Expression patterns of genes involved in starch biosynthesis during seed development in bread wheat (Triticum aestivum)

In-Silico Identification, Characterization and Expression Analysis of Genes Involved in Resistant Starch Biosynthesis in Potato (Solanum tuberosum L.) Varieties

Potato (Solanum tuberosum L.), an important horticultural crop is a member of the family Solanaceae and is mainly grown for consumption at global level. Starch, the principal component of tubers, is one of the significant elements for food and non-food-based applications. The genes associated with biosynthesis of starch have been investigated extensively over the last few decades. However, a complete regulation pathway of constituent of amylose and amylopectin are still not deeply explored. The current in-silico study of genes related to amylose and amylopectin synthesis and their genomic organization in potato is still lacking. In the current study, the nucleotide and amino acid arrangement in genome and twenty-two genes linked to starch biosynthesis pathway in potato were analysed. The genomic structure analysis was also performed to find out the structural pattern and phylogenetic relationship of genes. The genome mining and structure analysis identified ten specific motifs and phylogenetic analysis of starch biosynthesis genes divided them into three different clades on the basis of their functioning and phylogeny. Quantitative real-time PCR (qRT-PCR) of amylose biosynthesis pathway genes in three contrast genotypes revealed the down-gene expression that leads to identify potential cultivar for functional genomic approaches. These potential lines may help to achieve higher content of resistant starch.

Evaluation of altered starch mutants and identification of candidate genes responsible for starch variation in wheat

BACKGROUND

Induction of mutation through chemical mutagenesis is a novel approach for preparing diverse germplasm. Introduction of functional alleles in the starch biosynthetic genes help in the improvement of the quality and yield of cereals.

RESULTS

In the present study, a set of 350 stable mutant lines were used to evaluate dynamic variation of the total starch contents. A megazyme kits were used for measuring the total starch content, resistant starch, amylose, and amylopectin content. Analysis of variance showed significant variation (p < 0.05) in starch content within the population. Furthermore, two high starch mutants (JE0173 and JE0218) and two low starch mutants (JE0089 and JE0418) were selected for studying different traits. A multiple comparison test showed that significant variation in all physiological and morphological traits, with respect to the parent variety (J411) in 2019-2020 and 2020-2021. The quantitative expression of starch metabolic genes revealed that eleven genes of JE0173 and twelve genes of JE0218 had consistent expression in high starch mutant lines. Similarly, in low starch mutant lines, eleven genes of JE0089 and thirteen genes of JE0418 had consistent expression in all stages of seed development. An additional two candidate genes showed over-expression (PHO1, PUL) in the high starch mutant lines, indicating that other starch metabolic genes may also contribute to the starch biosynthesis. The overexpression of SSII, SSIII and SBEI in JE0173 may be due to presence of missense mutations in these genes and SSI also showed overexpression which may be due to 3-primer_UTR variant. These mutations can affect the other starch related genes and help to increase the starch content in this mutant line (JE0173).

CONCLUSIONS

This study screened a large scale of mutant population and identified mutants, could provide useful genetic resources for the study of starch biosynthesis and genetic improvement of wheat in the future. Further study will help to understand new genes which are responsible for the fluctuation of total starch.

Genetic Engineering of Starch Biosynthesis in Maize Seeds for Efficient Enzymatic Digestion of Starch during Bioethanol Production

Maize accumulates large amounts of starch in seeds which have been used as food for human and animals. Maize starch is an importantly industrial raw material for bioethanol production. One critical step in bioethanol production is degrading starch to oligosaccharides and glucose by α-amylase and glucoamylase. This step usually requires high temperature and additional equipment, leading to an increased production cost. Currently, there remains a lack of specially designed maize cultivars with optimized starch (amylose and amylopectin) compositions for bioethanol production. We discussed the features of starch granules suitable for efficient enzymatic digestion. Thus far, great advances have been made in molecular characterization of the key proteins involved in starch metabolism in maize seeds. The review explores how these proteins affect starch metabolism pathway, especially in controlling the composition, size and features of starch. We highlight the roles of key enzymes in controlling amylose/amylopectin ratio and granules architecture. Based on current technological process of bioethanol production using maize starch, we propose that several key enzymes can be modified in abundance or activities via genetic engineering to synthesize easily degraded starch granules in maize seeds. The review provides a clue for developing special maize cultivars as raw material in the bioethanol industry.

