STARCH SYNTHASE 4 is required for normal starch granule initiation in amyloplasts of wheat endosperm

Pre-anthesis phosphorus sprays boost reproductive and seed yield in heat-stressed Phaseolus vulgaris L

Climate change intensifies heat waves, threatening global food security. Common bean (Phaseolus vulgaris L.) is highly sensitive to heat waves during flowering, which disrupts source-sink relationships, reduces pollen viability, and diminishes seed yield. Phosphorus (P) deficiency further limits carbohydrate transport to reproductive tissues during heat waves. This study evaluated the efficacy of leaf P spraying in mitigating heat wave effects in common bean plants facing P deficiency (PD) and P sufficiency (PS) during flowering, i.e., phenological phases V4 to R6. A complete factorial experiment evaluated two substrate P availability during flowering (PD and PS), two leaf P spraying treatments applied during the pre-anthesis (without P spraying, -P; and with P spraying, +P), and two air temperature regimes during flowering: control (28/18 °C, day/night) and HW (38/28 °C, day/night). Preventive leaf P spraying improved photochemical efficiency, reduced oxidative stress, and avoided decreases in photosynthetic pigment content under heat wave. P-sprayed plants exhibited greater starch deposition in pollen grains and higher soluble carbohydrate concentration, showing less cellular damage caused by heat wave. As a result, leaf P spraying also increased seed yield under heat wave by sustaining pollen viability owing to improved supply of soluble carbohydrates to reproductive structures. These findings underscore the critical role of leaf P spraying in alleviating heat wave and impaired P acquisition in common bean during flowering by enhancing reproductive resilience and yield stability. This study highlights the potential of such a strategy of preventive leaf P spraying at critical phenological phases to improve crop performance under heat wave conditions caused by climate change.

An alternative pathway to starch granule initiation unraveled in Chlamydomonas reinhardtii

The initiation of starch granule synthesis remains a relatively poorly understood phenomenon. Recent advances enabled the establishment of a model explaining the synthesis of new starch granules in Arabidopsis thaliana. These characterization studies revealed the involvement of both a specific starch synthase isoform (SS4) and several nonenzymatic proteins in this process. In this work, we investigated whether the initiation of starch synthesis in the green microalgae Chlamydomonas reinhardtii involves the same machinery as those uncovered in the plant model. Our extensive phylogenetic analysis revealed that most of the key players that were identified in higher plants are not found in microalgae, suggesting that a different pathway is taking place. We showed that restoration of ADP-glucose synthesis in a mutant strain devoid of any endogenous primers allowed normal starch synthesis, revealing the existence of an initiation mechanism in Chlamydomonas. Our biochemical characterization studies revealed that starch synthase isoform 3 possesses the intrinsic capacity to initiate polysaccharide synthesis in vitro and could be one of the functions involved in starch initiation. Our work suggests that the initiation of starch synthesis in Chlamydomonas involves a different pathway from that described in Arabidopsis and that further efforts will be required to identify the proteins involved in this process.

Integrative transcriptogenomic analyses reveal the regulatory network underlying rice eating and cooking quality and identify a role for alpha-globulin in modulating starch and sucrose metabolism

Rice eating and cooking quality (ECQ) is significantly influenced by the physicochemical properties of rice starch. This study integrates whole-genome resequencing, transcriptomic data, and phenotypic analysis to identify the genetic factors that regulate transcript expression levels and contribute to phenotypic variation in rice ECQ traits. A TWAS (transcriptome-wide association study) identified 285 transcripts linked to 6 ECQ traits. Genome-wide mapping of these transcripts revealed 21 747 local eQTLs (expression quantitative trait loci) and 45 158 distal eQTLs. TWAS and eQTL analysis detected several known and novel genes, including starch synthesis-related genes, heat shock proteins, transcription factors, genes related to ATP accumulation, and UDP-glucosyltransferases, showcasing the complex genetic regulation of rice ECQ. WGCNA (weighted gene co-expression network analysis) uncovered key co-expression networks, including a module that links alpha-globulin1 (GLB1) to starch and sucrose metabolism. Genetic diversity analysis of the GLB1 gene across a Korean rice collection identified 26 haplotypes, with indica and aus forming 7 and 3 haplotypes, respectively, which showed significant phenotypic effects on ECQ traits. CRISPR-Cas9-created knockout lines validated these findings, demonstrating that loss of GLB1 function caused significant changes in seed storage proteins, reduced amylose content, altered starch granules, and modified pasting properties without affecting plant phenotypes. By integrating TWAS, eQTL mapping, haplotype analysis, gene expression networks, and CRISPR validation, this study establishes GLB1 as a regulator of ECQ, linking starch biosynthesis and protein accumulation pathways. This transcriptogenomic convergence approach provides novel insights into the genetic regulation of ECQ in rice, demonstrating its effectiveness for characterizing complex traits and enabling precision breeding.

