Progress in controlling starch structure by modifying starch-branching enzymes

Structural basis for rice starch multi-digestible fractions revealed by consecutive reaction kinetics model

BACKGROUND

Starch-based foods (e.g. rice) usually contain multiple starch fractions with distinct digestion rate constants, although their nature is currently unknown. The present study applied the recently developed consecutive reaction kinetics model to fit the in vitro digestion curves for starch fractions deconvoluted from the overall digestograms to differentiate their binding and catalysis rates to starch digestive enzymes. The fitting parameters were then correlated with starch molecular structures obtained from published data to understand starch structural features determining the binding and catalytic rate constants.

RESULTS

Binding and catalysis rates for the rapidly (RDF) and slowly digestible starch fraction (SDF) were controlled by distinct starch structural features. Typically, (i) the binding rate constant for RDF was negatively correlated with the amount of amylose short to intermediate chains, whereas it was positively correlated with the relative length of amylopectin intermediate chains; (ii) the catalysis rate constant for RDF was negatively correlated with the amount of amylose short to intermediate chains, relative length of amylose intermediate chains and amount of amylopectin long chains, whereas it was positively correlated with starch molecular size as well as relative length of amylopectin intermediate chains; (iii) and the catalysis rate constant for SDF was negatively correlated with the amount of amylopectin long chains, whereas it was positively correlated with starch molecular size.

CONCLUSION

These results provide a better understanding of the nature of different starch digestible fractions and the development of foods such as rice with slow starch digestibility. © 2023 Society of Chemical Industry.

Preparation of Neohesperidin-Taro Starch Complex as a Novel Approach to Modulate the Physicochemical Properties, Structure and In Vitro Digestibility

Neohesperidin (NH), a natural flavonoid, exerts multiple actions, such as antioxidant, antiviral, antiallergic, vasoprotective, anticarcinogenic and anti-inflammatory effects, as well as inhibition of tumor progression. In this study, the NH-taro starch complex is prepared, and the effects of NH complexation on the physicochemical properties, structure and in vitro digestibility of taro starch (TS) are investigated. Results showed that NH complexation significantly affected starch gelatinization temperatures and reduced its enthalpy value (ΔH). The addition of NH increased the viscosity and thickening of taro starch, facilitating shearing and thinning. NH binds to TS via hydrogen bonds and promotes the formation of certain crystalline regions in taro starch. SEM images revealed that the surface of NH-TS complexes became looser with the increasing addition of NH. The digestibility results demonstrated that the increase in NH (from 0.1% to 1.1%, weight based on starch) could raise RS (resistant starch) from 21.66% to 27.75% and reduce RDS (rapidly digestible starch) from 33.51% to 26.76% in taro starch. Our work provided a theoretical reference for the NH-taro starch complex's modification of physicochemical properties and in vitro digestibility with potential in food and non-food applications.

Disentangling the resistant mechanism of Fusarium wilt TR4 interactions with different cultivars and its elicitor application

Fusarium wilt of banana, especially Tropical Race 4 (TR4) is a major factor restricting banana production. Developing a resistant cultivar and inducing plant defenses by elicitor application are currently two of the best options to control this disease. Isotianil is a monocarboxylic acid amide that has been used as a fungicide to control rice blast and could potentially induce systemic acquired resistance in plants. To determine the control effect of elicitor isotianil on TR4 in different resistant cultivars, a greenhouse pot experiment was conducted and its results showed that isotianil could significantly alleviate the symptoms of TR4, provide enhanced disease control on the cultivars 'Baxi' and 'Yunjiao No.1' with control effect 50.14% and 56.14%, respectively. We compared the infection processes in 'Baxi' (susceptible cultivars) and 'Yunjiao No.1' (resistant cultivars) two cultivars inoculated with pathogen TR4. The results showed that TR4 hyphae could rapidly penetrate the cortex into the root vascular bundle for colonization, and the colonization capacity in 'Baxi' was significantly higher than that in 'Yunjiao No.1'. The accumulation of a large number of starch grains was observed in corms cells, and further analysis showed that the starch content in 'Yunjiao No. 1' as resistant cultivar was significantly higher than that in 'Baxi' as susceptible cultivar, and isotianil application could significantly increase the starch content in 'Baxi'. Besides, a mass of tyloses were observed in the roots and corms and these tyloses increased after application with isotianil. Furthermore, the total starch and tyloses contents and the control effect in the corms of 'Yunjiao No.1' was higher than that in the 'Baxi'. Moreover, the expression levels of key genes for plant resistance induction and starch synthesis were analyzed, and the results suggested that these genes were significantly upregulated at different time points after the application of isotianil. These results suggest that there are significant differences between cultivars in response to TR4 invasion and plant reactions with respect to starch accumulation, tyloses formation and the expression of plant resistance induction and starch synthesis related genes. Results also indicate that isotianil application may contribute to disease control by inducing host plant defense against TR4 infection and could be potentially used together with resistant cultivar as integrated approach to manage this destructive disease. Further research under field conditions should be included in the next phases of study.