Long-Term Soil Drought Limits Starch Accumulation by Altering Sucrose Transport and Starch Synthesis in Sweet Potato Tuberous Root

In this study, the influences of long-term soil drought with three levels [soil-relative water content (SRWC) (75 ± 5)%, as the control; SRWC (55 ± 5)%, mild drought; SRWC (45 ± 5)%, severe drought] were investigated on sucrose-starch metabolism in sweet potato tuberous roots (TRs) by pot experiment. Compared to the control, drought stress increased soluble sugar and sucrose content by 4-60% and 9-75%, respectively, but reduced starch accumulation by 30-66% through decreasing the starch accumulate rate in TRs. In the drought-treated TRs, the inhibition of sucrose decomposition was attributed to the reduced activities of acid invertase (AI) and alkaline invertase (AKI) and the IbA-INV3 expression, rather than sucrose synthase (SuSy), consequently leading to the increased sucrose content in TRs. In addition, starch synthesis was inhibited mainly by reducing ADP-glucose pyrophosphorylase (AGPase), granular starch synthase (GBSS) and starch branching enzyme (SBE) activities in TRs under drought stress, and AGPase was the rate-limiting enzyme. Furthermore, soil drought remarkably up-regulated the IbSWEET11, IbSWEET605, and IbSUT4 expressions in Jishu 26 TRs, while it down-regulated or had no significant differences in Xushu 32 and Ningzishu 1 TRs. These results suggested that the sucrose-loading capability in Jishu 26 TRs were stronger than that in Xushu 32 and Ningzishu 1 TRs. Moreover, IbA-INV3, IbAGPS1, IbAGPS2, IbGBSSI and IbSBEII play important roles in different drought-tolerant cultivars under drought stress.

Bibliometric Analysis of Functional Crops and Nutritional Quality: Identification of Gene Resources to Improve Crop Nutritional Quality through Gene Editing Technology

Food security and hidden hunger are two worldwide serious and complex challenges nowadays. As one of the newly emerged technologies, gene editing technology and its application to crop improvement offers the possibility to relieve the pressure of food security and nutrient needs. In this paper, we analyzed the research status of quality improvement based on gene editing using four major crops, including rice, soybean, maize, and wheat, through a bibliometric analysis. The research hotspots now focus on the regulatory network of related traits, quite different from the technical improvements to gene editing in the early stage, while the trends in deregulation in gene-edited crops have accelerated related research. Then, we mined quality-related genes that can be edited to develop functional crops, including 16 genes related to starch, 15 to lipids, 14 to proteins, and 15 to other functional components. These findings will provide useful reference information and gene resources for the improvement of functional crops and nutritional quality based on gene editing technology.

Comparative transcriptome analyses revealed key genes involved in high amylopectin biosynthesis in wheat

High amylopectin starch is an important modified starch for food processing industries. Despite a thorough understanding of starch biosynthesis pathway, the regulatory mechanism responsible for amylopectin biosynthesis is not well explored. The present study utilized transcriptome sequencing approach to understand the molecular basis of high amylopectin content in three high amylopectin mutant wheat lines ('TAC 6', 'TAC 358', and 'TAC 846') along with parent variety 'C 306'. Differential scanning calorimetry (DSC) of high amylopectin starch identified a high thermal transition temperature and scanning electron microscopy (SEM) revealed more spherical starch granules in mutant lines compared to parent variety. A set of 4455 differentially expressed genes (DEGs) were identified at two-fold compared to the parent variety in high amylopectin wheat mutants. At ten-fold, 279 genes, including two starch branching genes (SBEIIa and SBEIIb), were up-regulated and only 30 genes, including the starch debranching enzyme (DBE), were down-regulated. Among the genes, different isoforms of sucrose non-fermenting-1-related protein kinase-1 (TaSnRK1α2-3B and TaSnRK1α2-3D) and its regulatory subunit, sucrose non-fermenting-4 (SNF-4-2A, SNF-4-2B, and SNF-4-5D), were found to be highly up-regulated. Further, expression of the DEGs related to starch biosynthesis pathway and TaSnRK1α2 and SNF-4 was performed using qRT-PCR. High expression of TaSnRK1α2, SNF-4, and SBEII isoforms suggests their probable role in high amylopectin starch biosynthesis in grain endosperm.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03364-3.

