Effects of starch synthase IIa gene dosage on grain, protein and starch in endosperm of wheat

Starch phosphorylation regulates starch granule morphological homogeneity in Arabidopsis thaliana

Starch granule morphological homogeneity presents a gap in starch research. Transitory starch granules in wild-type plants are discoid, regardless of species. Notably, while the shape of starch granules can differ among mutants, it typically remains homogeneous within a genotype. We found an Arabidopsis thaliana mutant, dpe2sex4, lacking both the cytosolic disproportionating enzyme 2 (DPE2) and glucan phosphatase SEX4, showing an unprecedented bimodal starch granule diameter distribution when grown under a light/dark rhythm. dpe2sex4 contained 2 types of starch granules: large granules and small granules. In contrast to the double starch initiation in wheat (Triticum aestivum) endosperm, where A-type granules are initiated first and B-type granules are initiated later, dpe2sex4 small and large granules developed simultaneously in the same chloroplast. Compared with the large granules, the small granules had more branched amylopectin and less surface starch-phosphate, thus having a more compact structure that may hinder starch synthesis. During plant aging, the small granules barely grew. In in vitro experiments, fewer glucosyl residues were incorporated in small granules. Under continuous light, dpe2sex4 starch granules were morphologically homogeneous. Omitting the dark phase after a 2-wk light/dark cycle by moving plants into continuous light also reduced morphological variance between these 2 types of granules. These data shed light on the impact of starch phosphorylation on starch granule morphology homogeneity.

Rheology, stability, antioxidant properties, and curcumin release of oil-in-water Pickering emulsions stabilized by rice starch nanoparticles

Modification of rice starch nanoparticles (SNP) as an emulsifier in Pickering emulsions is reported in this work. The SNP was prepared by HCl hydrolysis with different resident times and subsequently modified via crosslinking by citric acid using various crosslinking times to improve the hydrophobicity of SNP. The modified SNP was used to prepare sunflower oil-in-water Pickering emulsions loaded with curcumin. The optimal hydrolysis conditions (2.2 M HCl, 6 days) produced SNP with a 21.87 ± 0.69 % yield and 45.56 ± 0.00 % crystallinity. The citric acid-modified SNP with a 6-h crosslinking period (SNP-M-6 h) had a water contact angle of 87.2°. The suitable Pickering emulsion containing 30 wt% curcumin-loaded sunflower oil was stabilized by 3.0 wt% SNP-M-6 h. This Pickering emulsion had shear thinning properties with a pseudoplastic fluid behavior and was characterized by a droplet size of 47.16 ± 4.22 μm with a high degree of stability over five weeks of storage. Furthermore, the curcumin release from the emulsion depended on the pH, and curcumin could maintain its free radical scavenging quality. A very beneficial property of the Pickering emulsion is that it can slowly release curcumin at low pH, but more rapid release at higher pH, making it a potentially excellent candidate for drug delivery through oral intake.

Determinants of resistant starch accumulation in wheat endosperm

A part of the big three cereal crops in the world, wheat has become a major constituent of the everyday food chain and is grown at a massive scale to meet global demands. This makes it an important crop from an economic as well as food security perspective. Selection of high-quality cultivars and consistent trait enhancement for such cultivars is crucial, and in light of new challenges from climate change, this has become an absolute necessity of time. In this regard, we conducted a detailed qualitative and quantitative trait analysis for multiple commercially viable varieties of wheat, and corresponding results were subjected to a series of critical statistical analyses. Final results have shown that five cultivars including Uqaab-2000, Faisalabad- 85, Anmol-19, NARC-2009, and Pirsabak-2004 depicts higher levels of various essential qualitative and quantitative traits (including Starch content, grain weight, RS content, Protein content, etc.) and are most viable varieties for further growth and trait enhancements to meet regional and global food challenges.

Quantitative Trait Locus Analysis and Identification of Candidate Genes Affecting Seed Size and Shape in an Interspecific Backcross Inbred Line Population of Gossypium hirsutum × Gossypium barbadense

Seed size and shape are key agronomic traits affecting seedcotton yield and seed quality in cotton (Gossypium spp.). However, the genetic mechanisms that regulate the seed physical traits in cotton are largely unknown. In this study, an interspecific backcross inbred line (BIL) population of 250 BC1F7 lines, derived from the recurrent parent Upland CRI36 (Gossypium hirsutum) and Hai7124 (Gossypium barbadense), was used to investigate the genetic basis of cotton seed physical traits via quantitative trait locus (QTL) mapping and candidate gene identification. The BILs were tested in five environments, measuring eight seed size and shape-related traits, including 100-kernel weight, kernel length width and their ratio, kernel area, kernel girth, kernel diameter, and kernel roundness. Based on 7,709 single nucleotide polymorphic (SNP) markers, a total of 49 QTLs were detected and each explained 2.91-35.01% of the phenotypic variation, including nine stable QTLs mapped in at least three environments. Based on pathway enrichment, gene annotation, genome sequence, and expression analysis, five genes encoding starch synthase 4, transcription factor PIF7 and MYC4, ubiquitin-conjugating enzyme E27, and THO complex subunit 4A were identified as candidate genes that might be associated with seed size and shape. Our research provides valuable information to improve seed physical traits in cotton breeding.