Preparation and Characterization of Extruded Yam Starch-Soy Protein Isolate Complexes and Their Effects on the Quality of Dough

Extrusion is a method of processing that changes the physicochemical and rheological properties of starch and protein under specific temperature and pressure conditions. In this study, twin-screw extrusion technology was employed to prepare yam starch-soy protein isolate complexes. The structure and properties of the complexes and their effects on the quality of dough were studied. The results showed changes in the X-ray diffraction, rheology, and in vitro digestibility of the complexes. The extruded starch-protein complex formed an A+V-type crystal structure with the addition of soy protein isolate. A small amount of soy protein isolate could improve the complex's viscoelasticity. As the content of soy protein isolate increased, the content of slow-digesting starch and resistant starch in the complexes increased, and the digestibility decreased. The microstructure of the dough indicated that the network structure of the puffed yam starch-protein complex dough was more uniform than that of the same amount of puffed yam starch. The moisture distribution of the dough showed that with the addition of extruded flour, the closely bound water content of the dough increased, and the weakly bound water content decreased. The hardness, gumminess, chewiness, and resilience of the dough decreased. In conclusion, extruded starch-protein complexes can improve dough quality and provide technical support for the broad application of yam.

Influence of Resistant Starch in Whole Rice on Human Gut Microbiota─From Correlation Implications to Possible Causal Mechanisms

Rice is the main staple food for a large population around the world, while it generally has a high glycemic index and low resistant starch (RS) content. Although many strategies have been applied to develop healthier rice products with increased RS contents, their actual effects on gut microbiota and human health remain elusive. In this review, currently available production methods of rice RS are briefly summarized, followed by a critical discussion on their interactions with gut microbiota and subsequent effects on human health, from correlation implications to causal mechanisms. Different contents, types, and structures of RS have been produced by strategies such as genetic manipulation and controlling cooking conditions. The difference can largely determine effects of rice RS on gut microbiota composition and metabolites by specific RS-gut microbiota interactions. This review can thus help the rice industry develop rice products with desirable RS contents and structures to generally improve human health.

Carbohydrate Repartitioning in the Rice Starch Branching Enzyme IIb Mutant Stimulates Higher Resistant Starch Content and Lower Seed Weight Revealed by Multiomics Analysis

The starch branching enzyme IIb mutant (be2b) in rice significantly increases the resistant starch (RS) content and leads to reduced seed weight. However, the underlying metabolic mechanisms remain unclear. Proteomic analysis indicated that upregulation of starch synthase IIa (SSIIa) and SSIIIa and downregulation of BEI and SSI were possibly responsible for the decreased short amylopectin chains (DP 6-15) and increased longer chains (DP > 16) of be2b starch. The upregulation of granule-bound starch synthase led to increased amylose content (AC). These changes in the amylopectin structure and AC accounted for the increased RS content. α-Amylase 2A showed the strongest upregulation (up to 8.45-fold), indicating that the loss of BEIIb activity enhanced starch degradation. Upregulation of glycolysis-related proteins stimulated carbohydrate repartitioning through glycerate-3-phosphate and promoted the accumulation of tricarboxylic acid cycle intermediates, amino acids, and fatty acids. The unexpected carbohydrate partitioning and enhanced starch degradation resulted in the reduced seed weight in the be2b mutant.