Using the dominant mutation gene Ae1-5180 (amylose extender) to develop high-amylose maize

Maize amylose is a type of high value-added starch used for medical, food, and chemical applications. Mutations in the starch branching enzyme (SBEIIb), with recessive ae (amylose extender) and dominant Ae1-5180 alleles, are the primary way to improve maize endosperm amylose content (AC). However, studies on Ae1-5180 mutation are scarce, and its roles in starch synthesis and breeding potential are unclear. We found that the AC of the Ae1-5180 mutant was 47.23%, and its kernels were tarnished and glassy and are easily distinguished from those of the wild type (WT), indicating that the dominant mutant has the classical characteristics of the ae mutant. Starch granules of Ae1-5180 became smaller, and higher in amount with irregular shape. The degree of amylopectin polymerisation changed to induce an increase in starch thermal stability. Compared with WT, the activity of granule-bound starch synthase and starch synthase was higher in early stages and lower in later stages, and other starch synthesis enzymes decreased during kernel development in the Ae1-5180 mutant. We successfully developed a marker (mu406) for the assisted selection of 17 Ae1-5180 near isogenic lines (NILs) according to the position of insertion of the Mu1 transposon in the SBEIIb promoter of Ae1-5180. JH214/Ae1-5180, CANS-1/Ae1-5180, CA240/Ae1-5180, and Z1698/Ae1-5180 have high breeding application potential with their higher AC (> 40%) and their 100-kernel weight decreased to < 25% compared to respective recurrent parents. Therefore, using the dominant Ae1-5180 mutant as a donor can detect the kernel phenotype and AC of Ae1-5180-NILs in advance, thereby accelerating the high-amylose breeding process.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-022-01323-7.

Ae1/Sbe1 maize-derived high amylose improves gut barrier function and ameliorates type II diabetes in high-fat diet-fed mice by increasing Akkermansia

Type II diabetes mellitus (T2DM) has its origins in chronic inflammation due to immune dysregulation. Improving chronic inflammation can significantly reduce the probability of T2DM and the rate of disease progression. Resistance to starch 2 (RSII) high-amylose maize starch (HAMS) has been widely implicated in the improvement and regulation of T2DM. However, its exact molecular mechanisms have not been fully discovered. Here, we used CRISPR/Cas9 technology to knock out two starch-branching enzyme genes, Ae1 and Sbe1, in maize to obtain mutants containing higher levels of HAMS. In experiments in which HAMS was fed to mice on a high-fat diet (HFD), we confirmed the function of HAMS in ameliorating hyperglycemia. Mechanistically, we found that HAMS improves the gut barrier function by increasing the Akkermansia abundance in the gut. This increase led to the alleviation of chronic inflammation in mice on a HFD, resulting in improved insulin sensitivity and a decrease in blood glucose.

Screening of Induced Mutants Led to the Identification of Starch Biosynthetic Genes Associated with Improved Resistant Starch in Wheat

Several health benefits are obtained from resistant starch, also known as healthy starch. Enhancing resistant starch with genetic modification has huge commercial importance. The variation of resistant starch content is narrow in wheat, in relation to which limited improvement has been attained. Hence, there is a need to produce a wheat population that has a wide range of variations in resistant starch content. In the present study, stable mutants were screened that showed significant variation in the resistant starch content. A megazyme kit was used for measuring the resistant starch content, digestible starch, and total starch. The analysis of variance showed a significant difference in the mutant population for resistant starch. Furthermore, four diverse mutant lines for resistant starch content were used to study the quantitative expression patterns of 21 starch metabolic pathway genes; and to evaluate the candidate genes for resistant starch biosynthesis. The expression pattern of 21 starch metabolic pathway genes in two diverse mutant lines showed a higher expression of key genes regulating resistant starch biosynthesis (GBSSI and their isoforms) in the high resistant starch mutant lines, in comparison to the parent variety (J411). The expression of SBEs genes was higher in the low resistant starch mutants. The other three candidate genes showed overexpression (BMY, Pho1, Pho2) and four had reduced (SSIII, SBEI, SBEIII, ISA3) expression in high resistant starch mutants. The overexpression of AMY and ISA1 in the high resistant starch mutant line JE0146 may be due to missense mutations in these genes. Similarly, there was a stop_gained mutation for PHO2; it also showed overexpression. In addition, the gene expression analysis of 21 starch metabolizing genes in four different mutants (low and high resistant starch mutants) shows that in addition to the important genes, several other genes (phosphorylase, isoamylases) may be involved and contribute to the biosynthesis of resistant starch. There is a need to do further study about these new genes, which are responsible for the fluctuation of resistant starch in the mutants.