Longer Bread Shelf-Life and Improved Noodle-Making Properties Imparted by a Novel Wheat Genotype Are Stable in Different Genetic Backgrounds

A recently developed wheat variety, known as 5-5 wheat, which has inactive GBSSI-B1, GBSSI-D1, SSIIa-B1, and SSIIa-D1 isozymes, accumulates a novel type of starch, affecting bread texture and leading to reduction in bread staling. These properties are potentially useful for commercial bakery products; thus, the 5-5 genotype represents a new resource for wheat breeding. In this study, the 5-5 alleles were backcrossed into the hard wheat variety "Minaminokaori" and the soft wheat variety "Shirogane-Komugi", which are both leading Japanese wheat varieties. In comparison to their parental varieties, the two 5-5 near-isogenic lines (NILs) showed a decrease in amylose levels, an increase in the proportion of short chains of amylopectin, a lower gelatinization temperature and enthalpy change, a higher peak viscosity and breakdown viscosity as measured by a Rapid Visco Analyzer, a reduced retrogradation rate, and an increase in grain hardness. Importantly, the 5-5 NILs also showed lower bread crumb firmness and reduced hardening after storage for 2 days at either 20 °C or 7 °C. Considering the results obtained here along with those from the original line, it is clear that the 5-5 genotype can generate specific changes in starch characteristics and staling regardless of its genetic background. Thus, we renamed the 5-5 wheat lines "Slow Staling" (SS) wheat. As expected, our results indicated that the hard wheat SS NIL was more suitable for bread-making. On the other hand, we found that white salted noodle made with the SS NIL of the soft wheat variety had a relatively shorter cooking time, a softer texture, and a reduction in textural changes during storage, all of which are potentially useful for noodle manufacturers.

Effects of Two Starch Synthase IIa Isoforms on Grain Components and Other Grain Traits in Barley

Starch biosynthesis in cereal crops is a complex pathway regulated by multiple starch synthetic enzymes. Starch synthase IIa (SSIIa) is well-known to be one of the major starch synthases and is very important in amylopectin biosynthesis. It has significant effects on grain composition and kernel traits. However, there are few reports on the association of natural variation of SSIIa in barley and grain composition and characteristics. In this work, two SSIIa isoforms were first identified as SSIIa^H and SSIIa^L by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis, mass spectrometry, and Western blotting. Sequence analysis of the SSIIa gene demonstrated that a 33 bp insertion coding a peptide of APPSSVVPAKK caused different SSIIa, e.g., SSIIa^H and SSIIa^L. Based on this molecular difference, a polymerase chain reaction marker was developed, which could be used to screen different SSIIa genotypes easily. Kernel hardness of SSIIa^L genotypes was significantly higher than that of SSIIa^H Chinese barley cultivars. The proportion of SSIIa^L genotypes was extremely low in Australian barley cultivars (5/24) and much higher in Tibetan hull-less barley cultivars (46/74), consistent with the end-use requirements of barley grain. This study provided new information in barley endosperm starch synthesis and indicated that it is valuable for choosing the preferred SSIIa genotype according to the end-use requirements.

Effects of potassium sulfate on swelling, gelatinizing and pasting properties of three rice starches from different sources

This study evaluates the effects of potassium sulfate (K₂SO₄) on the swelling, gelatinization, and pasting properties of indica rice starch (IRS), japonica rice starch (JRS), and waxy rice starch (WRS). As a result, the gelatinization temperatures (T_p), swelling capacities, and pasting viscosities of rice starches in water followed the order of WRS > JRS > IRS, showing positive correlations to amylopectin content and molecular weight. At K₂SO₄ concentration of 0.05-0.6 M, T_p increased by 10-13 ℃ due to a more compact structure of starch granules resulting from increased interactions of starch chains with K⁺. However, the swelling capacity decreased with increasing K₂SO₄ concentration and followed the order of WRS < JRS < IRS, which decreased sharply from 27.3 to 2.5 g/g for WRS. K₂SO₄ dramatically reduced the pasting viscosity of starch pastes due to the decreased swelling capacity. This study provides the scientific basis for rice starch processing with K₂SO₄.

