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

Impact of Variation in Amylose Content on Durum Wheat cv. Svevo Technological and Starch Properties

Reserve starch, the main component of durum wheat semolina, is constituted of two glucan homopolymers (amylose and amylopectin) that differ in their chemical structure. Amylose is mainly a linear structure formed of α-1,4-linked glucose units, with a lower polymerization degree, whereas amylopectin is a highly branched structure of α-1,4-chains linked by α-1,6-bonds. Variation of the amylose/amylopectin ratio has a profound effect on the starch properties which may impact the wheat technological and nutritional characteristics and their possible use in the food and non-food sector. In this work a set of genotypes, with a range of amylose from 14.9 to 57.8%, derived from the durum wheat cv. Svevo was characterised at biochemical and rheological level and used to produce pasta to better understand the role of amylose content in a common genetic background. A negative correlation was observed between amylose content and semolina swelling power, starch peak viscosity, and pasta stickiness. A worsening of the firmness was observed in the low amylose pasta compared to the control (cv. Svevo), whereas no difference was highlighted in the high amylose samples. The resistant starch was higher in the high amylose (HA) pasta compared to the control and low amylose (LA) pasta. Noteworthy, the extent of starch digestion was reduced in the HA pasta while the LA genotypes offered a higher starch digestion, suggesting other possible applications.

Genetic and phenotypic characterization of rice grain quality traits to define research strategies for improving rice milling, appearance, and cooking qualities in Latin America and the Caribbean

Rice (Oryza sativa L.)grain quality is a set of complex interrelated traits that include grain milling, appearance, cooking, and edible properties. As consumer preferences in Latin America and the Caribbean evolve, determining what traits best capture regional grain quality preferences is fundamental for breeding and cultivar release. In this study, a genome-wide association study (GWAS), marker-assisted selection (MAS), and genomic selection (GS) were evaluated to help guide the development of new breeding strategies for rice grain quality improvement. For this purpose, 284 rice lines representing over 20 yr of breeding in Latin America and the Caribbean were genotyped and phenotyped for 10 different traits including grain milling, appearance, cooking, and edible quality traits. Genetic correlations among the 10 traits ranged from -0.83 to 0.85. A GWAS identified 19 significant marker/trait combinations associated with eight grain quality traits. Four functional markers, three located in the Waxy and one in the starch synthase IIa genes, were significantly associated with six grain-quality traits. These markers individually explained 51-75% of the phenotypic variance depending on the trait, clearly indicating their potential utility for MAS. Cross-validation studies to evaluate predictive abilities of four different GS models for each of the 10 quality traits were conducted and predictive abilities ranged from 0.3 to 0.72. Overall, the machine learning model random forest had the highest predictive abilities and was especially effective for traits where large effect quantitative trait loci were identified. This study provides the foundation for deploying effective molecular breeding strategies for grain quality in Latin American rice breeding programs.

Protein phosphorylation regulates maize endosperm starch synthase IIa activity and protein-protein interactions

Starch synthesis is an elaborate process employing several isoforms of starch synthases (SSs), starch branching enzymes (SBEs) and debranching enzymes (DBEs). In cereals, some starch biosynthetic enzymes can form heteromeric complexes whose assembly is controlled by protein phosphorylation. Previous studies suggested that SSIIa forms a trimeric complex with SBEIIb, SSI, in which SBEIIb is phosphorylated. This study investigates the post-translational modification of SSIIa, and its interactions with SSI and SBEIIb in maize amyloplast stroma. SSIIa, immunopurified and shown to be free from other soluble starch synthases, was shown to be readily phosphorylated, affecting V_max but with minor effects on substrate K_d and K_m values, resulting in a 12-fold increase in activity compared with the dephosphorylated enzyme. This ATP-dependent stimulation of activity was associated with interaction with SBEIIb, suggesting that the availability of glucan branching limits SSIIa and is enhanced by physical interaction of the two enzymes. Immunoblotting of maize amyloplast extracts following non-denaturing polyacrylamide gel electrophoresis identified multiple bands of SSIIa, the electrophoretic mobilities of which were markedly altered by conditions that affected protein phosphorylation, including protein kinase inhibitors. Separation of heteromeric enzyme complexes by GPC, following alteration of protein phosphorylation states, indicated that such complexes are stable and may partition into larger and smaller complexes. The results suggest a dual role for protein phosphorylation in promoting association and dissociation of SSIIa-containing heteromeric enzyme complexes in the maize amyloplast stroma, providing new insights into the regulation of starch biosynthesis in plants.

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.