Starch Branching Enzyme 1 Is Important for Amylopectin Synthesis and Cyst Reactivation in Toxoplasma gondii

Toxoplasma gondii (T. gondii) bradyzoites facilitate chronic infections that evade host immune response. Furthermore, reactivation in immunocompromised individuals causes severe toxoplasmosis. The presence of abundant granules containing the branched starch amylopectin is major characteristic of bradyzoites that is nearly absent from tachyzoites that drive acute disease. T. gondii genome encodes to potential Starch branching enzyme 1 (SBE1) that creates branching during amylopectin biosynthesis. However, the physiological function of the amylopectin in T. gondii remains unclear. In this study, we generated a SBE1 knockout parasites and revealed that deletion of SBE1 caused amylopectin synthesis defects while having no significant impact on the growth of tachyzoites under normal culture conditions in vitro as well as virulence and brain cyst formation. Nevertheless, SBE1 knockout decreased the influx of exogenous glucose and reduced tachyzoites proliferation in nutrition-deficient conditions. Deletion of SBE1 together with the α-amylase (α-AMY), responsible for starch digestion, abolished amylopectin production and attenuated virulence while restoring brain cyst formation. In addition, cysts with defective amylopectin metabolism showed abnormal morphology and were avirulent to mice. In conclusion, SBE1 is essential for the synthesis of amylopectin, which serves as energy storage during the development and reactivation of bradyzoites. IMPORTANCE Toxoplasmosis has become a global, serious public health problem due to the extensiveness of the host. There are great differences in the energy metabolism in the different stages of infection. The most typical difference is the abundant accumulation of amylopectin granules in bradyzoites, which is almost absent in tachyzoites. Until now, the physiological functions of amylopectin have not been clearly elucidated. We focused on starch branching enzyme 1 (SBE1) in the synthesis pathway to reveal the exact physiological significance of amylopectin. Our study clarified the role of SBE1 in the synthesis pathway and amylopectin in tachyzoites and bradyzoites, and demonstrated that amylopectin, as an important carbon source, was critical to parasites growth under an unfavorable environment and the reactivation of bradyzoites to tachyzoites. The findings obtained from our study provides a new avenue for the development of Toxoplasma vaccines and anti-chronic toxoplasmosis drugs.

Combined effects of starch fine molecular structures and storage temperatures on long-term rice amylopectin retrogradation property

Though the retrogradation property as affected by starch fine molecular structures has been widely investigated, it remains largely unexplored how concurrent starch structures and storage conditions e.g. temperature tailor the starch retrogradation property. The amylopectin long-term retrogradation for 8 different rice starches with a broad range of amylose content was thus investigated under different storage temperatures. Results showed that gelatinized starch stored at -20 °C generally had a narrower melting temperature range from differential scanning calorimetry, while larger cells and thicker cell walls in the gel matrix than that stored at 4 °C. Different linear correlations were found between starch fine molecular structures and amylopectin retrogradation parameters when starch was stored under different temperatures. For example, the melting enthalpy of retrograded starch double helices was negatively correlated with the amount of amylose intermediate chains at 4 °C, while positively correlated with the relative length of amylopectin short chains at -20 °C. Under both temperatures, rice starch R250 had the highest retrogradation enthalpy. These results could help the rice industry improve both the nutritional and textural attributes of cooked rice by selecting starch with desirable molecular structures and optimizing the storage conditions for rice after cooking.

Consecutive reaction kinetics model reveals the nature of long-term rice amylopectin retrogradation characteristics

Starch retrogradation involves nucleation and crystal growth steps, while their relative contribution to the overall retrogradation kinetics and relations with starch fine molecular structures have not been elucidated. In this study, a consecutive reaction kinetics model (CRK) was developed to fit long-term retrogradation kinetics curves for 10 rice starches with distinct molecular structures. Starch chain-length distributions (CLDs) and melting enthalpy kinetics curves for these starches were obtained from our published data. It was shown that these melting enthalpy kinetics curves can be satisfactorily deconvoluted by the CRK model into a combination of nucleation and crystal growth curves. Correlation analysis between CRK model-fitted parameters with starch CLDs showed that starch retrogradation nucleation and crystal growth steps were controlled by distinct starch fine molecular structures. These results have practical applications, as it enables a separate regulation of nucleation and crystal growth steps during retrogradation process of starch-based foods for desirable nutritional properties.