Challenges of translating Arabidopsis insights into crops

The significance of research conducted on Arabidopsis thaliana cannot be overstated. This focus issue showcases how insights from Arabidopsis have opened new areas of biology and directly advanced our understanding of crops. Here, experts intimately involved in bridging between Arabidopsis and crops share their perspectives on the challenges and opportunities for translation. First, we examine the translatability of genetic modules from Arabidopsis into maize, emphasizing the need to publish well-executed translational experiments, regardless of outcome. Second, we highlight the landmark success of HB4, the first GM wheat cultivar on the market, whose abiotic tolerance is borne from direct translation and based on strategies first outlined in Arabidopsis. Third, we discuss the decades-long journey to engineer oilseed crops capable of producing omega-3 fish oils, with Arabidopsis serving as a critical intermediary. Fourth, we explore how direct translation of starch synthesizing proteins characterized in Arabidopsis helped uncover novel mechanisms and functions in crops, with potential valuable applications. Finally, we illustrate how shared molecular factors between Arabidopsis and barley exhibit distinct molecular wiring as exemplified in cuticular and stomatal development. Together, these vignettes underscore the pivotal role of Arabidopsis as a foundational model plant while highlighting the challenges of translating discoveries into field-ready, commercial cultivars with enhanced knowledge-based traits.

Enhancing quality traits in staple crops: current advances and future perspectives

Staple crops such as rice, wheat and maize are crucial for global food security; however, improving their quality remains a significant challenge. This review summarizes recent advances in enhancing crop quality, focusing on key areas such as the molecular mechanisms underlying endosperm filling initiation, starch granule synthesis, protein body formation, and the interactions between carbon and nitrogen metabolism. It also highlights ten unresolved questions related to starch-protein spatial distribution, epigenetic regulation, and the environmental impacts on quality traits. The integration of multi-omics approaches, and rational design strategies presents opportunities to develop high-yield "super-crop" varieties with enhanced nutritional value, better processing characteristics, and attributes preferred by consumers. Addressing these challenges is crucial to promote sustainable agriculture and achieve the dual objectives of food security and environmental conservation.

Roles of sugar metabolism and transport in flower development

Flowers, which are primarily heterotrophic, thrive on photosynthates transported to the floral receptacle through the phloem. Once phloem unloading occurs, carbohydrates are distributed to various flower organs to support growth and development. This brief review summarizes how flowers acquire carbohydrates and transport them to different organs and tissues through the coordinated actions of transporters and enzymes, as well as the developmental issues that arise from carbohydrate imbalances. It will also discuss recently discovered transcription factors that regulate carbohydrate utilization in anthers and pistils. Additionally, the review provides an overview of the role of sugars as signaling molecules regulating floral organ development and the interaction between sugars and hormones.

Branched oligosaccharides cause atypical starch granule initiation in Arabidopsis chloroplasts