Protein targeting to starch 1, a functional protein of starch biosynthesis in wheat (Triticum aestivum L.)

TaPTST1, a wheat homolog of AtPTST1 containing CBM can interact with GBSSI and regulate starch metabolism in wheat endosperm. In cereal endosperm, native starch comprising amylose and amylopectin is synthesized by the coordinated activities of several pathway enzymes. Amylose in starch influences its physio-chemical properties resulting in several human health benefits. The Granule-Bound Starch Synthase I (GBSSI) is the most abundant starch-associated protein. GBSSI lacks dedicated Carbohydrate-binding module (CBM). Previously, Protein Targeting To Starch 1 (PTST1) was identified as a crucial protein for the localization of GBSSI to the starch granules in Arabidopsis. The function of its homologous protein in the wheat endosperm is not known. In this study, TaPTST1, an AtPTST1 homolog, containing a CBM and a coiled-coil domain was identified in wheat. Protein-coding nucleotide sequence of TaPTST1 from Indian wheat variety 'C 306' was cloned and characterized. Homology modelling and molecular docking suggested the potential interaction of TaPTST1 with glucans and GBSSI. The TaPTST1 expression was higher in wheat grain than the other tissues, suggesting a grain-specific function. In vitro binding assays demonstrated different binding affinities of TaPTST1 for native starch, amylose, and amylopectin. Furthermore, the immunoaffinity pull-down assay revealed that TaPTST1 directly interacts with GBSSI, and the interaction is mediated by a coiled-coil domain. The direct protein-protein interaction was further confirmed by bimolecular fluorescence complementation assay (BiFC) in planta. Based on our findings we postulate a functional role for TaPTST1 in starch metabolism by targeting GBSSI to starch granules in wheat endosperm.

Goyal A, Chaudhury A. Molecular and Transcriptional Regulation of Seed Development in Cereals: Present Status and Future Prospects

Cereals are a rich source of vitamins, minerals, carbohydrates, fats, oils and protein, making them the world’s most important source of nutrition. The influence of rising global population, as well as the emergence and spread of disease, has the major impact on cereal production. To meet the demand, there is a pressing need to increase cereal production. Optimal seed development is a key agronomical trait that contributes to crop yield. The seed development and maturation is a complex process that includes not only embryo and endosperm development, but also accompanied by huge physiological, biochemical, metabolic, molecular and transcriptional changes. This chapter discusses the growth of cereal seed and highlights the novel biological insights, with a focus on transgenic and new molecular breeding, as well as biotechnological intervention strategies that have improved crop yield in two major cereal crops, primarily wheat and rice, over the last 21 years (2000–2021).

Gene Expression and Proteomics Studies Suggest an Involvement of Multiple Pathways Under Day and Day-Night Combined Heat Stresses During Grain Filling in Wheat

