New perspectives on the role of α- and β-amylases in transient starch synthesis

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.

Polyploid GWAS reveals the basis of molecular marker development for complex breeding traits including starch content in the storage roots of sweet potato

Given the importance of prioritizing genome-based breeding of sweet potato to enable the promotion of food and nutritional security for future human societies, here, we aimed to dissect the genetic basis of storage root starch content (SC) when associated with a complex set of breeding traits including dry matter (DM) rate, storage root fresh weight (SRFW), and anthocyanin (AN) content in a mapping population containing purple-fleshed sweet potato. A polyploid genome-wide association study (GWAS) was extensively exploited using 90,222 single-nucleotide polymorphisms (SNPs) obtained from a bi-parental 204 F₁ population between 'Konaishin' (having high SC but no AN) and 'Akemurasaki' (having high AN content but moderate SC). Through the comparison of polyploid GWAS on the whole set of the 204 F₁, 93 high-AN-containing F₁, and 111 low-AN-containing F₁ populations, a total of two (consists of six SNPs), two (14 SNPs), four (eight SNPs), and nine (214 SNPs) significantly associated signals were identified for the variations of SC, DM, SRFW, and the relative AN content, respectively. Of them, a novel signal associated with SC, which was most consistent in 2019 and 2020 in both the 204 F₁ and 111 low-AN-containing F₁ populations, was identified in homologous group 15. The five SNP markers associated with homologous group 15 could affect SC improvement with a degree of positive effect (~4.33) and screen high-starch-containing lines with higher efficiency (~68%). In a database search of 62 genes involved in starch metabolism, five genes including enzyme genes granule-bound starch synthase I (IbGBSSI), α-amylase 1D, α-amylase 1E, and α-amylase 3, and one transporter gene ATP/ADP-transporter were located on homologous group 15. In an extensive qRT-PCR of these genes using the storage roots harvested at 2, 3, and 4 months after field transplantation in 2022, IbGBSSI, which encodes the starch synthase isozyme that catalyzes the biosynthesis of amylose molecule, was most consistently elevated during starch accumulation in sweet potato. These results would enhance our understanding of the underlying genetic basis of a complex set of breeding traits in the starchy roots of sweet potato, and the molecular information, particularly for SC, would be a potential platform for molecular marker development for this trait.

Phosphoglucoisomerase Is an Important Regulatory Enzyme in Partitioning Carbon out of the Calvin-Benson Cycle

Phosphoglucoisomerase (PGI) isomerizes fructose 6-phosphate (F6P) and glucose 6-phosphate (G6P) in starch and sucrose biosynthesis. Both plastidic and cytosolic isoforms are found in plant leaves. Using recombinant enzymes and isolated chloroplasts, we have characterized the plastidic and cytosolic isoforms of PGI. We have found that the Arabidopsis plastidic PGI K m for G6P is three-fold greater compared to that for F6P and that erythrose 4-phosphate is a key regulator of PGI activity. Additionally, the K m of spinach plastidic PGI can be dynamically regulated in the dark compared to the light and increases by 200% in the dark. We also found that targeting Arabidopsis cytosolic PGI into plastids of Nicotiana tabacum disrupts starch accumulation and degradation. Our results, in combination with the observation that plastidic PGI is not in equilibrium, indicates that PGI is an important regulatory enzyme that restricts flow and acts as a one-way valve preventing backflow of G6P into the Calvin-Benson cycle. We propose the PGI may be manipulated to improve flow of carbon to desired targets of biotechnology.

Carbohydrate reserves and seed development: an overview

Seeds are one of the most important food sources, providing humans and animals with essential nutrients. These nutrients include carbohydrates, lipids, proteins, vitamins and minerals. Carbohydrates are one of the main energy sources for both plant and animal cells and play a fundamental role in seed development, human nutrition and the food industry. Many studies have focused on the molecular pathways that control carbohydrate flow during seed development in monocot and dicot species. For this reason, an overview of seed biodiversity focused on the multiple metabolic and physiological mechanisms that govern seed carbohydrate storage function in the plant kingdom is required. A large number of mutants affecting carbohydrate metabolism, which display defective seed development, are currently available for many plant species. The physiological, biochemical and biomolecular study of such mutants has led researchers to understand better how metabolism of carbohydrates works in plants and the critical role that these carbohydrates, and especially starch, play during seed development. In this review, we summarize and analyze the newest findings related to carbohydrate metabolism's effects on seed development, pointing out key regulatory genes and enzymes that influence seed sugar import and metabolism. Our review also aims to provide guidelines for future research in the field and in this way to assist seed quality optimization by targeted genetic engineering and classical breeding programs.