Plant chloroplasts store starch during the day, which acts as a source of carbohydrates and energy at night. Starch granule initiation relies on the elongation of malto-oligosaccharide primers. In Arabidopsis thaliana, PROTEIN TARGETING TO STARCH 2 (PTST2) and STARCH SYNTHASE 4 (SS4) are essential for the selective binding and elongation of malto-oligosaccharide primers, respectively, and very few granules are initiated in their absence. However, the precise origin and metabolism of the primers remain unknown. Potential origins of malto-oligosaccharide primers include de novo biosynthesis or their release from existing starch granules. For example, the endoamylase α-AMYLASE 3 (AMY3) can cleave a range of malto-oligosaccharides from the granule surface during starch degradation at night, some of which are branched. In the Arabidopsis double mutant deficient in the two debranching enzymes ISOAMYLASE 3 (ISA3) and LIMIT DEXTRINASE (LDA), branched malto-oligosaccharides accumulate in the chloroplast stroma. Here, we reveal that the isa3 lda double mutant shows a substantial increase in granule number per chloroplast, caused by these branched malto-oligosaccharides. The amy3 isa3 lda triple mutant, which lacks branched malto-oligosaccharides, has far fewer granules than isa3 lda, and its granule numbers are barely higher than in the wild type. Plants lacking both ISA3 and LDA and either PTST2 or SS4 show granule over-initiation, indicating that this process occurs independently of the recently described granule initiation pathway. Our findings provide insight into how and where starch granules are initiated. This knowledge can be used to alter granule number and morphological characteristics, traits known to affect starch properties.

Wheat2035: Integrating pan-omics and advanced biotechnology for future wheat design

Wheat (Triticum aestivum) production is vital for global food security, providing energy and protein to millions of people worldwide. Recent advancements in wheat research have led to significant increases in production, fueled by technological and scientific innovation. Here, we summarize the major advancements in wheat research, particularly the integration of biotechnologies and a deeper understanding of wheat biology. The shift from multi-omics to pan-omics approaches in wheat research has greatly enhanced our understanding of the complex genome, genomic variations, and regulatory networks to decode complex traits. We also outline key scientific questions, potential research directions, and technological strategies for improving wheat over the next decade. Since global wheat production is expected to increase by 60% in 2050, continued innovation and collaboration are crucial. Integrating biotechnologies and a deeper understanding of wheat biology will be essential for addressing future challenges in wheat production, ensuring sustainable practices and improved productivity.

Effects of exogenous brassinosteroids on the starch structure, physicochemical properties and digestibility of wheat under high-temperature stress at the early grain-filling stage

The starch synthesis of wheat was seriously affected by high temperature stress, which affected its yield and quality. At the same time, plant hormones are often used to regulate crop production and mitigate environmental stress in times of adversity. However, whether Brassinosteroids (BRs) can alleviate the adverse effects of high-temperature stress on wheat starch and its internal mechanism are still unclear. Therefore, a two-year field experiment was carried out from 2020 to 2022 to explore the effects of exogenous BRs spraying on the wheat yield, morphological structure and physicochemical properties of starch under high-temperature stress at the early grain-filling stage. The results showed that exogenous spraying of BRs under high-temperature stress increased the activity of key enzymes involved in starch synthesis, the contents of amylose (25.34 %) and amylopectin (11.57 %) were increased on average in two years, and ultimately increased the yield. On the other hand, BRs increased the number of B-type (49.90 %) and C-type starch granules (23.19 %), decreased the number of A-type starch granules (6.38 %) under high-temperature stress, and reduced the chain length of B1 (5.04 %) and B2 (4.88 %) chains of amylopectin. Moreover, BRs decreased crystallinity (25.40 %), gelatinization enthalpy (4.76 %) and 1047/1022 cm-1 (1.25 %), thereby improving the cooking quality of the starch. Through further analysis of the rheological properties and digestibility of starch, BRs improved the processing quality of starch and increased the content of resistant starch (RS, 15.77 %), which provided insights for the processing and utilization of food. In summary, under high-temperature stress, exogenous BRs increased wheat yield and improved starch quality. We recommend BRs as an effective management measure to mitigate high-temperature stress to cope with future climate change.