The impact of the indica rice SSIIa allele on the apparent high amylose starch from rice grain with downregulated japonica SBEIIb

Catalytically active indica SSIIa allele in high amylose rice with down-regulated japonica SBEIIb can increase starch content and modify the starch structure and properties without changing its amylose content. Rice (Oryza sativa) genotypes with inactive starch synthase IIa (SSIIa) with recessive variants of starch branching enzyme IIb (SBEIIb) exhibit a range of alterations in grain phenotype, starch granule morphology, starch granule bound proteins, starch structure, and functional properties. However, the interactions between the two enzymes have not been thoroughly investigated yet. We analysed recombinant rice lines having down-regulated SBEIIb expression (SBEIIb^DR) with either indica or japonica type SSIIa (SSIIa^ind or SSIIa^jap). In SBEIIb^DR rice starch granules, the increased abundance of two protein bands (SSI and SSIIa) was found with eight additional protein bands not generally associated with starch granules. The amount of SSIIa was higher in SSIIa^indSBEIIb^DR than SSIIa^japSBEIIb^DR, which indicated that indica type SSIIa, possibly in the monomer form, was extensively involved in starch biosynthesis in the SBEIIb^DR endosperm. Furthermore, SSIIa^indSBEIIb^DR grains had higher total starch content and higher starch swelling power than SSIIa^japSBEIIb^DR lines, but the amylopectin gelatinization temperatures and enthalpy and the apparent amylose content remained similar. In summary, this work suggests that SSIIa^ind can partly compensate for the alteration of starch synthesis resulting from the SBEIIb down-regulation in japonica background without reducing its amylose content. The study provides insight into the starch structural and textural improvements of high amylose starch.

Effects of low temperature at stem elongation stage on the development, morphology, and physicochemical properties of wheat starch

The low temperature in spring is a meteorological problem in wheat production because it leads to yield reduction and alteration of wheat quality. In this study, an artificial low-temperature treatment (LTT) at the stem elongation stage was implemented to investigate the starch granule development and physical and chemical properties of wheat starch. Results showed that the agronomic traits of spike, such as spike length, spikelet number per spike, and grain number per spikelet, decreased after LTT. LTT promoted the development of starch granules in developing grains at 15 days post-anthesis, resulting in increased B-type granules and reduced C-type granules with an irregular shape in mature grains. The swelling power of the starch granules was reduced, but their solubility was enhanced by LTT. The proportion of the amorphous structure within the granules increased, but the crystalline component decreased after LTT, leading to a significant reduction in the relative degree of crystallinity and double- and single-helix structures. Three types of hydrolysis showed that starch in LTT was easy to hydrolyze, resulting in a high degree of hydrolysis. The findings suggest that low temperature at the stem elongation stage can not only reduce the yield parameters of spike but also alter starch accumulation, thereby affecting the processability and structure characteristics of starch in wheat grains.

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.

Can Manipulation of Durum Wheat Amylose Content Reduce the Glycaemic Index of Spaghetti?

Resistant starch (RS) in foods has positive benefits for potentially alleviating lifestyle diseases. RS is correlated positively with starch amylose content. This study aimed to see what level of amylose in durum wheat is needed to lower pasta GI. The silencing of starch synthases IIa (SSIIa) and starch branching enzymes IIa (SBEIIa), key genes involved in starch biosynthesis, in durum wheat cultivar Svevo was performed and spaghetti was prepared and evaluated. The SSIIa and SBEIIa mutants have a 28% and 74% increase in amylose and a 2.8- and 35-fold increase in RS, respectively. Cooked pasta was softer, with higher cooking loss but lower stickiness compared to Svevo spaghetti, and with acceptable appearance and colour. In vitro starch digestion extent (area under the digestion curve) was decreased in both mutants, but much more in SBEIIa, while in vivo GI was only significantly reduced from 50 to 38 in SBEIIa. This is the first study of the glycaemic response of spaghetti prepared from SBEIIa and SSIIa durum wheat mutants. Overall pasta quality was acceptable in both mutants but the SBEIIa mutation provides a clear glycaemic benefit and would be much more appealing than wholemeal spaghetti. We suggest a minimum RS content in spaghetti of ~7% is needed to lower GI which corresponded to an amylose content of ~58%.

Upregulated Lipid Biosynthesis at the Expense of Starch Production in Potato (Solanum tuberosum) Vegetative Tissues via Simultaneous Downregulation of ADP-Glucose Pyrophosphorylase and Sugar Dependent1 Expressions