A review of starch biosynthesis in cereal crops and its potential breeding applications in rice (Oryza Sativa L.)

Starch provides primary storage of carbohydrates, accounting for approximately 85% of the dry weight of cereal endosperm. Cereal seeds contribute to maximum annual starch production and provide the primary food for humans and livestock worldwide. However, the growing demand for starch in food and industry and the increasing loss of arable land with urbanization emphasizes the urgency to understand starch biosynthesis and its regulation. Here, we first summarized the regulatory signaling pathways about leaf starch biosynthesis. Subsequently, we paid more attention to how transcriptional factors (TFs) systematically respond to various stimulants via the regulation of the enzymes during starch biosynthesis. Finally, some strategies to improve cereal yield and quality were put forward based on the previous reports. This review would collectively help to design future studies on starch biosynthesis in cereal crops.

A structural explanation for the mechanism and specificity of plant branching enzymes I and IIb

Branching enzymes (BEs) are essential in the biosynthesis of starch and glycogen and play critical roles in determining the fine structure of these polymers. The substrates of these BEs are long carbohydrate chains that interact with these enzymes via multiple binding sites on the enzyme’s surface. By controlling the branched-chain length distribution, BEs can mediate the physiological properties of starch and glycogen moieties; however, the mechanism and structural determinants of this specificity remain mysterious. In this study, we identify a large dodecaose binding surface on rice BE I (BEI) that reaches from the outside of the active site to the active site of the enzyme. Mutagenesis activity assays confirm the importance of this binding site in enzyme catalysis, from which we conclude that it is likely the acceptor chain binding site. Comparison of the structures of BE from Cyanothece and BE1 from rice allowed us to model the location of the donor-binding site. We also identified two loops that likely interact with the donor chain and whose sequences diverge between plant BE1, which tends to transfer longer chains, and BEIIb, which transfers exclusively much shorter chains. When the sequences of these loops were swapped with the BEIIb sequence, rice BE1 also became a short-chain transferring enzyme, demonstrating the key role these loops play in specificity. Taken together, these results provide a more complete picture of the structure, selectivity, and activity of BEs.

Understanding Wheat Starch Metabolism in Properties, Environmental Stress Condition, and Molecular Approaches for Value-Added Utilization

Wheat starch is one of the most important components in wheat grain and is extensively used as the main source in bread, noodles, and cookies. The wheat endosperm is composed of about 70% starch, so differences in the quality and quantity of starch affect the flour processing characteristics. Investigations on starch composition, structure, morphology, molecular markers, and transformations are providing new and efficient techniques that can improve the quality of bread wheat. Additionally, wheat starch composition and quality are varied due to genetics and environmental factors. Starch is more sensitive to heat and drought stress compared to storage proteins. These stresses also have a great influence on the grain filling period and anthesis, and, consequently, a negative effect on starch synthesis. Sucrose metabolizing and starch synthesis enzymes are suppressed under heat and drought stress during the grain filling period. Therefore, it is important to illustrate starch and sucrose mechanisms during plant responses in the grain filling period. In recent years, most of these quality traits have been investigated through genetic modification studies. This is an attractive approach to improve functional properties in wheat starch. The new information collected from hybrid and transgenic plants is expected to help develop novel starch for understanding wheat starch biosynthesis and commercial use. Wheat transformation research using plant genetic engineering technology is the main purpose of continuously controlling and analyzing the properties of wheat starch. The aim of this paper is to review the structure, biosynthesis mechanism, quality, and response to heat and drought stress of wheat starch. Additionally, molecular markers and transformation studies are reviewed to elucidate starch quality in wheat.

A kinetics-based decomposition approach to reveal the nature of starch asymmetric gelatinization thermograms at non-isothermal conditions

Starch gelatinization under non-isothermal conditions with limited moisture content is a common industrial process involved in the processing of many starchy foods, while the nature of its asymmetric differential scanning calorimetry thermograms is still undefinable. This study for the first time developed a kinetics-based mathematical model, which could yield a parameterization of gelatinization thermograms that are essentially the same as experimental ones. Even more, the model is capable of decomposing gelatinization thermograms into kinetics-based sub-patterns, and revealing hidden features. By applying this newly developed methodology to nine starches with different plant origins and correlated with their lamellar structures, the results indicated that distinctly arranged groups (sub-components) of semi-crystalline lamellae with different thermal stabilities are existed in the native starch granules. This gives ways to better understand starch structure-property relations, and suggests useful directions for food manufactures to produce functional foods by understanding and differentially controlling the starch gelatinization sub-components.

