improved phenotypes, biomass, photosynthesis parameters (Pn, ΨPSII, and NPQ), osmolytes contents (i.e. proline and soluble sugar), and enhanced antioxidant enzyme (AE) activity following salt stress treatment. A set of genes associated with proline synthesis (i.e., NtP5CS1 and NtP5CS2) and encoding AEs (i.e., NtSOD2, NtCAT1, and NtPOD4) were upregulated in the salt-challenged transgenic lines of TaZFP1 expression. Additionally, the transgenic lines exhibited similar stomata movement patterns and leaf water retention properties under salinity conditions compared to those induced by exogenous abscisic acid (ABA) treatment, suggesting that the TaZFP1-mediated salt response is dependent on the ABA signaling. High throughput RNAseq analysis revealed significant alteration of gene transcription in transgenic lines upon salt stress. Among them, the differentially expressed genes (DEGs) represented by the gene ontology (GO) terms were associated with organic acid, carboxylic acid, carbohydrate, and coenzyme as well as organonitrogen compounds, translation, peptide metabolism, and peptide biosynthesis. A set of upregulated DEGs were found to be thylakoid- and photosystem-associated, which is consistent with the TaZFP1-mediated improvement in photosynthesis in salt-stressed transgenic lines. Our investigation indicated that the TaZFP1-mediated salt tolerance is ascribed to the regulation of gene functions related to photosynthesis, osmolytes metabolism and ROS homeostasis mediated by ABA signaling.

Brain Glycogen Structure and Its Associated Proteins: Past, Present and Future

This chapter reviews the history of glycogen-related research and discusses in detail the structure, regulation, chemical properties and subcellular distribution of glycogen and its associated proteins, with particular focus on these aspects in brain tissue.

Starch degradation, abscisic acid and vesicular trafficking are important elements in callose priming by indole-3-carboxylic acid in response to Plectosphaerella cucumerina infection

A fast callose accumulation has been shown to mediate defence priming in certain plant-pathogen interactions, but the events upstream of callose assembly following chemical priming are poorly understood, mainly because those steps comprise sugar transfer to the infection site. β-Amino butyric acid (BABA)-induced resistance in Arabidopsis against Plectosphaerella cucumerina is known to be mediated by callose priming. Indole-3-carboxylic acid (ICOOH, also known as I3CA) mediates BABA-induced resistance in Arabidopsis against P. cucumerina. This indolic compound is found in a common fingerprint of primed metabolites following treatments with various priming stimuli. In the present study, we show that I3CA induces resistance in Arabidopsis against P. cucumerina and primes enhancement of callose accumulation. I3CA treatment increased abscisic acid (ABA) levels before infection with P. cucumerina. An intact ABA synthesis pathway is needed to activate a starch amylase (BAM1) to trigger augmented callose deposition against P. cucumerina during I3CA-IR. To verify the relevance of the BAM1 amylase in I3CA-IR, knockdown mutants and overexpressors of the BAM1 gene were tested. The mutant bam1 was impaired to express I3CA-IR, but complemented 35S::BAM1-YFP lines in the background of bam1 restored an intact I3CA-IR and callose priming. Therefore, a more active starch metabolism is a committed step for I3CA-IR, inducing callose priming in adult plants. Additionally, I3CA treatments induced expression of the ubiquitin ligase ATL31 and syntaxin SYP131, suggesting that vesicular trafficking is relevant for callose priming. As a final element in the callose priming, an intact Powdery Mildew resistant4 (PMR4) gene is also essential to fully express I3CA-IR.

EARLY STARVATION1 specifically affects the phosphorylation action of starch-related dikinases

Starch phosphorylation by starch-related dikinases glucan, water dikinase (GWD) and phosphoglucan, water dikinase (PWD) is a key step in starch degradation. Little information is known about the precise structure of the glucan substrate utilized by the dikinases and about the mechanisms by which these structures may be influenced. A 50-kDa starch-binding protein named EARLY STARVATION1 (ESV1) was analyzed regarding its impact on starch phosphorylation. In various in vitro assays, the influences of the recombinant protein ESV1 on the actions of GWD and PWD on the surfaces of native starch granules were analyzed. In addition, we included starches from various sources as well as truncated forms of GWD. ESV1 preferentially binds to highly ordered, α-glucans, such as starch and crystalline maltodextrins. Furthermore, ESV1 specifically influences the action of GWD and PWD at the starch granule surface. Starch phosphorylation by GWD is decreased in the presence of ESV1, whereas the action of PWD increases in the presence of ESV1. The unique alterations observed in starch phosphorylation by the two dikinases are discussed in regard to altered glucan structures at the starch granule surface.