Triacylglycerol is a major component of vegetable oil in seeds and fruits of many plants, but its production in vegetative tissues is rather limited. It would be intriguing and important to explore any possibility to expand current oil production platforms, for example from the plant vegetative tissues. By expressing a suite of transgenes involved in the triacylglycerol biosynthesis, we have previously observed substantial accumulation of triacylglycerol in tobacco (Nicotiana tabacum) leaf and potato (Solanum tuberosum) tuber. In this study, simultaneous RNA interference (RNAi) downregulation of ADP-glucose pyrophosphorylase (AGPase) and Sugar-dependent1 (SDP1), was able to increase the accumulation of triacylglycerol and other lipids in both wild type potato and the previously generated high oil potato line 69. Particularly, a 16-fold enhancement of triacylglycerol production was observed in the mature transgenic tubers derived from the wild type potato, and a two-fold increase in triacylglycerol was observed in the high oil potato line 69, accounting for about 7% of tuber dry weight, which is the highest triacylglycerol accumulation ever reported in potato. In addition to the alterations of lipid content and fatty acid composition, sugar accumulation, starch content of the RNAi potato lines in both tuber and leaf tissues were also substantially changed, as well as the tuber starch properties. Microscopic analysis further revealed variation of lipid droplet distribution and starch granule morphology in the mature transgenic tubers compared to their parent lines. This study reflects that the carbon partitioning between lipid and starch in both leaves and non-photosynthetic tuber tissues, respectively, are highly orchestrated in potato, and it is promising to convert low-energy starch to storage lipids via genetic manipulation of the carbon metabolism pathways.

Combining mutations at genes encoding key enzymes involved in starch synthesis affects the amylose content, carbohydrate allocation and hardness in the wheat grain

Modifications to the composition of starch, the major component of wheat flour, can have a profound effect on the nutritional and technological characteristics of the flour's end products. The starch synthesized in the grain of conventional wheats (Triticum aestivum) is a 3:1 mixture of the two polysaccharides amylopectin and amylose. Altering the activity of certain key starch synthesis enzymes (GBSSI, SSIIa and SBEIIa) has succeeded in generating starches containing a different polysaccharide ratio. Here, mutagenesis, followed by a conventional marker-assisted breeding exercise, has been used to generate three mutant lines that produce starch with an amylose contents of 0%, 46% and 79%. The direct and pleiotropic effects of the multiple mutation lines were identified at both the biochemical and molecular levels. Both the structure and composition of the starch were materially altered, changes which affected the functionality of the starch. An analysis of sugar and nonstarch polysaccharide content in the endosperm suggested an impact of the mutations on the carbon allocation process, suggesting the existence of cross-talk between the starch and carbohydrate synthesis pathways.

Alternative splicing results in a lack of starch synthase IIa-D in Chinese wheat landrace

We evaluated the SGP-1 protein composition of 368 Chinese wheat landraces using SDS-PAGE. The SGP-D1 null type was identified in three accessions (Xiaoqingmang, Pushanbamai, and P119). An 18-bp deletion and 9-bp variation were found at the junction region of the 7th intron and 8th exon, leading to deletion of the intron-exon junction recognition site AG when aligned the 8261-bp DNA sequence of TaSSIIa-D in Pushanbamai with that of Chinese Spring. Four cDNA types with mis-spliced isoforms were subsequently detected through amplification of TaSSIIa-D cDNAs. Among these, nine type II cDNAs with a 16-bp deletion in the 8th exon were detected, indicating that the major transcriptional pattern of TaSSIIa in Pushanbamai is type II. In the type IV cDNA, a 97-bp sequence remains undeleted in the end of the 5th exon. The amylose content in Pushanbamai was significantly higher than that in all control lines under field conditions, which suggested that deletion of SGP-D1 has an efficient impact on amylose content. As the TaSSIIa gene plays an important role in regulating the content of amylose, it is anticipated that these natural variants of TaSSIIa-D will provide useful resources for quality improvement in wheat.

Evaluation of the Molecular Structural Parameters of Normal Rice Starch and Their Relationships with Its Thermal and Digestion Properties

The molecular structural parameters of six normal rice starches with different amylose contents were investigated through their iodine absorption spectra and gel permeation chromatography of fully branched and debranched starches. The thermal and digestion properties of starches were also determined and their relationships with molecular structural parameters were analyzed. Results showed that the molecular structural parameters of maximum absorption wavelength, blue value (BV), optical density 620 nm/550 nm (OD 620/550), amylose, intermediate component, and amylopectin, including its short branch-chains, long branch-chains, and branching degree, had high correlation in different determining methods. The intermediate component of starch was significantly positively related to amylose and negatively related to amylopectin, and the amylopectin branching degree was significantly positively related to amylopectin content and negatively related to amylose content. The gelatinization temperatures and enthalpy of native starch were significantly positively related to BV, OD 620/550, and amylose content and negatively related to amylopectin short branch-chains. The gelatinization temperatures and enthalpy of retrograded starch were significantly negatively related to amylopectin branching degree. The digestions of gelatinized and retrograded starches were significantly negatively related to the BV, OD 620/550, amylose, and intermediate component and positively related to amylopectin and its short branch-chains and branching degree.

Comparative Mapping and Candidate Gene Analysis of SSIIa Associated with Grain Amylopectin Content in Barley (Hordeum vulgare L.)