Functional Genomic Validation of the Roles of Soluble Starch Synthase IIa in Japonica Rice Endosperm

The enzyme starch synthase IIa (SSIIa) in cereals has catalytic and regulatory roles during the synthesis of amylopectin that influences the functional properties of the grain. Rice endosperm SSIIa is more active in indica accessions compared to japonica lines due to functional SNP variations in the coding region of the structural gene. In this study, downregulating the expression of japonica-type SSIIa in Nipponbare endosperm resulted in either shrunken or opaque grains with an elevated proportion of A-type starch granules. Shrunken seeds had severely reduced starch content and could not be maintained in succeeding generations. In comparison, the opaque grain morphology was the result of weaker down-regulation of SSIIa which led to an elevated proportion of short-chain amylopectin (DP 6-12) and a concomitant reduction in the proportion of medium-chain amylopectin (DP 13-36). The peak gelatinization temperature of starch and the estimated glycemic score of cooked grain as measured by the starch hydrolysis index were significantly reduced. These results highlight the important role of medium-chain amylopectin in influencing the functional properties of rice grains, including its digestibility. The structural, regulatory and nutritional implications of down-regulated japonica-type SSIIa in rice endosperm are discussed.

Mutations in sorghum SBEIIb and SSIIa affect alkali spreading value, starch composition, thermal properties and flour viscosity

Seven novel alleles of SBEIIb and one allele of SSIIa co-segregated with the ASV phenotype and contributed to distinct starch quality traits important for food-processing applications. Sorghum is an important food crop for millions of people in Africa and Asia. Whole-genome re-sequencing of sorghum EMS mutants exhibiting an alkali spreading value (ASV) phenotype revealed candidate SNPs in Sobic.004G163700 and Sobic.010G093400. Comparative genomics identified Sobic.010G093400 as a starch synthase IIa and Sobic.004G163700 as a starch branching enzyme IIb. Segregation analyses showed that mutations in Sobic.010G093400 or Sobic.004G163700 co-segregated with the ASV phenotype. Mutants in SSIIa exhibited no change in amylose content but expressed lower final viscosity and lower starch gelatinization temperature (GT) than starches from non-mutant plants. The sbeIIb mutants exhibited significantly higher amylose levels and starch GT and lower viscosity compared to non-mutant starches and ssIIa mutants. Mutations in SBEIIb had a dosage-dependent effect on amylose content. Double mutants of sbeIIb and ssIIa resembled their sbeIIb parent in amylose content, starch thermal properties and viscosity profiles. These variants will provide opportunities to produce sorghum varieties with modified starch end-use qualities important for the beer brewing and baking industries and specialty foods for humans with diabetes.

Three Major Nucleotide Polymorphisms in the Waxy Gene Correlated with the Amounts of Extra-long Chains of Amylopectin in Rice Cultivars with S or L-type Amylopectin

Extra-long chains (ELC) of amylopectin in rice endosperm are synthesized by granule-bound starch synthase I encoded by the Waxy (Wx) gene, which primarily synthesizes amylose. Previous studies showed that single nucleotide polymorphisms (SNP) in intron 1 and exon 6 of the Wx gene influences ELC amount. However, whether these SNPs are conserved among rice cultivars and if any other SNPs are present in the Wx gene remained unknown. Here, we sequenced the Wx gene from 17 rice cultivars with S or L-type amylopectin, including those with known ELC content and those originating in China with unique starch properties, as well as typical japonica and indica cultivars. In addition to the two SNPs described above, an additional SNP correlating with ELC content was found in exon 10. Low ELC cultivars (<3.0 %) had thymine at the splicing donor site of intron 1, Tyr224 in exon 6, and Pro415 in exon 10. Cultivars with moderate ELC content (4.1–6.9 %) had guanine at the splicing donor site of intron 1, Ser224 in exon 6, and Pro415 in exon 10. Cultivars with high ELC content (7.7–13.9 %) had guanine at the splicing donor site of intron 1, Tyr224 in exon 6, and Ser415 in exon 10. The chain length distribution pattern of amylopectin was correlated with the amounts of SSIIa found in starch granules and gelatinization temperature, but not with ELC content. The combinations of SNPs in the Wx gene found in this study may provide useful information for screening specific cultivars with different ELC content.