Recent weather fluctuations imposing heat stress at the time of wheat grain filling cause frequent losses in grain yield and quality. Field-based studies for understanding the effect of terminal heat stress on wheat are complicated by the effect of multiple confounding variables. In the present study, the effect of day and day-night combined heat stresses during the grain-filling stage was studied using gene expression and proteomics approaches. The gene expression analysis was performed by using real-time quantitative PCR (RT-qPCR). The expression of genes related to the starch biosynthetic pathway, starch transporters, transcription factors, and stress-responsive and storage proteins, at four different grain developmental stages, indicated the involvement of multiple pathways. Under the controlled conditions, their expression was observed until 28 days after anthesis (DAA). However, under the day stress and day-night stress, the expression of genes was initiated earlier and was observed until 14 DAA and 7 DAA, respectively. The protein profiles generated using two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS/MS) showed a differential expression of the proteins belonging to multiple pathways that included the upregulation of proteins related to the translation, gliadins, and low-molecular-weight (LMW) glutenins and the downregulation of proteins related to the glycolysis, photosynthesis, defense, and high-molecular-weight (HMW) glutenins. Overall, the defense response to the day heat stress caused early gene expression and day-night heat stress caused suppression of gene expression by activating multiple pathways, which ultimately led to the reduction in grain-filling duration, grain weight, yield, and processing quality.

Understanding the regulatory relationship of abscisic acid and bZIP transcription factors towards amylose biosynthesis in wheat

Starch is biosynthesized during seed development and this process is regulated by many bZIP proteins in bread wheat. Abscisic acid (ABA), an important phyto-hormone involved in various physiological processes mediated by bZIPs in plants including seed development. The 'Group A' TabZIP transcription factors play important roles in the ABA signaling pathway in Arabidopsis, rice and other cereal crops but their role in regulation of amylose biosynthesis in wheat is limited. In this study 83 'Group A' TabZIPs were characterized by gene expression analysis in wheat amylose mutants. A set of 17 TabZIPs was selected on the basis of differential expression (> 2 fold) in low and high amylose mutants from RNA-seq data and validated by qRT PCR. Based on qRT PCR and correlation analysis out of the 17 TabZIPs six differentially expressed candidate TabZIPs were identified, involving in high amylose biosynthesis. The TabZIP175.2, identified as upregulated in all high amylose lines and TabZIP90.2, TabZIP129.1, TabZIP132.2, TabZIP143 and TabZIP159.2 were found downregulated in all low amylose lines, after exogenous supply of ABA. Proximal promoter analysis of starch pathway genes revealed the presence of ABA-responsive elements (ABREs) that are putative binding sites for bZIPs. Collectively, these findings indicated the involvement of putative six candidate TabZIPs as transcriptional regulators of amylose related genes via an ABA-dependent pathway in wheat. This study could help the investigators to make an informed decision to edit wheat genome for high/low amylose content using gene-editing technologies.

Over-Expressing TaSPA-B Reduces Prolamin and Starch Accumulation in Wheat (Triticum aestivum L.) Grains

Starch and prolamin composition and content are important indexes for determining the processing and nutritional quality of wheat (Triticum aestivum L.) grains. Several transcription factors (TFs) regulate gene expression during starch and protein biosynthesis in wheat. Storage protein activator (TaSPA), a member of the basic leucine zipper (bZIP) family, has been reported to activate glutenin genes and is correlated to starch synthesis related genes. In this study, we generated TaSPA-B overexpressing (OE) transgenic wheat lines. Compared with wild-type (WT) plants, the starch content was slightly reduced and starch granules exhibited a more polarized distribution in the TaSPA-B OE lines. Moreover, glutenin and ω- gliadin contents were significantly reduced, with lower expression levels of related genes (e.g., By15, Dx2, and ω-1,2 gliadin gene). RNA-seq analysis identified 2023 differentially expressed genes (DEGs). The low expression of some DEGs (e.g., SUSase, ADPase, Pho1, Waxy, SBE, SSI, and SS II a) might explain the reduction of starch contents. Some TFs involved in glutenin and starch synthesis might be regulated by TaSPA-B, for example, TaPBF was reduced in TaSPA-B OE-3 lines. In addition, dual-luciferase reporter assay indicated that both TaSPA-B and TaPBF could transactivate the promoter of ω-1,2 gliadin gene. These results suggest that TaSPA-B regulates a complex gene network and plays an important role in starch and protein biosynthesis in wheat.