Evidence of Dextrin Hydrolyzing Enzymes in Cascade Hops ( Humulus lupulus)

Dry-hopping, the addition of hops to beer during or after fermentation, is a common practice in brewing to impart hoppy flavor to beer. Previously assumed to be inert ingredients, recent evidence suggests that hops contain biologically active compounds that may also extract into beer and complicate the brewing process by altering the final composition of beer. Experiments described herein provide evidence of microbial and/or plant-derived enzymes associated with hops ( Humulus lupulus) which can impact beer quality by influencing the composition of fermentable and nonfermentable carbohydrates in dry-hopped beer. Fully attenuated and packaged commercial lager beer was dry-hopped at a rate of 10 g hops/L beer with pelletized Cascade hops, dosed with 10⁶ cells/mL of ale yeast, and incubated at 20 °C. Real extract of the treated beer declined significantly within several days with a reduction of 1 °P (% w/w) after 5 days and then slowly to a total reduction of approximately 2 °P after 40 days. When fully fermented, this was equivalent to the production of an additional 4.75% (v/v) of CO₂ and an additional 1.3% (v/v) of alcohol. The refermentation of beer driven by dry-hopping was attributed to the low but persistent activities of several starch degrading enzymes present in Cascade hops including amyloglucosidase, α-amylase, β-amylase, and limit dextrinase. The effect of hop-derived enzymes on beer was time, temperature, and dose-dependent. Characterizing bioactive enzymes in hops will help hop suppliers and brewers to address the unexpected quality and safety issues surrounding hopping practices in beer.

On the Role of Catabolic Enzymes in Biosynthetic Models of Glycogen Molecular Weight Distributions

Glycogen and starch are complex branched polymers of glucose that serve as units of glucose storage in animals and plants, respectively. Changes in the structure of these molecules have been linked to changes in their respective functional properties. Enzymatic models of starch synthesis have provided valuable insights into the biosynthetic origins of starch structure and functional properties but have not successfully been applied to glycogen despite the structural similarities between the two polymers. Modifications to biosynthetic models of starch structure were tested for applicability to glycogen. Mathematical evidence is provided showing the necessity (which has hitherto been in doubt) of considering the effects of catabolic (degradative) enzymes in biosynthesis-based approaches that seek to accurately describe the molecular weight distributions of individual chains of glycogen formed in vivo through glycogenesis. This finding also provides future direction for inferring the dependence of enzyme activities on substrate chain length from in vivo data.

The glucose 6-phosphate shunt around the Calvin-Benson cycle

It is just over 60 years since a cycle for the regeneration of the CO2-acceptor used in photosynthesis was proposed. In this opinion paper, we revisit the origins of the Calvin-Benson cycle that occurred at the time that the hexose monophosphate shunt, now called the pentose phosphate pathway, was being worked out. Eventually the pentose phosphate pathway was separated into two branches, an oxidative branch and a non-oxidative branch. It is generally thought that the Calvin-Benson cycle is the reverse of the non-oxidative branch of the pentose phosphate pathway but we describe crucial differences and also propose that some carbon routinely passes through the oxidative branch of the pentose phosphate pathway. This creates a futile cycle but may help to stabilize photosynthesis. If it occurs it could explain a number of enigmas including the lack of complete labelling of the Calvin-Benson cycle intermediates when carbon isotopes are fed to photosynthesizing leaves.

Aspergillus Oryzae S2 α-Amylase Domain C Involvement in Activity and Specificity: In Vivo Proteolysis, Molecular and Docking Studies

We previously reported that Aspergillus oryzae strain S2 had produced two α-amylase isoforms named AmyA and AmyB. The apparent molecular masses revealed by SDS-PAGE were 50 and 42 kDa, respectively. Yet AmyB has a higher catalytic efficiency. Based on a monitoring study of the α-amylase production in both the presence and absence of different protease inhibitors, a chymotrypsin proteolysis process was detected in vivo generating AmyB. A. oryzae S2 α-amylase gene was amplified, cloned and sequenced. The sequence analysis revealed nine exons, eight introns and an encoding open reading frame of 1500 bp corresponding to AmyA isoform. The amino-acid sequence analysis revealed aY371 potential chymotrypsin cleaving site, likely to be the AmyB C-Terminal end and two other potential sites at Y359, and F379. A zymogram with a high acrylamide concentration was used. It highlighted two other closed apparent molecular mass α-amylases termed AmyB1 and AmyB2 reaching40 kDa and 43 kDa. These isoforms could be possibly generated fromY359, and F379secondary cut, respectively. The molecular modeling study showed that AmyB preserved the (β/α)8 barrel domain and the domain B but lacked the C-terminal domain C. The contact map analysis and the docking studies strongly suggested a higher activity and substrate binding affinity for AmyB than AmyA which was previously experimentally exhibited. This could be explained by the easy catalytic cleft accessibility.