Wheat MYOSIN-RESEMBLING CHLOROPLAST PROTEIN controls B-type starch granule initiation timing during endosperm development

Molecular factors that contribute to the diverse spatial and temporal patterns of starch granule initiation between species and organs are poorly understood. Wheat (Triticum sp.) endosperm contains both large A-type granules initiated during early grain development and small B-type granules that initiate about 10 to 15 days later. Here, we identify that the MYOSIN-RESEMBLING CHLOROPLAST PROTEIN (MRC) is required for the correct timing of B-type granule initiation in wheat endosperm during grain development. MRC is expressed in the endosperm exclusively in early grain development, before B-type granule initiation. We isolated three independent TILLING mutants of tetraploid wheat (Triticum turgidum cv. 'Kronos') with premature stop or missense mutations in the A-genome homeolog, which we showed to be the only active homeolog in tetraploid wheat due to a disruption of the B-genome homeolog. The mrc mutants had significantly smaller A-type granules and a higher relative volume of B-type granules in the endosperm than the wild type. Whereas B-type granules initiated 15 to 20 days post-anthesis (dpa) in the wild type, they appeared as early as 10 dpa in the mrc-1 mutant. These results suggest a temporal role for MRC in repressing B-type granule initiation, providing insight into how the distinct biochemical mechanisms that control A- and B-type granule initiation are regulated. This role of MRC in the wheat endosperm is distinct from the previously described role of Arabidopsis (Arabidopsis thaliana) MRC in promoting granule initiation in leaves, providing an example of functional diversification among granule initiation proteins.

Nuclear factor-Y-polycomb repressive complex2 dynamically orchestrates starch and seed storage protein biosynthesis in wheat

The endosperm in cereal grains is instrumental in determining grain yield and seed quality, as it controls starch and seed storage protein (SSP) production. In this study, we identified a specific nuclear factor-Y (NF-Y) trimeric complex in wheat (Triticum aestivum L.), consisting of TaNF-YA3-D, TaNF-YB7-B, and TaNF-YC6-B, and exhibiting robust expression within the endosperm during grain filling. Knockdown of either TaNF-YA3 or TaNF-YC6 led to reduced starch but increased gluten protein levels. TaNF-Y indirectly boosted starch biosynthesis genes by repressing TaNAC019, a repressor of cytosolic small ADP-glucose pyrophosphorylase 1a (TacAGPS1a), sucrose synthase 2 (TaSuS2), and other genes involved in starch biosynthesis. Conversely, TaNF-Y directly inhibited the expression of Gliadin-γ-700 (TaGli-γ-700) and low molecular weight-400 (TaLMW-400). Furthermore, TaNF-Y components interacted with SWINGER (TaSWN), the histone methyltransferase subunit of Polycomb repressive complex 2 (PRC2), to repress TaNAC019, TaGli-γ-700, and TaLMW-400 expression through trimethylation of histone H3 at lysine 27 (H3K27me3) modifications. Notably, weak mutation of FERTILIZATION INDEPENDENT ENDOSPERM (TaFIE), a core PRC2 subunit, reduced starch but elevated gliadin and LMW-GS contents. Intriguingly, sequence variation within the TaNF-YB7-B coding region was linked to differences in starch and SSP content. Distinct TaNF-YB7-B haplotypes affect its interaction with TaSWN-B, influencing the repression of targets like TaNAC019 and TaGli-γ-700. Our findings illuminate the intricate molecular mechanisms governing TaNF-Y-PRC2-mediated epigenetic regulation for wheat endosperm development. Manipulating the TaNF-Y complex holds potential for optimizing grain yield and enhancing grain quality.

Exploring the synergy of enzymes, nutrients, and gene networks in rice starch granule biogenesis

Introduction

Rice is a primary food source almost for more than 50% of the total world's population. Glycemic index (GI) is high in most of the rice varieties, limiting their consumption by diabetic and obese people. As a result, developing new rice varieties with low GI necessitates a thorough understanding of starch biogenesis gene expression and its interrelationship.

Methods

A total 200 rice genotypes were analyzed for total starch content (TSC), amylopectin content (APC), and amylose content (AC). The clustering of these rice genotypes was done based on their AC. Further, these genotypes were categorized into three groups up to 10% amylose-low, 10-26% amylose-medium, and more than 26% amylose-high. Among them, six genotypes 1 from low AC (NJ-72), 2 from medium AC (UPRI-2003-18, PRR-126), and 3 from high AC (RNRM-7, Urvashi and Ananga) were selected. The genotypes selected from the medium and high AC groups were having 2% amylose variation among themselves respectively and they were further used to study the level of RS, protein content (PC), fatty acid (FA) profiles, and granule morphology along with low group sample.