Amylopectin concentration in barley endosperm has important effects on grain quality and end-use. In this study, quantitative trait locus (QTL) analysis together with genome-wide association studies (GWAS) were performed to identify markers linked to grain amylopectin content respectively using a doubled haploid (DH) population of 178 lines and a collection of 185 diverse barley germplasms both genotyped by genotyping-by-sequencing (GBS). A stable QTL on chromosome 7H and 11 associated single nucleotide polymorphisms (SNPs) were detected. In the co-localized region, the SSIIa (SSII-3) gene was predicted as the candidate gene. Then we isolated and characterized biparental SSIIa alleles of the DH population, investigated the expression pattern by quantitative real-time PCR (qRT-PCR), and revealed that a 33-bp deletion in exon 2 is responsible for reducing SSIIa transcript, thus resulting in a reduced amylopectin content. A sequence-based molecular marker was developed for the SSIIa allele and validated the effectivity, which would provide help for barley breeding.

Novel mutant alleles of the starch synthesis gene TaSSIVb-D result in the reduction of starch granule number per chloroplast in wheat

BACKGROUND

Transient starch provides carbon and energy for plant growth, and its synthesis is regulated by the joint action of a series of enzymes. Starch synthesis IV (SSIV) is one of the important starch synthase isoforms, but its impact on wheat starch synthesis has not yet been reported due to the lack of mutant lines.

RESULTS

Using the TILLING approach, we identified 54 mutations in the wheat gene TaSSIVb-D, with a mutation density of 1/165 Kb. Among these, three missense mutations and one nonsense mutation were predicted to have severe impacts on protein function. In the mutants, TaSSIVb-D was significantly down-regulated without compensatory increases in the homoeologous genes TaSSIVb-A and TaSSIVb-B. Altered expression of TaSSIVb-D affected granule number per chloroplast; compared with wild type, the number of chloroplasts containing 0-2 granules was significantly increased, while the number containing 3-4 granules was decreased. Photosynthesis was affected accordingly; the maximum quantum yield and yield of PSII were significantly reduced in the nonsense mutant at the heading stage.

CONCLUSIONS

These results indicate that TaSSIVb-D plays an important role in the formation of transient starch granules in wheat, which in turn impact the efficiency of photosynthesis. The mutagenized population created in this study allows the efficient identification of novel alleles of target genes and could be used as a resource for wheat functional genomics.

Novel mutant alleles of the starch synthesis gene TaSSIVb-D result in the reduction of starch granule number per chloroplast in wheat

BACKGROUND

Transient starch provides carbon and energy for plant growth, and its synthesis is regulated by the joint action of a series of enzymes. Starch synthesis IV (SSIV) is one of the important starch synthase isoforms, but its impact on wheat starch synthesis has not yet been reported due to the lack of mutant lines.

RESULTS

Using the TILLING approach, we identified 54 mutations in the wheat gene TaSSIVb-D, with a mutation density of 1/165 Kb. Among these, three missense mutations and one nonsense mutation were predicted to have severe impacts on protein function. In the mutants, TaSSIVb-D was significantly down-regulated without compensatory increases in the homoeologous genes TaSSIVb-A and TaSSIVb-B. Altered expression of TaSSIVb-D affected granule number per chloroplast; compared with wild type, the number of chloroplasts containing 0-2 granules was significantly increased, while the number containing 3-4 granules was decreased. Photosynthesis was affected accordingly; the maximum quantum yield and yield of PSII were significantly reduced in the nonsense mutant at the heading stage.

CONCLUSIONS

These results indicate that TaSSIVb-D plays an important role in the formation of transient starch granules in wheat, which in turn impact the efficiency of photosynthesis. The mutagenized population created in this study allows the efficient identification of novel alleles of target genes and could be used as a resource for wheat functional genomics.

Crystalline, thermal and swelling properties of starches from single-segment substitution lines with different Wx alleles in rice (Oryza sativa L.)

BACKGROUND

In rice, five common Wx alleles, wx, Wx^t , Wx^g1 , Wx^g2 and Wx^g3 , have been identified according to their apparent amylose content (AAC) phenotypes. Previous studies revealed that this Wx allelic variation may also affect other starch properties. However, so far, to what extent the five Wx alleles influence the crystalline structure, thermal and swelling properties of rice starch is still unclear. For this purpose, a set of single-segment substitution lines (SSSLs) harboring five different Wx alleles, varying widely in AAC, was used for comparative studies.

RESULTS

The crystalline structure, thermal properties and swelling behavior of starches from the SSSLs varied widely depending on Wx genotype. Effects of different Wx alleles on relative crystallinity followed the order wx > Wx^t > Wx^g1 = Wx^g2 > Wx^g3 . The glutinous and Wx^t genotype starches showed higher gelatinization temperatures and enthalpy compared with other Wx genotypes. The order for swelling power was wx > Wx^t > Wx^g1 > Wx^g2 > Wx^g3 , while the order for degree of solubility was Wx^g3 > Wx^g1 > Wx^g2 > Wx^t = wx. Correlation analysis indicated that AAC was significantly and negatively correlated with relative crystallinity (r = -0.996, P < 0.01) and swelling power (r = -0.982, P < 0.01).