Progress in controlling starch structure by modifying starch-branching enzymes

This paper reviews the progress of development of plants with desirable starch structure by modifying starch branching enzymes. Starch-branching enzyme (SBE) is responsible for the creation of branches during starch biosynthesis in plastids, and is a major determinant of the final fine structure and physical properties of the starch. Multiple isoforms of SBE have been found in plants, with each playing a different role in amylopectin synthesis. Different methods have been used to develop desirable starch structures by modifying the SBE activity. These can involve changing its expression level (either up-regulation or down-regulation), genetically modifying the activity of the SBE itself, and varying the length of its transferred chains. Changing the activity and the transferred chain length of SBE has been less studied than changing the expression level of SBE in vivo. This article reviews and summarizes new tools for developing plants producing the next generation of starches.

Transcriptional Analyses of Mandarins Seriously Infected by 'Candidatus Liberibacter asiaticus'

A range of leaf symptoms, including blotchy mottle, yellowing, and small, upright leaves with a variety of chlorotic patterns resembling those induced by zinc deficiencies, are associated with huanglongbing (HLB, yellow shoot disease), a worldwide destructive citrus disease. HLB is presumably caused by the phloem-limited fastidious prokaryotic α-proteobacterium ‘Candidatus Liberibacter spp.’ Previous studies focused on the proteome and transcriptome analyses of citrus 5 to 35 weeks after ‘Ca. L. spp.’ inoculation. In this study, gene expression profiles were analyzed from mandarin Citrus reticulate Blanco cv. jiaogan leaves after a 2 year infection with ‘Ca. L. asiaticus’. The Affymetrix microarray analysis explored 2,017 differentially expressed genes. Of the 1,364 genes had known functions, 938 (46.5%) were up-regulated. Genes related to photosynthesis, carbohydrate metabolic, and structure were mostly down-regulated, with rates of 92.7%, 61.0%, and 80.2%, respectively. Genes associated with oxidation-reduction and transport were mostly up-regulated with the rates of 75.0% and 64.6%, respectively. Our data analyses implied that the infection of ‘Ca. L. asiaticus’ could alter hormone crosstalk, inducing the jasmine acid pathway and depressing the ethylene and salicylic acid pathways in the citrus host. This study provides an enhanced insight into the host response of citrus to ‘Ca. L. asiaticus’ infection at a two-years infection stage.

The biosynthesis, structure and gelatinization properties of starches from wild and cultivated African rice species (Oryza barthii and Oryza glaberrima)

The molecular structure and gelatinization properties of starches from domesticated African rice (Oryza glaberrima) and its wild progenitor (Oryza barthii) are determined and comparison made with Asian domesticated rice (Oryza sativa), the commonest commercial rice. This suggests possible enzymatic processes contributing to the unique traits of the African varieties. These have similar starch structures, including smaller amylose molecules, but larger amounts of amylose chains across the whole amylose chain-length distribution, and higher amylose contents, than O. sativa. They also show a higher proportion of two- and three-lamellae spanning amylopectin branch chains (degree of polymerization 34-100) than O. sativa, which contributes to their higher gelatinization temperatures. Fitting amylopectin chain-length distribution with a biosynthesis-based mathematical model suggests that the reason for this difference might be because O. glaberrima and O. barthii have more active SSIIIa and/or less active SBEIIb enzymes.

Two-dimensional macromolecular distributions reveal detailed architectural features in high-amylose starches

Two-dimensional (2D) structural distributions based on macromolecular size and branch chain-length are obtained for three maize starches with different amylose contents (one normal and two high-amylose varieties). Data were obtained using an analytical methodology combining chemical fractionation, enzymatic debranching, and offline 2D size-exclusion chromatography with multiple detection. The 2D distributions reveal novel features in the branching structure of high-amylose maize starches. Normal maize starch shows well-resolved structural topologies, corresponding to the amylopectin and amylose macromolecular populations. However, high-amylose maize starches exhibit very complex topologies with significant features between those of amylose and amylopectin, showing the presence of distinct intermediate components. These have the macromolecular size of amylose but similar branching structure to amylopectin, except for a higher proportion of longer branches. These structural features of the intermediate components can be related to a less tightly controlled biosynthesis of the branching structures in high-amylose maize starch mutants, which may prevent these molecules from maturing into full-size amylopectin. This altered macromolecular branched architecture of high-amylose starches probably contribute to their better nutritional properties.