Modification of starch composition, structure and properties through editing of TaSBEIIa in both winter and spring wheat varieties by CRISPR/Cas9

Foods high in amylose content and resistant starch (RS) offer great potential to improve human health and lower the risk of serious noninfectious diseases. Common wheat (Triticum aestivum L.) is a major staple food crop globally. However, the RS contents in the grains of modern wheat varieties are low. Here, we report the generation of high-amylose wheat through targeted mutagenesis of TaSBEIIa in a modern winter wheat cv Zhengmai 7698 (ZM) and a spring wheat cv Bobwhite by CRISPR/Cas9, respectively. We generated a series of transgene-free mutant lines either with partial or triple-null TasbeIIa alleles in ZM and Bobwhite, respectively. Analyses of starch composition, structure and properties revealed that the effects of partial or triple-null alleles were dosage dependent with triple-null lines demonstrated more profound impacts on starch composition, fine structures of amylopectin and physiochemical and nutritional properties. The flours of triple-null lines possessed significantly increased amylose, RS, protein and soluble pentosan contents which benefit human health. Baking quality analyses indicated that the high-amylose flours may be used as additives or for making cookies. Collectively, we successfully modified the starch composition, structure and properties through targeted mutagenesis of TaSBEIIa by CRISPR/Cas9 in both winter and spring wheat varieties and generated transgene-free high-amylose wheat. Our finding provides deep insights on the role of TaSBEIIa in determining starch composition, structure, properties and end-use quality in different genetic backgrounds and improving RS content with multiple breeding and end-use applications in cereal crop species through genome editing for health benefits.

Comparative proteomic analysis provides insight into the molecular mechanism of vegetative growth advantage in allotriploid Populus

Though allotriploid poplar shows a salient vegetative growth advantage that impacts biomass and lumber yield, the proteomic data of Populus allotriploids have not been scrutinized for identifying the underlying molecular mechanisms. We conducted a large-scale label-free proteomics profiling of the 5th, 10th, and 25th leaves of allotriploids and diploids, and identified 4587 protein groups. Among 932 differentially expressed proteins (DEPs), 22 are transcription factors (TFs) that could regulate vegetative growth advantage in allotriploids. The DEPs involved in light reaction, Calvin cycle, and photorespiration, protein synthesis, sucrose synthesis, starch synthesis, and starch decomposition displayed elevated expression in Populus allotriploids. However, the DEPs functioning in sucrose decomposition, tricarboxylic acid (TCA) cycle, and protein degradation exhibited significantly downregulated expression. The alternations of these DEPs augmented efficiency of photosynthesis, carbon fixation, sucrose and starch accumulation, and decreased capacity of carbohydrate consumption, leading to larger volume of biomass and vigorous growth in Populus allotriploids.

Effects of acid hydrolysis on the evolution of starch fine molecular structures and gelatinization properties

Effects of acid hydrolysis on amylose molecular structures and their relations to starch gelatinization properties were investigated. First-order kinetics models were applied to fit the evolution curve of starch chain-length and molecular size by acid hydrolysis treatment. Results showed that a single hydrolysis phase was involved in the degradation of waxy maize starch chains, while two distinct phases existed for the degradation of maize, high amylose maize and sago starch chains. The fast hydrolysis phase involved degradation of amylose chains with DP > ~300 and amylopectin long intra-cluster branches, while amylose chains with DP < ~300 was involved in the slow hydrolysis phase. Amylose molecules with DP ~ 300 were proposed to impact starch gelatinization properties by interaction with cut-off amylopectin double helices and formation of amylose crystallites/entanglements. This study could help food industry precisely control amylose molecular structures by acid hydrolysis treatment to develop starchy foods with desirable properties.

Causal relations among starch fine molecular structure, lamellar/crystalline structure and in vitro digestion kinetics of native rice starch

Causal relations among starch fine molecular structures, lamellar/crystalline structures, and the in vitro digestion kinetics of native rice starches.