Results

Resistant starch (RS) content ranged from 0.33-2.75%, and fatty acid profiling revealed high levels of palmitic, linoleic, and oleic acids. The degree of crystallinity and APC% were found to be positively correlated. Ananga, the genotype with the highest RS, displayed compact starch granules. Further, NJ-72 showing low RS and Ananga with high RS were selected for investigation of enzymatic activities of starch biosynthesis, metabolites accumulation, and expressions of 20 starch biogenesis genes in developing endosperm. Starch branching enzymes (SBE) and starch synthase (SS) activities peaked at 13 days after anthesis (DAA), while starch debranching enzymes (DBE) were most active at 18 DAA. In Ananga, TSC, AC, APC, and RS levels progressively increased from 3 to 23 DAA. Ananga showed 1.25-fold upregulation of granule-bound starch synthase I (GBSSI) at 18DAA. Higher expressions of SSI and SBEIIb were observed in NJ-72 at 13DAA. PUL2 was predominantly expressed followed by ISA1. GBSSI was positively correlated with both AC and RS while SS, SBE, and DBE were positively related to APC.

Conclusion

This research could lead to the development of rice varieties with improved nutritional qualities, such as higher RS content, which is beneficial for human health due to its role in lowering glycemic response and promoting gut health. Additionally, the study provides insights into how the modulation of key genes and enzymes can affect starch composition, offering strategies to breed rice varieties tailored for specific dietary needs or industrial applications.

Mutations in starch BRANCHING ENZYME 2a suppress the traits caused by the loss of ISOAMYLASE1 in barley

KEY MESSAGE

The hvbe2a mutations restore the starch-deficient phenotype caused by the hvisa1 and hvflo6 mutations in barley endosperm. The genetic interactions among starch biosynthesis genes can be exploited to alter starch properties, but they remain poorly understood due to the various combinations of mutations to be tested. Here, we isolated two novel barley mutants defective in starch BRANCHING ENZYME 2a (hvbe2a-1 and hvbe2a-2) based on the starch granule (SG) morphology. Both hvbe2a mutants showed elongated SGs in the endosperm and increased resistant starch content. hvbe2a-1 had a base change in HvBE2a gene, substituting the amino acid essential for its enzyme activity, while hvbe2a-2 is completely missing HvBE2a due to a chromosomal deletion. Further genetic crosses with barley isoamylase1 mutants (hvisa1) revealed that both hvbe2a mutations could suppress defects in endosperm caused by hvisa1, such as reduction in starch, increase in phytoglycogen, and changes in the glucan chain length distribution. Remarkably, hvbe2a mutations also transformed the endosperm SG morphology from the compound SG caused by hvisa1 to bimodal simple SGs, resembling that of wild-type barley. The suppressive impact was in competition with floury endosperm 6 mutation (hvflo6), which could enhance the phenotype of hvisa1 in the endosperm. In contrast, the compound SG formation induced by the hvflo6 hvisa1 mutation in pollen was not suppressed by hvbe2a mutations. Our findings provide new insights into genetic interactions in the starch biosynthetic pathway, demonstrating how specific genetic alterations can influence starch properties and SG morphology, with potential applications in cereal breeding for desired starch properties.

The miR159a-DUO1 module regulates pollen development by modulating auxin biosynthesis and starch metabolism in citrus