CONCLUSION

The present results provide new knowledge about the influence of different Wx alleles on the structural and physicochemical properties of rice starch. © 2016 Society of Chemical Industry.

The impact of the SSIIa null mutations on grain traits and composition in durum wheat

Starch represents a major nutrient in the human diet providing essentially a source of energy. More recently the modification of its composition has been associated with new functionalities both at the nutritional and technological level. Targeting the major starch biosynthetic enzymes has been shown to be a valuable strategy to manipulate the amylose-amylopectin ratio in reserve starch. In the present work a breeding strategy aiming to produce a set of SSIIa (starch synthases IIa) null durum wheat is described. We have characterized major traits such as seed weight, total starch, amylose, protein and β-glucan content in a set of mutant families derived from the introgression of the SSIIa null trait into Svevo, an elite Italian durum wheat cultivar. A large degree of variability was detected and used to select wheat lines with either improved quality traits or agronomic performances. Semolina of a set of two SSIIa null lines showed new rheological behavior and an increased content of all major dietary fiber components, namely arabinoxylans, β-glucans and resistant starch. Furthermore the investigation of gene expression highlighted important differences in some genes involved in starch and β-glucans biosynthesis.

Genetic Diversity and Genome Wide Association Study of β-Glucan Content in Tetraploid Wheat Grains

Non-starch polysaccharides (NSPs) have many health benefits, including immunomodulatory activity, lowering serum cholesterol, a faecal bulking effect, enhanced absorption of certain minerals, prebiotic effects and the amelioration of type II diabetes. The principal components of the NSP in cereal grains are (1,3;1,4)-β-glucans and arabinoxylans. Although (1,3;1,4)-β-glucan (hereafter called β-glucan) is not the most representative component of wheat cell walls, it is one of the most important types of soluble fibre in terms of its proven beneficial effects on human health. In the present work we explored the genetic variability of β-glucan content in grains from a tetraploid wheat collection that had been genotyped with a 90k-iSelect array, and combined this data to carry out an association analysis. The β-glucan content, expressed as a percentage w/w of grain dry weight, ranged from 0.18% to 0.89% across the collection. Our analysis identified seven genomic regions associated with β-glucan, located on chromosomes 1A, 2A (two), 2B, 5B and 7A (two), confirming the quantitative nature of this trait. Analysis of marker trait associations (MTAs) in syntenic regions of several grass species revealed putative candidate genes that might influence β-glucan levels in the endosperm, possibly via their participation in carbon partitioning. These include the glycosyl hydrolases endo-β-(1,4)-glucanase (cellulase), β-amylase, (1,4)-β-xylan endohydrolase, xylanase inhibitor protein I, isoamylase and the glycosyl transferase starch synthase II.

Using the Hexaploid Nature of Wheat To Create Variability in Starch Characteristics

In hexaploid crops, such as bread wheat, it should be possible to fine-tune phenotypic traits by identifying wild-type and null genes from each of the three genomes and combining them in a calculated manner. Here, we demonstrate this with gene combinations for two starch synthesis genes, SSIIa and GBSSI. Lines with inactive copies of both enzymes show a very dramatic change in phenotype, so to create intermediate phenotypes, we used marker-assisted selection to develop near-isogenic lines (NILs) carrying homozygous combinations of null alleles. For both genes, gene dosage effects follow the order B > D ≥ A; therefore, we completed detailed analysis of starch characteristics for NIL 3-3, which is null for the B-genome copy of the SSIIa and GBSSI genes, and NIL 5-5, which has null mutations in the B- and D-genome-encoded copies of both of these genes. The effects of the combinations on phenotypic traits followed the order expected on the basis of genotype, with NIL 5-5 showing the largest differences from the wild type, while NIL 3-3 characteristics were intermediate between NIL 5-5 and the wild type. Differences among genotypes were significant for many starch characteristics, including percent amylose, chain length distribution, gelatinization temperature, retrogradation, and pasting properties, and these differences appeared to translate into improvements in end-product quality, since bread made from type 5-5 flour showed a 3 day lag in staling.

Suppression of glucan, water dikinase in the endosperm alters wheat grain properties, germination and coleoptile growth

Starch phosphate ester content is known to alter the physicochemical properties of starch, including its susceptibility to degradation. Previous work producing wheat (Triticum aestivum) with down-regulated glucan, water dikinase, the primary gene responsible for addition of phosphate groups to starch, in a grain-specific manner found unexpected phenotypic alteration in grain and growth. Here, we report on further characterization of these lines focussing on mature grain and early growth. We find that coleoptile length has been increased in these transgenic lines independently of grain size increases. No changes in starch degradation rates during germination could be identified, or any major alteration in soluble sugar levels that may explain the coleoptile growth modification. We identify some alteration in hormones in the tissues in question. Mature grain size is examined, as is Hardness Index and starch conformation. We find no evidence that the increased growth of coleoptiles in these lines is connected to starch conformation or degradation or soluble sugar content and suggest these findings provide a novel means of increasing coleoptile growth and early seedling establishment in cereal crop species.