Abnormal glycogen chain length pattern, not hyperphosphorylation, is critical in Lafora disease

Lafora disease (LD) is a fatal progressive epilepsy essentially caused by loss-of-function mutations in the glycogen phosphatase laforin or the ubiquitin E3 ligase malin. Glycogen in LD is hyperphosphorylated and poorly hydrosoluble. It precipitates and accumulates into neurotoxic Lafora bodies (LBs). The leading LD hypothesis that hyperphosphorylation causes the insolubility was recently challenged by the observation that phosphatase-inactive laforin rescues the laforin-deficient LD mouse model, apparently through correction of a general autophagy impairment. We were for the first time able to quantify brain glycogen phosphate. We also measured glycogen content and chain lengths, LBs, and autophagy markers in several laforin- or malin-deficient mouse lines expressing phosphatase-inactive laforin. We find that: (i) in laforin-deficient mice, phosphatase-inactive laforin corrects glycogen chain lengths, and not hyperphosphorylation, which leads to correction of glycogen amounts and prevention of LBs; (ii) in malin-deficient mice, phosphatase-inactive laforin confers no correction; (iii) general impairment of autophagy is not necessary in LD We conclude that laforin's principle function is to control glycogen chain lengths, in a malin-dependent fashion, and that loss of this control underlies LD.

Modification of Cassava Root Starch Phosphorylation Enhances Starch Functional Properties

Cassava (Manihot esculenta Crantz) is a root crop used as a foodstuff and as a starch source in industry. Starch functional properties are influenced by many structural features including the relative amounts of the two glucan polymers amylopectin and amylose, the branched structure of amylopectin, starch granule size and the presence of covalent modifications. Starch phosphorylation, where phosphates are linked either to the C3 or C6 carbon atoms of amylopectin glucosyl residues, is a naturally occurring modification known to be important for starch remobilization. The degree of phosphorylation has been altered in several crops using biotechnological approaches to change expression of the starch-phosphorylating enzyme GLUCAN WATER DIKINASE (GWD). Interestingly, this frequently alters other structural features of starch beside its phosphate content. Here, we aimed to alter starch phosphorylation in cassava storage roots either by manipulating the expression of the starch phosphorylating or dephosphorylating enzymes. Therefore, we generated transgenic plants in which either the wild-type potato GWD (StGWD) or a redox-insensitive version of it were overexpressed. Further plants were created in which we used RNAi to silence each of the endogenous phosphoglucan phosphatase genes STARCH EXCESS 4 (MeSEX4) and LIKE SEX4 2 (MeLSF), previously discovered by analyzing leaf starch metabolism in the model species Arabidopsis thaliana. Overexpressing the potato GWD gene (StGWD), which specifically phosphorylates the C6 position, increased the total starch-bound phosphate content at both the C6 and the C3 positions. Silencing endogenous LSF2 gene (MeLSF2), which specifically dephosphorylates the C3 position, increased the ratio of C3:C6 phosphorylation, showing that its function is conserved in storage tissues. In both cases, other structural features of starch (amylopectin structure, amylose content and starch granule size) were unaltered. This allowed us to directly relate the physicochemical properties of the starch to its phosphate content or phosphorylation pattern. Starch swelling power and paste clarity were specifically influenced by total phosphate content. However, phosphate position did not significantly influence starch functional properties. In conclusion, biotechnological manipulation of starch phosphorylation can specifically alter certain cassava storage root starch properties, potentially increasing its value in food and non-food industries.