Novel intron length polymorphic (ILP) markers from starch biosynthesis genes reveal genetic relationships in Indian wheat varieties and related species

Introns experience lesser selection pressure, thus are liable for higher polymorphism. Intron Length Polymorphic (ILP) markers designed from exon-flanking introns exploits this polymorphic potential and have been proved to be a robust co-dominant marker in eukaryotes. Wheat is among the most consumed cereal crop by majority of the word population. It is a rich source of calories in the form of stored starch. In the current study, starch biosynthesis genes were mined for development of ILP markers and their subsequent utilization for genetic characterization of popular Indian wheat varieties and transferability to wild relatives. Sixty-one markers generated 122 alleles and showed 77-88.5% transferability (mean PIC: 0.36) to the related species. A subset of markers showed clear genetic distinctions (Avg. genetic dissimilarity = 0.42) among Indian wheat varieties, signifying the importance of novel ILPs. 'Kenphad25' showed maximum genetic dissimilarity with 'K 8962' (0.82), while maximum genetic similarity was observed between 'Safed Lerma' and 'RAJ 4037' (0.1). This is the first report of ILP markers in wheat and will be a useful genomic resource for future germplasm conservation and molecular breeding studies.

High night temperature induced changes in grain starch metabolism alters starch, protein, and lipid accumulation in winter wheat

Unlike sporadic daytime heat spikes, a consistent increase in night-time temperatures can potentially derail the genetic gains being achieved. Ten winter wheat genotypes were exposed to six different night-time temperatures (15-27°C) during flowering and grain-filling stages in controlled environment chambers. We identified the night-time temperature of 23^o C as the critical threshold beyond which a consistent decline in yields and quality was observed. Confocal laser scanning micrographs of central endosperm, bran, and germ tissue displayed differential accumulation of protein, lipid, and starch with increasing night-time temperatures. KS07077M-1 recorded a decrease in starch and an increase in protein and lipid in central endosperm with increasing night-time temperatures, whereas the same was significantly lower in the tolerant SY Monument. Expression analysis of genes encoding 21 enzymes (including isoforms) involved in grain-starch metabolism in developing grains revealed a high night-time temperature (HNT)-induced reduction in transcript levels of adenosine diphosphate glucose pyrophosphorylase small subunit involved in starch synthesis and a ≥2-fold increase in starch degrading enzymes isoamylase III, alpha-, and beta-amylase. The identified critical threshold, grain compositional changes, and the key enzymes in grain starch metabolism that lead to poor starch accumulation in grains establish the foundational knowledge for enhancing HNT tolerance in wheat.

The synergistic effects of TaAGP.L-B1 and TaSSIVb-D mutations in wheat lead to alterations of gene expression patterns and starch content in grain development

Starch is synthesized from a series of reactions catalyzed by enzymes. ADP-glucose pyrophosphorylase (AGPase) initiates the synthesis pathway and synthesizes ADP-glucose, the substrate of starch synthase (SS), of which SSIV is an isoform. Mutations of the AGPase subunit and SSIV-coding genes affect starch content and cause variation in the number of granules. Here, we pyramided the functional mutation alleles of the AGPase subunit gene TaAGP.L-B1 and the SSIV-coding gene TaSSIVb-D to elucidate their synergistic effects on other key starch biosynthesis genes and their impact on starch content. Both the TaAGP.L-B1 and TaSSIVb-D genes were expressed in wheat grain development, and the expression level of TaAGP.L-B1 was higher than that of TaSSIVb-D. The TaAGP.L-B1 gene was downregulated in the agp.L-B1 single and agp.L-B1/ssIV-D double mutants at 12 to 18 days after flowering (DAF). TaSSIVb-D expression was significantly reduced at 6 DAF in both ssIV-D single and double mutants. In the agp.L-B1/ssIV-D double mutant, TaGBSSII was upregulated, while TaAGPSS, TaSSI, and TaSBEII were downregulated. Under the interaction of these genes, the total starch and amylopectin contents were significantly decreased in agp.L-B1 and agp.L-B1/ssIV-D mutants. The results suggested that the mutations of TaAGP.L-B1 and TaSSIVb-D genes resulted in variation in the expression patterns of the other four starch synthetic genes and led to a reduction in starch and amylopectin contents. These mutants could be used further as germplasm for resistant starch analysis.