A combined action of amylose and amylopectin fine molecular structures in determining the starch pasting and retrogradation property

Starch fine molecular structures are of essentially important in determining its pasting and retrogradation properties. In this study, 10 different starches from various botanical sources were selected to investigate the combined action of amylose and amylopectin molecules in determining the starch physicochemical properties. Correlation between starch structural parameters with the pasting and retrogradation properties showed that amylose and amylopectin CLDs do not affect these properties in isolation. Such as, the amount of amylose long chains and amylopectin short chains are both positively correlated with the melting temperatures and enthalpy of retrograded starches. Furthermore, relatively longer amylose short to medium chains can result in higher trough and breakdown viscosity, while higher amount of amylopectin medium to long chains result in higher peak viscosity. The results help a better understanding of the importance of amylose and amylopectin fine molecular structures in determining starch functional properties.

Molecular identification of the key starch branching enzyme-encoding gene SBE2.3 and its interacting transcription factors in banana fruits

Starch branching enzyme (SBE) has rarely been studied in common starchy banana fruits. For the first time, we report here the molecular characterization of seven SBE (MaSBE) and six SBE (MbSBE) genes in the banana A- and B-genomes, respectively, which could be classified into three distinct subfamilies according to genome-wide identification. Systematic transcriptomic analysis revealed that six MaSBEs and six MbSBEs were expressed in the developing banana fruits of two different genotypes, BaXi Jiao (BX, AAA) and Fen Jiao (FJ, AAB), among which MaSBE2.3 and MbSBE2.3 were highly expressed. Transient silencing of MaSBE2.3 expression in banana fruit discs led to a significant decrease in its transcription, which coincides with significant reductions in total starch and amylopectin contents compared to those of empty vector controls. The suggested functional role of MaSBE2.3 in banana fruit development was corroborated by its transient overexpression in banana fruit discs, which led to significant enhancements in total starch and amylopectin contents. A number of transcription factors, including three auxin response factors (ARF2/12/24) and two MYBs (MYB3/308), that interact with the MaSBE2.3 promoter were identified by yeast one-hybrid library assays. Among these ARFs and MYBs, MaARF2/MaMYB308 and MaARF12/MaARF24/MaMYB3 were demonstrated via a luciferase reporter system to upregulate and downregulate the expression of MaSBE2.3, respectively.

Cas9-mediated mutagenesis of potato starch-branching enzymes generates a range of tuber starch phenotypes

We investigated whether Cas9-mediated mutagenesis of starch-branching enzymes (SBEs) in tetraploid potatoes could generate tuber starches with a range of distinct properties. Constructs containing the Cas9 gene and sgRNAs targeting SBE1, SBE2 or both genes were introduced by Agrobacterium-mediated transformation or by PEG-mediated delivery into protoplasts. Outcomes included lines with mutations in all or only some of the homoeoalleles of SBE genes and lines in which homoeoalleles carried several different mutations. DNA delivery into protoplasts resulted in mutants with no detectable Cas9 gene, suggesting the absence of foreign DNA. Selected mutants with starch granule abnormalities had reductions in tuber SBE1 and/or SBE2 protein that were broadly in line with expectations from genotype analysis. Strong reduction in both SBE isoforms created an extreme starch phenotype, as reported previously for low-SBE potato tubers. HPLC-SEC and 1 H NMR revealed a decrease in short amylopectin chains, an increase in long chains and a large reduction in branching frequency relative to wild-type starch. Mutants with strong reductions in SBE2 protein alone had near-normal amylopectin chain-length distributions and only small reductions in branching frequency. However, starch granule initiation was enormously increased: cells contained many granules of <4 μm and granules with multiple hila. Thus, large reductions in both SBEs reduce amylopectin branching during granule growth, whereas reduction in SBE2 alone primarily affects numbers of starch granule initiations. Our results demonstrate that Cas9-mediated mutagenesis of SBE genes has the potential to generate new, potentially valuable starch properties without integration of foreign DNA into the genome.