A comprehensive investigation of starch degradation process and identification of a transcriptional activator MabHLH6 during banana fruit ripening

Although starch degradation has been well studied in model systems such as Arabidopsis leaves and cereal seeds, this process in starchy fruits during ripening, especially in bananas, is largely unknown. In this study, 38 genes encoding starch degradation-related proteins were identified and characterized from banana fruit. Expression analysis revealed that 27 candidate genes were significantly induced during banana fruit ripening, with concomitant conversion of starch-to-sugars. Furthermore, iTRAQ-based proteomics experiments identified 18 starch degradation-associated enzymes bound to the surface of starch granules, of which 10 were markedly up-regulated during ripening. More importantly, a novel bHLH transcription factor, MabHLH6, was identified based on a yeast one-hybrid screening using MaGWD1 promoter as a bait. Transcript and protein levels of MabHLH6 were also increased during fruit ripening. Electrophoretic mobility shift assays, chromatin immunoprecipitation and transient expression experiments confirmed that MabHLH6 activates the promoters of 11 starch degradation-related genes, including MaGWD1, MaLSF2, MaBAM1, MaBAM2, MaBAM8, MaBAM10, MaAMY3, MaAMY3C, MaISA2, MaISA3 and MapGlcT2-2 by recognizing their E-box (CANNTG) motifs present in the promoters. Collectively, these findings suggest that starch degradation during banana fruit ripening may be attributed to the complex actions of numerous enzymes related to starch breakdown at transcriptional and translational levels, and that MabHLH6 may act as a positive regulator of this process via direct activation of a series of starch degradation-related genes.

Alpha-Glucan, Water Dikinase 1 Affects Starch Metabolism and Storage Root Growth in Cassava (Manihot esculenta Crantz)

Regulation of storage root development by source strength remains largely unknown. The cassava storage root delay (srd) T-DNA mutant postpones storage root development but manifests normal foliage growth as wild-type plants. The SRD gene was identified as an orthologue of α-glucan, water dikinase 1 (GWD1), whose expression is regulated under conditions of light/dark cycles in leaves and is associated with storage root development. The GWD1-RNAi cassava plants showed both retarded plant and storage root growth, as a result of starch excess phenotypes with reduced photosynthetic capacity and decreased levels of soluble saccharides in their leaves. These leaves contained starch granules having greatly increased amylose content and type C semi-crystalline structures with increased short chains that suggested storage starch. In storage roots of GWD1-RNAi lines, maltose content was dramatically decreased and starches with much lower phosphorylation levels showed a drastically reduced β-amylolytic rate. These results suggested that GWD1 regulates transient starch morphogenesis and storage root growth by decreasing photo-assimilation partitioning from the source to the sink and by starch mobilization in root crops.

The analysis of the different functions of starch-phosphorylating enzymes during the development of Arabidopsis thaliana plants discloses an unexpected role for the cytosolic isoform GWD2

The genome of Arabidopsis thaliana encodes three glucan, water dikinases. Glucan, water dikinase 1 (GWD1; EC 2.7.9.4) and phosphoglucan, water dikinase (PWD; EC 2.7.9.5) are chloroplastic enzymes, while glucan, water dikinase 2 (GWD2) is cytosolic. Both GWDs and PWD catalyze the addition of phosphate groups to amylopectin chains at the surface of starch granules, changing its physicochemical properties. As a result, GWD1 and PWD have a positive effect on transitory starch degradation at night. Because of its cytosolic localization, GWD2 does not have the same effect. Single T-DNA mutants of either GWD1 or PWD or GWD2 have been analyzed during the entire life cycle of A. thaliana. We report that the three dikinases are all important for proper seed development. Seeds from gwd2 mutants are shrunken, with the epidermal cells of the seed coat irregularly shaped. Moreover, gwd2 seeds contain a lower lipid to protein ratio and are impaired in germination. Similar seed phenotypes were observed in pwd and gwd1 mutants, except for the normal morphology of epidermal cells in gwd1 seed coats. The gwd1, pwd and gwd2 mutants were also very similar in growth and flowering time when grown under continuous light and all three behaved differently from wild-type plants. Besides pinpointing a novel role of GWD2 and PWD in seed development, this analysis suggests that the phenotypic features of the dikinase mutants in A. thaliana cannot be explained solely in terms of defects in leaf starch degradation at night.

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.