Rice Mutants Lacking Starch Synthase I or Branching Enzyme IIb Activity Altered Starch Biosynthetic Protein Complexes

Amylopectin, the major component of starch, is synthesized by synergistic activity of multiple isozymes of starch synthases (SSs) and branching enzymes (BEs). The frequency and length of amylopectin branches determine the functionality of starch. In the rice endosperm, BEIIb generates short side chains of amylopectin and SSI elongates those branches, which can be further elongated by SSIIa. Absence of these enzymes greatly affects amylopectin structure. SSI, SSIIa, and BEIIb associate with each other and with other starch biosynthetic enzymes although SSIIa is low activity in japonica rice. The aim of the current study was to understand how the activity of starch biosynthetic enzyme complexes is compensated in the absence of SSI or BEIIb, and whether the compensatory effects are different in the absence of BEIIb or in the presence of inactive BEIIb. Interactions between starch biosynthetic enzymes were analyzed using one ss1 null mutant and two be2b japonica rice mutants (a mutant producing inactive BEIIb and a mutant that did not produce BEIIb). Soluble proteins extracted from the developing rice seeds were separated by gel filtration chromatography. In the absence of BEIIb activity, BEIIa was eluted in a broad molecular weight range (60-700 kDa). BEIIa in the wild-type was eluted with a mass below 300 kDa. Further, majority of inactive BEIIb co-eluted with SSI, SSIIa, and BEI, in a mass fraction over 700 kDa, whereas only small amounts of these isozymes were found in the wild-type. Compared with the be2b lines, the ss1 mutant showed subtle differences in protein profiles, but the amounts of SSIIa, SSIVb, and BEI in the over-700-kDa fraction were elevated. Immunoprecipitation revealed reduced association of SSIIa and BEIIb in the ss1 mutant, while the association of BEIIb with SSI, SSIIa, SSIVb, BEI, and BEIIa were more pronounced in the be2b mutant that produced inactive BEIIb enzyme. Mass spectrometry and western blotting revealed that SSI, SSIIa, SSIIIa, BEI, BEIIa, starch phosphorylase 1, and pullulanase were bound to the starch granules in the be2b mutants, but not in the wild-type and ss1 mutant. These results will aid the understanding of the mechanism of amylopectin biosynthesis.

Crop resistant starch and genetic improvement: a review of recent advances

Resistant starch (RS), as a healthy dietary fiber, meets with great human favor along with the rapid development and improvement of global living standards. RS shows direct effects in reducing postprandial blood glucose levels, serum cholesterol levels and glycemic index. Therefore, RS plays an important role in preventing and improving non-communicable diseases, such as obesity, diabetes, colon cancer, cardiovascular diseases and chronic kidney disease. In addition, RS leads to its potential applied value in the development of high-quality foodstuffs, such as bread, noodles and dumplings. This paper reviews the recent advances in RS research, focusing mainly on RS classification and measurement, formation, quantitative trait locus mapping, genome-wide association studies, molecular marker development and genetic improvement through induced mutations, plant breeding combined with marker-assisted selection and genetic transformation. Challenges and perspectives on further RS research are also discussed.

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.

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

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

Does Late Maturity Alpha-Amylase Impact Wheat Baking Quality?

Late maturity α-amylase (LMA) and pre-harvest sprouting (PHS) are both recognized as environmentally induced grain quality defects resulting from abnormally high levels of α-amylase. LMA is a more recently identified quality issue that is now receiving increasing attention worldwide and whose prevalence is now seen as impeding the development of superior quality wheat varieties. LMA is a genetic defect present in specific wheat genotypes and is characterized by elevated levels of the high pI TaAMY1 α-amylase, triggered by environmental stress during wheat grain development. TaAMY1 remains present in the aleurone through the harvest, lowering Falling Number (FN) at receival, causing a down-grading of the grain, often to feed grade, thus reducing the farmers' income. This downgrading is based on the assumption within the grain industry that, as for PHS, a low FN represents poor quality grain. Consequently any wheat line possessing low FN or high α-amylase levels is automatically considered a poor bread wheat despite there being no published evidence to date, to show that LMA is detrimental to end product quality. To evaluate the validity of this assumption a comprehensive evaluation of baking properties was performed from LMA prone lines using a subset of tall non-Rht lines from a multi-parent advanced generation inter-cross (MAGIC) wheat population grown at three different sites. LMA levels were determined along with quality parameters including end product functionality such as oven spring, bread loaf volume and weight, slice area and brightness, gas cell number and crumb firmness. No consistent or significant phenotypic correlation was found between LMA related FN and any of the quality traits. This manuscript provides for the first time, compelling evidence that LMA has limited impact on bread baking end product functionality.