MutMapPlus identified novel mutant alleles of a rice starch branching enzyme IIb gene for fine-tuning of cooked rice texture

Physicochemical properties of storage starch largely determine rice grain quality and food characteristics. Therefore, modification of starch property is effective to fine-tune cooked rice textures. To obtain new resources with modified starch property as breeding materials, we screened a mutant population of a japonica cultivar Nipponbare and found two independent mutant lines, altered gelatinization (age)1 and age2, with moderate changes in starch gelatinization property. A combination of conventional genetic analyses and the latest mapping method, MutMapPlus, revealed that both of these lines harbour novel independent mutant alleles of starch branching enzyme IIb (BEIIb) gene. In age1, amino acid substitution of Met-723 to Lys completely abolished BEIIb enzyme activity without significant reduction in its protein level. A transposon insertion in an intron of BEIIb gene reduced BEIIb protein level and activity in age2. Production of a series of the mutant lines by combining age alleles and indica-type starch synthase IIa allele established stepwise alteration of the physicochemical properties of starch including apparent amylose content, thermal property, digestibility by α-amylase and branched structures of amylopectin. Consistent with the alteration of starch properties, the results of a sensory evaluation test demonstrated that warm cooked rice of the mutants showed a variety of textures without marked reduction in overall palatability. These results suggest that a series of the mutant lines are capable of manipulation of cooked rice textures.

Generation of High-Amylose Rice through CRISPR/Cas9-Mediated Targeted Mutagenesis of Starch Branching Enzymes

Cereals high in amylose content (AC) and resistant starch (RS) offer potential health benefits. Previous studies using chemical mutagenesis or RNA interference have demonstrated that starch branching enzyme (SBE) plays a major role in determining the fine structure and physical properties of starch. However, it remains a challenge to control starch branching in commercial lines. Here, we use CRISPR/Cas9 technology to generate targeted mutagenesis in SBEI and SBEIIb in rice. The frequencies of obtained homozygous or bi-allelic mutant lines with indels in SBEI and SBEIIb in T0 generation were from 26.7 to 40%. Mutations in the homozygous T0 lines stably transmitted to the T1 generation and those in the bi-allelic lines segregated in a Mendelian fashion. Transgene-free plants carrying only the frame-shifted mutagenesis were recovered in T1 generation following segregation. Whereas no obvious differences were observed between the sbeI mutants and wild type, sbeII mutants showed higher proportion of long chains presented in debranched amylopectin, significantly increased AC and RS content to as higher as 25.0 and 9.8%, respectively, and thus altered fine structure and nutritional properties of starch. Taken together, our results demonstrated for the first time the feasibility to create high-amylose rice through CRISPR/Cas9-mediated editing of SBEIIb.

Bound Substrate in the Structure of Cyanobacterial Branching Enzyme Supports a New Mechanistic Model

Branching enzyme (BE) catalyzes the formation of α-1,6-glucosidic linkages in amylopectin and glycogen. The reaction products are variable, depending on the organism sources, and the mechanistic basis for these different outcomes is unclear. Although most cyanobacteria have only one BE isoform belonging to glycoside hydrolase family 13, Cyanothece sp. ATCC 51142 has three isoforms (BE1, BE2, and BE3) with distinct enzymatic properties, suggesting that investigations of these enzymes might provide unique insights into this system. Here, we report the crystal structure of ligand-free wild-type BE1 (residues 5-759 of 1-773) at 1.85 Å resolution. The enzyme consists of four domains, including domain N, carbohydrate-binding module family 48 (CBM48), domain A containing the catalytic site, and domain C. The central domain A displays a (β/α)₈-barrel fold, whereas the other domains adopt β-sandwich folds. Domain N was found in a new location at the back of the protein, forming hydrogen bonds and hydrophobic interactions with CBM48 and domain A. Site-directed mutational analysis identified a mutant (W610N) that bound maltoheptaose with sufficient affinity to enable structure determination at 2.30 Å resolution. In this structure, maltoheptaose was bound in the active site cleft, allowing us to assign subsites -7 to -1. Moreover, seven oligosaccharide-binding sites were identified on the protein surface, and we postulated that two of these in domain A served as the entrance and exit of the donor/acceptor glucan chains, respectively. Based on these structures, we propose a substrate binding model explaining the mechanism of glycosylation/deglycosylation reactions catalyzed by BE.