Turgor-responsive starch phosphorylation in Oryza sativa stems: A primary event of starch degradation associated with grain-filling ability

Grain filling ability is mainly affected by the translocation of carbohydrates generated from temporarily stored stem starch in most field crops including rice (Oryza sativa L.). The partitioning of non-structural stem carbohydrates has been recognized as an important trait for raising the yield ceiling, yet we still do not fully understand how carbohydrate partitioning occurs in the stems. In this study, two rice subspecies that exhibit different patterns of non-structural stem carbohydrates partitioning, a japonica-dominant cultivar, Momiroman, and an indica-dominant cultivar, Hokuriku 193, were used as the model system to study the relationship between turgor pressure and metabolic regulation of non-structural stem carbohydrates, by combining the water status measurement with gene expression analysis and a dynamic prefixed 13C tracer analysis using a mass spectrometer. Here, we report a clear varietal difference in turgor-associated starch phosphorylation occurred at the initiation of non-structural carbohydrate partitioning. The data indicated that starch degradation in Hokuriku 193 stems occurred at full-heading, 5 days earlier than in Momiroman, contributing to greater sink filling. Gene expression analysis revealed that expression pattern of the gene encoding α-glucan, water dikinase (GWD1) was similar between two varieties, and the maximum expression level in Hokuriku 193, reached at full heading (4 DAH), was greater than in Momiroman, leading to an earlier increase in a series of amylase-related gene expression in Hokuriku 193. In both varieties, peaks in turgor pressure preceded the increases in GWD1 expression, and changes in GWD1 expression was correlated with turgor pressure. Additionally, a threshold is likely to exist for GWD1 expression to facilitate starch degradation. Taken together, these results raise the possibility that turgor-associated starch phosphorylation in cells is responsible for the metabolism that leads to starch degradation. Because the two cultivars exhibited remarkable varietal differences in the pattern of non-structural carbohydrate partitioning, our findings propose that the observed difference in grain-filling ability originated from turgor-associated regulation of starch phosphorylation in stem parenchyma cells. Further understanding of the molecular mechanism of turgor-regulation may provide a new selection criterion for breaking the yield barriers in crop production.

Simultaneous silencing of isoamylases ISA1, ISA2 and ISA3 by multi-target RNAi in potato tubers leads to decreased starch content and an early sprouting phenotype

Isoamylases hydrolyse (1–6)-alpha-D-glucosidic linkages in starch and are involved in both starch granule formation and starch degradation. In plants, three isoamylase isoforms with distinct functions in starch synthesis (ISA1 and ISA2) and degradation (ISA3) have been described. Here, we created transgenic potato plants with simultaneously decreased expression of all three isoamylases using a chimeric RNAi construct targeting all three isoforms. Constitutive expression of the hairpin RNA using the 35S CaMV promoter resulted in efficient silencing of all three isoforms in leaves, growing tubers, and sprouting tubers. Neither plant growth nor tuber yield was effected in isoamylase-deficient potato lines. Interestingly, starch metabolism was found to be impaired in a tissue-specific manner. While leaf starch content was unaffected, tuber starch was significantly reduced. The reduction in tuber starch content in the transgenic plants was accompanied by a decrease in starch granules size, an increased sucrose content and decreased hexose levels. Despite the effects on granule size, only little changes in chain length composition of soluble and insoluble glucose polymers were detected. The transgenic tubers displayed an early sprouting phenotype that was accompanied by an increased level of sucrose in parenchyma cells below the outgrowing bud. Since high sucrose levels promote sprouting, we propose that the increased number of small starch granules may cause an accelerated turnover of glucan chains and hence a more rapid synthesis of sucrose. This observation links alterations in starch structure/degradation with developmental processes like meristem activation and sprout outgrowth in potato tubers.

Highly phosphorylated functionalized rice starch produced by transgenic rice expressing the potato GWD1 gene

Starch phosphorylation occurs naturally during starch metabolism in the plant and is catalysed by glucan water dikinases (GWD1) and phosphoglucan water dikinase/glucan water dikinase 3 (PWD/GWD3). We generated six stable individual transgenic lines by over-expressing the potato GWD1 in rice. Transgenic rice grain starch had 9-fold higher 6-phospho (6-P) monoesters and double amounts of 3-phospho (3-P) monoesters, respectively, compared to control grain. The shape and topography of the transgenic starch granules were moderately altered including surface pores and less well defined edges. The gelatinization temperatures of both rice flour and extracted starch were significantly lower than those of the control and hence negatively correlated with the starch phosphate content. The 6-P content was positively correlated with amylose content and relatively long amylopectin chains with DP25-36, and the 3-P content was positively correlated with short chains of DP6-12. The starch pasting temperature, peak viscosity and the breakdown were lower but the setback was higher for transgenic rice flour. The 6-P content was negatively correlated with texture adhesiveness but positively correlated with the cohesiveness of rice flour gels. Our data demonstrate a way forward to employ a starch bioengineering approach for clean modification of starch, opening up completely new applications for rice starch.