Analysis of chloroplast differences in leaves of rice isonuclear alloplasmic lines

The chloroplast being an important organelle of plant cells could possibly be associated with plant cytoplasmic male sterility (CMS). To better understand the correlation between (CMS) and chloroplast, we presented a comprehensive analysis based on the changes of photosynthetic parameters, chloroplasts ultrastructure, soluble sugar and starch content, the relative expression of sugar and starch metabolism genes, and chloroplast genome in rice isonuclear alloplasmic CMS lines at the flowering stage. Leaf gas exchange parameters did not affect by CMS lines (M2BS and M2A), although intercellular CO₂ concentration (C _i) was influenced in both M2BS and M2A. Ultrastructural observation results indicated that many starch granules were observed in the chloroplast of CMS lines, especially bigger size in M2BS, while few ones in M2B. Only the chloroplasts of M2A contained some additional number of lipoids compared with those of the other two lines (M2B and M2BS). Soluble sugar and starch contents in CMS lines (M2BS and M2A) were significantly higher than those in maintainer line (M2B) (p < 0.01). The relative expression of sugar and starch metabolism genes indicated the imbalance of starch and sugar synthesis and decomposition may lead to accumulation of starch granules and demonstrated the presence of cytoplasmic effects. Moreover, chloroplast genome sequencing results showed similarity in both CMS lines, which revealed different single nucleotide polymorphisms (SNPs) and insertion/deletion (InDels) models compared with their maintainer line. Those models were located in psbD, rpoC2, rpl33, psbB, ndhA, ndhH, and intergenic regions. These findings, aligned with the possible association of CMS characteristics with cpDNA and genetically close relationship among both CMS lines, may contribute for future research.

Molecular aspects of sucrose transport and its metabolism to starch during seed development in wheat: A comprehensive review

Wheat is one of the most important crops globally, and its grain is mainly used for human food, accounting for 20% of the total dietary calories. It is also used as animal feed and as a raw material for a variety of non-food and non-feed industrial products such as a feedstock for the production of bioethanol. Starch is the major constituent of a wheat grain, as a result, it is considered as a critical determinant of wheat yield and quality. The amount and composition of starch deposited in wheat grains is controlled primarily by sucrose transport from source tissues to the grain and its conversion to starch. Therefore, elucidation of the molecular mechanisms regulating these physiological processes provides important opportunities to improve wheat starch yield and quality through biotechnological approaches. This review comprehensively discusses the current understanding of the molecular aspects of sucrose transport and sucrose-to-starch metabolism in wheat grains. It also highlights the advances and prospects of starch biotechnology in wheat.

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.

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

The starch-rich endosperms of the Poaceae, which includes wild grasses and their domesticated descendents the cereals, have provided humankind and their livestock with the bulk of their daily calories since the dawn of civilization up to the present day. There are currently unprecedented pressures on global food supplies, largely resulting from population growth, loss of agricultural land that is linked to increased urbanization, and climate change. Since cereal yields essentially underpin world food and feed supply, it is critical that we understand the biological factors contributing to crop yields. In particular, it is important to understand the biochemical pathway that is involved in starch biosynthesis, since this pathway is the major yield determinant in the seeds of six out of the top seven crops grown worldwide. This review outlines the critical stages of growth and development of the endosperm tissue in the Poaceae, including discussion of carbon provision to the growing sink tissue. The main body of the review presents a current view of our understanding of storage starch biosynthesis, which occurs inside the amyloplasts of developing endosperms.

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.

Differences in specificity and compensatory functions among three major starch synthases determine the structure of amylopectin in rice endosperm

The lengths of amylopectin-branched chains are precise and influence the physicochemical properties of starch, which determine starch functionality. Three major isozymes of starch synthases (SSs), SSI, SSII(a), and SSIII(a), are primarily responsible for amylopectin chain elongation in the storage tissues of plants. To date, the majority of reported rice mutants were generated using japonica cultivars, which have almost inactive SSIIa. Although three SSs share some overlapping chain length preferences, whether they complement each other remains unknown due to the absence of suitable genetic combinations of materials. In this study, rice ss1/SS2a/SS3a and SS1/SS2a/ss3a were newly generated, and the chain length distribution patterns of all the possible combinations of presence and absence of SSI, SSIIa, and SSIIIa activities were compared. This study demonstrated that SSIIa can complement most SSI functions that use glucan chains with DP 6-7 to generate DP 8-12 chains but cannot fully compensate for the elongation of DP 16-19 chains. This suggests that SSIIa preferentially elongates outer but not inner chains of amylopectin. In addition, the results revealed that neither SSI nor SSIIIa compensate for SSIIa. Neither SSI nor SSIIa compensate for elongation of DP >30 by SSIIIa. SSIIa could not resolve the pleiotropic increase of SSI caused by the absence of SSIIIa; instead, SSIIa further elongated those branches elongated by SSI. These results revealed compensatory differences among three major SS isozymes responsible for lengths of amylopectin branches.

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.

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.

Formation of starch in plant cells

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