Carbon balance and source-sink metabolic changes in winter wheat exposed to high night-time temperature

Carbon loss under high night-time temperature (HNT) leads to significant reduction in wheat yield. Growth chamber studies were carried out using six winter wheat genotypes, to unravel postheading HNT (23°C)-induced alterations in carbon balance, source-sink metabolic changes, yield, and yield-related traits compared with control (15°C) conditions. Four of the six tested genotypes recorded a significant increase in night respiration after 4 days of HNT exposure, with all the cultivars regulating carbon loss and demonstrating different degree of acclimation to extended HNT exposure. Metabolite profiling indicated carbohydrate metabolism in spikes and activation of the TriCarboxylic Acid (TCA) cycle in leaves as important pathways operating under HNT exposure. A significant increase in sugars, sugar-alcohols, and phosphate in spikes of the tolerant genotype (Tascosa) indicated osmolytes and membrane protective mechanisms acting against HNT damage. Enhanced night respiration under HNT resulted in higher accumulation of TCA cycle intermediates like isocitrate and fumarate in leaves of the susceptible genotype (TX86A5606). Lower grain number due to lesser productive spikes and reduced grain weight due to shorter grain-filling duration determined HNT-induced yield loss in winter wheat. Traits and mechanisms identified will help catalyze the development of physiological and metabolic markers for breeding HNT-tolerant wheat.

Pivotal role of bZIPs in amylose biosynthesis by genome survey and transcriptome analysis in wheat (Triticum aestivum L.) mutants

Starch makes up 70% of the wheat grain, and is an important source of calories for humans, however, the overconsumption of wheat starch may contribute to nutrition-associated health problems. The challenge is to develop resistant starch including high amylose wheat varieties with health benefits. Adapting advance genomic approaches in EMS-induced mutant lines differing in amylose content, basic leucine zipper (bZIP) regulatory factors that may play role in controlling amylose biosynthesis were identified in wheat. bZIP transcription factors are key regulators of starch biosynthesis genes in rice and maize, but their role in regulating these genes in wheat is poorly understood. A genome-wide survey identified 370 wheat bZIPs, clustered in 11 groups, showing variations in amino acids composition and predicted physicochemical properties. Three approaches namely, whole transcriptome sequencing, qRT-PCR, and correlation analysis in contrasting high and low amylose mutants and their parent line identified 24 candidate bZIP (positive and negative regulators), suggesting bZIPs role in high amylose biosynthesis. bZIPs positive role in high amylose biosynthesis is not known. In silico interactome studies of candidate wheat bZIP homologs in Arabidopsis and rice identified their putative functional role. The identified bZIPs are involved in stress-related pathways, flower and seed development, and starch biosynthesis. An in-depth analysis of molecular mechanism of novel candidate bZIPs may help in raising and improving high amylose wheat varieties.

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.

TaGW2-6A allelic variation contributes to grain size possibly by regulating the expression of cytokinins and starch-related genes in wheat

Functional allelic variants of TaGW2 - 6A produce large grains, possibly via changes in endosperm cells and dry matter by regulating the expression of cytokinins and starch-related genes via the ubiquitin-proteasome system. In wheat, TaGW2-6A coding region allelic variants are closely related to the grain width and weight, but how this region affects grain development has not been fully elucidated; thus, we explored its influence on grain development based mainly on histological and grain filling analyses. We found that the insertion type (NIL31) TaGW2-6A allelic variants exhibited increases in cell numbers and cell size, thereby resulting in a larger (wider) grain size with an accelerated grain milk filling rate, and increases in grain width and weight. We also found that cytokinin (CK) synthesis genes and key starch biosynthesis enzyme AGPase genes were significantly upregulated in the TaGW2-6A allelic variants, while CK degradation genes and starch biosynthesis-negative regulators were downregulated in the TaGW2-6A allelic variants, which was consistent with the changes in cells and grain filling. Thus, we speculate that TaGW2-6A allelic variants are linked with CK signaling, but they also influence the accumulation of starch by regulating the expression of related genes via the ubiquitin-proteasome system to control the grain size and grain weight.

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.