Transitory Starch Metabolism in Guard Cells: Unique Features for a Unique Function

The pathway and timing of starch turnover in guard cells differs from mesophyll cells and is linked to stomatal opening in the light.

Starch as a determinant of plant fitness under abiotic stress

Contents 943 I. 943 II. 944 III. 945 IV. 945 V. 948 VI. 949 950 References 950 SUMMARY: Abiotic stresses, such as drought, high salinity and extreme temperatures, pose one of the most important constraints to plant growth and productivity in many regions of the world. A number of investigations have shown that plants, including several important crops, remobilize their starch reserve to release energy, sugars and derived metabolites to help mitigate the stress. This is an essential process for plant fitness with important implications for plant productivity under challenging environmental conditions. In this Tansley insight, we evaluate the current literature on starch metabolism in response to abiotic stresses, and discuss the key enzymes involved and how they are regulated.

Inhibition of Arabidopsis chloroplast β-amylase BAM3 by maltotriose suggests a mechanism for the control of transitory leaf starch mobilisation

Starch breakdown in leaves at night is tightly matched to the duration of the dark period, but the mechanism by which this regulation is achieved is unknown. In Arabidopsis chloroplasts, β-amylase BAM3 hydrolyses transitory starch, producing maltose and residual maltotriose. The aim of the current research was to investigate the regulatory and kinetic properties of BAM3. The BAM3 protein was expressed in Escherichia coli and first assayed using a model substrate. Enzyme activity was stimulated by treatment with dithiothreitol and was increased 40% by 2–10 μM Ca²⁺ but did not require Mg²⁺. In order to investigate substrate specificity and possible regulatory effects of glucans, we developed a GC-MS method to assay reaction products. BAM3 readily hydrolysed maltohexaose with a K_m of 1.7 mM and K_cat of 4300 s^-1 but activity was 3.4-fold lower with maltopentaose and was negligible with maltotetraose. With maltohexaose or amylopectin as substrates and using [UL-¹³C₁₂]maltose in an isotopic dilution method, we discovered that BAM3 activity is inhibited by maltotriose at physiological (mM) concentrations, but not by maltose. In contrast, the extracellular β-amylase of barley is only weakly inhibited by maltotriose. Our results may explain the impaired starch breakdown in maltotriose-accumulating mutants such as dpe1 which lacks the chloroplast disproportionating enzyme (DPE1) metabolising maltotriose to glucose. We hypothesise that the rate of starch breakdown in leaves can be regulated by inhibition of BAM3 by maltotriose, the concentration of which is determined by DPE, which is in turn influenced by the stromal concentration of glucose. Since the plastid glucose transporter pGlcT catalyses facilitated diffusion between stroma and cytosol, changes in consumption of glucose in the cytosol are expected to lead to concomitant changes in plastid glucose and maltotriose, and hence compensatory changes in BAM3 activity.

Engineering Potato Starch with a Higher Phosphate Content

Phosphate esters are responsible for valuable and unique functionalities of starch for industrial applications. Also in the cell phosphate esters play a role in starch metabolism, which so far has not been well characterized in storage starch. Laforin, a human enzyme composed of a carbohydrate-binding module and a dual-specificity phosphatase domain, is involved in the dephosphorylation of glycogen. To modify phosphate content and better understand starch (de)phosphorylation in storage starch, laforin was engineered and introduced into potato (cultivar Kardal). Interestingly, expression of an (engineered) laforin in potato resulted in significantly higher phosphate content of starch, and this result was confirmed in amylose-free potato genetic background (amf). Modified starches exhibited altered granule morphology and size compared to the control. About 20–30% of the transgenic lines of each series showed red-staining granules upon incubation with iodine, and contained higher phosphate content than the blue-stained starch granules. Moreover, low amylose content and altered gelatinization properties were observed in these red-stained starches. Principle component and correlation analysis disclosed a complex correlation between starch composition and starch physico-chemical properties. Ultimately, the expression level of endogenous genes involved in starch metabolism was analysed, revealing a compensatory response to the decrease of phosphate content in potato starch. This study provides a new perspective for engineering starch phosphate content in planta by making use of the compensatory mechanism in the plant itself.