The different effects of starch synthase IIa mutations or variation on endosperm amylose content of barley, wheat and rice are determined by the distribution of starch synthase I and starch branching enzyme IIb between the starch granule and amyloplast stroma

The distribution of starch synthase I and starch branching enzyme IIb between the starch granule and amyloplast stroma plays an important role in determining endosperm amylose content of cereal grains. Starch synthase IIa (SSIIa) catalyses the polymerisation of intermediate length glucan chains of amylopectin in the endosperm of cereals. Mutations of SSIIa genes in barley and wheat and inactive SSIIa variant in rice induce similar effects on the starch structure and the amylose content, but the severity of the phenotypes is different. This study compared the levels of transcripts and partitioning of proteins of starch synthase I (SSI) and starch branching enzyme IIb (SBEIIb) inside and outside the starch granules in the developing endosperms of these ssIIa mutants and inactive SSIIa variant. Pleiotropic effects on starch granule-bound proteins suggested that the different effects of SSIIa mutations on endosperm amylose content of barley, wheat and rice are determined by the distribution of SSI and SBEIIb between the starch granule and amyloplast stroma in cereals. Regulation of starch synthesis in ssIIa mutants and inactive SSIIa variant may be at post-translational level or the altered amylopectin structure deprives the affinity of SSI and SBEIIb to amylopectin.

TILLING mutants of durum wheat result in a high amylose phenotype and provide information on alternative splicing mechanisms

The amylose/amylopectin ratio has a major influence over the properties of starch and determines its optimal end use. Here, high amylose durum wheat has been bred by combining knock down alleles at the two homoelogous genes encoding starch branching enzyme IIa (SBEIIa-A and SBEIIa-B). The complete silencing of these genes had a number of pleiotropic effects on starch synthesis: it affected the transcriptional activity of SBEIIb, ISA1 (starch debranching enzyme) and all of the genes encoding starch synthases (SSI, SSIIa, SSIII and GBSSI). The starch produced by grain of the double SBEIIa mutants was high in amylose (up to ∼1.95 fold that of the wild type) and contained up to about eight fold more resistant starch. A single nucleotide polymorphism adjacent to the splice site at the end of exon 10 of the G364E mutant copies of both SBEIIa-A and SBEIIa-B resulted in the loss of a conserved exonic splicing silencer element. Its starch was similar to that of the SBEIIa double mutant. G364E SBEIIa pre-mRNA was incorrectly processed, resulting in the formation of alternative, but non-functional splicing products.

Effect of wide variation of the Waxy gene on starch properties in hull-less barley from Qinghai-Tibet plateau in China

Granule-bound starch synthase I (GBSS I) plays an important role in the synthesis of amylose and in the determination of starch properties in barley grains. Genomic DNAs for the Waxy gene encoding GBSS I protein were sequenced from 34 barley accessions or lines from Qinghai-Tibet plateau in China, to identify Waxy gene nucleotide variations and study the roles of polymorphic sites of the Waxy gene on expression levels of Waxy transcripts and GBSS I proteins and on resulting starch properties. A total of 116 DNA polymorphic sites were identified within the barley Waxy gene, which divided the studied accessions into 11 haplotypes. Among 33 nucleotide polymorphic sites in coding regions, 5 SNPs in three exons were found to play different roles on the expression level of the Waxy transcript and the GBSS I protein and on the amylose content and starch properties. One SNP G(3935)-to-T substitution in the 10th exon in the accession Z999 (HP II-2) caused a high expression level of the Waxy transcript and the GBSS I protein and the amylose free phenotype. The other SNP alteration was a C(2453)-to-T in the fifth exon in the accession Z1191 (HP I-5), which drastically reduced the expression level of the Waxy transcript and the GBSS I protein and, finally, produced the amylose free phenotype. Three SNPs in the seventh exon in the accession Z1337 (HP I-6) did not significantly change the level of Waxy transcript, the GBSS I protein, and starch properties, except obviously reducing the breakdown value of starch viscosity and extending the peak time. A total of 84 DNA polymorphic sites were found in the noncoding regions. A 403 bp deletion at 5'UTR in the accession Z1979 (HP I-3) had low transcript level, low GBSS I protein level, and low amylose content due to the deletion of cis-acting DNA regulatory elements. A 191 bp insertion and a 15 bp insertion in the first intron and second exons, respectively, may be closely related to a higher transcript level of the Waxy gene and significant differences in some starch properties of the Waxy I DNA group as compared to the Waxy II DNA group. This study indicates the specific variations of the Waxy gene have a great effect on amylose synthesis and starch properties of hull-less barley, which could be very useful to produce new barley with variable starch properties.