Development of EMS-induced mutation population for amylose and resistant starch variation in bread wheat (Triticum aestivum) and identification of candidate genes responsible for amylose variation

BACKGROUND

Starch is a major part of cereal grain. It comprises two glucose polymer fractions, amylose (AM) and amylopectin (AP), that make up about 25 and 75 % of total starch, respectively. The ratio of the two affects processing quality and digestibility of starch-based food products. Digestibility determines nutritional quality, as high amylose starch is considered a resistant or healthy starch (RS type 2) and is highly preferred for preventive measures against obesity and related health conditions. The topic of nutrition security is currently receiving much attention and consumer demand for food products with improved nutritional qualities has increased. In bread wheat (Triticum aestivum L.), variation in amylose content is narrow, hence its limited improvement. Therefore, it is necessary to produce wheat lines or populations showing wide variation in amylose/resistant starch content. In this study, a set of EMS-induced M4 mutant lines showing dynamic variation in amylose/resistant starch content were produced. Furthermore, two diverse mutant lines for amylose content were used to study quantitative expression patterns of 20 starch metabolic pathway genes and to identify candidate genes for amylose biosynthesis.

RESULTS

A population comprising 101 EMS-induced mutation lines (M4 generation) was produced in a bread wheat (Triticum aestivum) variety. Two methods of amylose measurement in grain starch showed variation in amylose content ranging from ~3 to 76 % in the population. The method of in vitro digestion showed variation in resistant starch content from 1 to 41 %. One-way ANOVA analysis showed significant variation (p < 0.05) in amylose and resistant starch content within the population. A multiple comparison test (Dunnett's test) showed that significant variation in amylose and resistant starch content, with respect to the parent, was observed in about 89 and 38 % of the mutant lines, respectively. Expression pattern analysis of 20 starch metabolic pathway genes in two diverse mutant lines (low and high amylose mutants) showed higher expression of key genes of amylose biosynthesis (GBSSI and their isoforms) in the high amylose mutant line, in comparison to the parent. Higher expression of amylopectin biosynthesis (SBE) was observed in the low amylose mutant lines. An additional six candidate genes showed over-expression (BMY, SPA) and reduced-expression (SSIII, SBEI, SBEIII, ISA3) in the high amylose mutant line, indicating that other starch metabolic genes may also contribute to amylose biosynthesis.

CONCLUSION

In this study a set of 101 EMS-induced mutant lines (M4 generation) showing variation in amylose and resistant starch content in seed were produced. This population serves as useful germplasm or pre-breeding material for genome-wide study and improvement of starch-based processing and nutrition quality in wheat. It is also useful for the study of the genetic and molecular basis of amylose/resistant starch variation in wheat. Furthermore, gene expression analysis of 20 starch metabolic genes in the two diverse mutant lines (low and high amylose mutants) indicates that in addition to key genes, several other genes (such as phosphorylases, isoamylases, and pullulanases) may also be involved in contributing to amylose/amylopectin biosynthesis.

Genome-Wide QTL Mapping for Wheat Processing Quality Parameters in a Gaocheng 8901/Zhoumai 16 Recombinant Inbred Line Population

Dough rheological and starch pasting properties play an important role in determining processing quality in bread wheat (Triticum aestivum L.). In the present study, a recombinant inbred line (RIL) population derived from a Gaocheng 8901/Zhoumai 16 cross grown in three environments was used to identify quantitative trait loci (QTLs) for dough rheological and starch pasting properties evaluated by Mixograph, Rapid Visco-Analyzer (RVA), and Mixolab parameters using the wheat 90 and 660 K single nucleotide polymorphism (SNP) chip assays. A high-density linkage map constructed with 46,961 polymorphic SNP markers from the wheat 90 and 660 K SNP assays spanned a total length of 4121 cM, with an average chromosome length of 196.2 cM and marker density of 0.09 cM/marker; 6596 new SNP markers were anchored to the bread wheat linkage map, with 1046 and 5550 markers from the 90 and 660 K SNP assays, respectively. Composite interval mapping identified 119 additive QTLs on 20 chromosomes except 4D; among them, 15 accounted for more than 10% of the phenotypic variation across two or three environments. Twelve QTLs for Mixograph parameters, 17 for RVA parameters and 55 for Mixolab parameters were new. Eleven QTL clusters were identified. The closely linked SNP markers can be used in marker-assisted wheat breeding in combination with the Kompetitive Allele Specific PCR (KASP) technique for improvement of processing quality in bread wheat.