Analysis of the functional interaction of Arabidopsis starch synthase and branching enzyme isoforms reveals that the cooperative action of SSI and BEs results in glucans with polymodal chain length distribution similar to amylopectin

Starch synthase (SS) and branching enzyme (BE) establish the two glycosidic linkages existing in starch. Both enzymes exist as several isoforms. Enzymes derived from several species were studied extensively both in vivo and in vitro over the last years, however, analyses of a functional interaction of SS and BE isoforms are missing so far. Here, we present data from in vitro studies including both interaction of leaf derived and heterologously expressed SS and BE isoforms. We found that SSI activity in native PAGE without addition of glucans was dependent on at least one of the two BE isoforms active in Arabidopsis leaves. This interaction is most likely not based on a physical association of the enzymes, as demonstrated by immunodetection and native PAGE mobility analysis of SSI, BE2, and BE3. The glucans formed by the action of SSI/BEs were analysed using leaf protein extracts from wild type and be single mutants (Atbe2 and Atbe3 mutant lines) and by different combinations of recombinant proteins. Chain length distribution (CLD) patterns of the formed glucans were irrespective of SSI and BE isoforms origin and still independent of assay conditions. Furthermore, we show that all SS isoforms (SSI-SSIV) were able to interact with BEs and form branched glucans. However, only SSI/BEs generated a polymodal distribution of glucans which was similar to CLD pattern detected in amylopectin of Arabidopsis leaf starch. We discuss the impact of the SSI/BEs interplay for the CLD pattern of amylopectin.

Suppression of starch synthase I expression affects the granule morphology and granule size and fine structure of starch in wheat endosperm

Studies in Arabidopsis and rice suggest that manipulation of starch synthase I (SSI) expression in wheat may lead to the production of wheat grains with novel starch structure and properties. This work describes the suppression of SSI expression in wheat grains using RNAi technology, which leads to a low level of enzymatic activity for SSI in the developing endosperm, and a low abundance of SSI protein inside the starch granules of mature grains. The amylopectin fraction of starch from the SSI suppressed lines showed an increased frequency of very short chains (degree of polymerization, dp 6 and 7), a lower proportion of short chains (dp 8-12), and more intermediate chains (dp 13-20) than in the grain from their negative segregant lines. In the most severely affected line, amylose content was significantly increased, the morphology of starch granules was changed, and the proportion of B starch granules was significantly reduced. The change of the fine structure of the starch in the SSI-RNAi suppression lines alters the gelatinization temperature, swelling power, and viscosity of the starch. This work demonstrates that the roles of SSI in the determination of starch structure and properties are similar among different cereals and Arabidopsis.

Improving cereal grain carbohydrates for diet and health

Starch and cell wall polysaccharides (dietary fibre) of cereal grains contribute to the health benefits associated with the consumption of whole grain cereal products, including reduced risk of obesity, type 2 diabetes, cardiovascular disease and colorectal cancer. The physiological bases for these effects are reviewed in relation to the structures and physical properties of the polysaccharides and their behaviour (including digestion and fermentation) in the gastro-intestinal tract. Strategies for modifying the content and composition of grain polysaccharides to increase their health benefits are discussed, including exploiting natural variation and using mutagenesis and transgenesis to generate further variation. These studies will facilitate the development of new types of cereals and cereal products to face the major health challenges of the 21st century.

Use of advanced recombinant lines to study the impact and potential of mutations affecting starch synthesis in barley

The effects on barley starch and grain properties of four starch synthesis mutations were studied during the introgression of the mutations from diverse backgrounds into an elite variety. The lys5f (ADPglucose transporter), wax (granule-bound starch synthase), isa1 (debranching enzyme isoamylase 1) and sex6 (starch synthase IIa) mutations were introgressed into NFC Tipple to give mutant and wild-type BC₂F₄ families with different genomic contributions of the donor parent. Comparison of starch and grain properties between the donor parents, the BC₂F₄ families and NFC Tipple allowed the effects of the mutations to be distinguished from genetic background effects. The wax and sex6 mutations had marked effects on starch properties regardless of genetic background. The sex6 mutation conditioned low grain weight and starch content, but the wax mutation did not. The lys5 mutation conditioned low grain weight and starch content, but exceptionally high β-glucan contents. The isa1 mutation promotes synthesis of soluble α-glucan (phytoglycogen). Its introgression into NFC Tipple increased grain weight and total α-glucan content relative to the donor parent, but reduced the ratio of phytoglycogen to starch. This study shows that introgression of mutations into a common, commercial background provides new insights that could not be gained from the donor parent.