Achieving seedlessness in citrus varieties is one of the important objectives of citrus breeding. Male sterility associated with abnormal pollen development is an important factor in seedlessness. However, our understanding of the regulatory mechanism underlying the seedlessness phenotype in citrus is still limited. Here, we determined that the miR159a-DUO1 module played an important role in regulating pollen development in citrus, which further indirectly modulated seed development and fruit size. Both the overexpression of csi-miR159a and the knocking out of DUO1 in Hong Kong kumquat (Fortunella hindsii) resulted in small and seedless fruit phenotypes. Moreover, pollen was severely aborted in both transgenic lines, with arrested pollen mitotic I and abnormal pollen starch metabolism. Through additional cross-pollination experiments, DUO1 was proven to be the key target gene for miR159a to regulate male sterility in citrus. Based on DNA affinity purification sequencing (DAP-seq), RNA-seq, and verified interaction assays, YUC2/YUC6, SS4 and STP8 were identified as downstream target genes of DUO1, those were all positively regulated by DUO1. In transgenic F. hindsii lines, the miR159a-DUO1 module down-regulated the expression of YUC2/YUC6, which decreased indoleacetic acid (IAA) levels and modulated auxin signaling to repress pollen mitotic I. The miR159a-DUO1 module reduced the expression of the starch synthesis gene SS4 and sugar transport gene STP8 to disrupt starch metabolism in pollen. Overall, this work reveals a new mechanism by which the miR159a-DUO1 module regulates pollen development and elucidates the molecular regulatory network underlying male sterility in citrus.

Creating a zero amylose barley with high soluble sugar content by genome editing

Amylose biosynthesis is strictly associated with granule-bound starch synthase I (GBSSI) encoded by the Waxy gene. Mutagenesis of single bases in the Waxy gene, which induced by CRISPR/Cas9 genome editing, caused absence of intact GBSSI protein in grain of the edited line. The amylose and amylopectin contents of waxy mutants were zero and 31.73%, while those in the wild type were 33.50% and 39.00%, respectively. The absence of GBSSI protein led to increase in soluble sugar content to 37.30% compared with only 10.0% in the wild type. Sucrose and β-glucan, were 39.16% and 35.40% higher in waxy mutants than in the wild type, respectively. Transcriptome analysis identified differences between the wild type and waxy mutants that could partly explain the reduction in amylose and amylopectin contents and the increase in soluble sugar, sucrose and β-glucan contents. This waxy flour, which showed lower final viscosity and setback, and higher breakdown, could provide more option for food processing.

Initiation of B-type starch granules in wheat endosperm requires the plastidial α-glucan phosphorylase PHS1

The plastidial α-glucan phosphorylase (PHS1) can elongate and degrade maltooligosaccharides (MOSs), but its exact physiological role in plants is poorly understood. Here, we discover a specialized role of PHS1 in establishing the unique bimodal characteristic of starch granules in wheat (Triticum spp.) endosperm. Wheat endosperm contains large A-type granules that initiate at early grain development and small B-type granules that initiate in later grain development. We demonstrate that PHS1 interacts with B-GRANULE CONTENT1 (BGC1), a carbohydrate-binding protein essential for normal B-type granule initiation. Mutants of tetraploid durum wheat (Triticum turgidum) deficient in all homoeologs of PHS1 had normal A-type granules but fewer and larger B-type granules. Grain size and starch content were not affected by the mutations. Further, by assessing granule numbers during grain development in the phs1 mutant and using a double mutant defective in both PHS1 and BGC1, we demonstrate that PHS1 is exclusively involved in B-type granule initiation. The total starch content and number of starch granules per chloroplast in leaves were not affected by loss of PHS1, suggesting that its role in granule initiation in wheat is limited to the endosperm. We therefore propose that the initiation of A- and B-type granules occurs via distinct biochemical mechanisms, where PHS1 plays an exclusive role in B-type granule initiation.

Increasing amyloplast size in wheat endosperm through mutation of PARC6 affects starch granule morphology

The determination of starch granule morphology in plants is poorly understood. The amyloplasts of wheat endosperm contain large discoid A-type granules and small spherical B-type granules. To study the influence of amyloplast structure on these distinct morphological types, we isolated a mutant in durum wheat (Triticum turgidum) defective in the plastid division protein PARC6, which had giant plastids in both leaves and endosperm. Endosperm amyloplasts of the mutant contained more A- and B-type granules than those of the wild-type. The mutant had increased A- and B-type granule size in mature grains, and its A-type granules had a highly aberrant, lobed surface. This morphological defect was already evident at early stages of grain development and occurred without alterations in polymer structure and composition. Plant growth and grain size, number and starch content were not affected in the mutants despite the large plastid size. Interestingly, mutation of the PARC6 paralog, ARC6, did not increase plastid or starch granule size. We suggest TtPARC6 can complement disrupted TtARC6 function by interacting with PDV2, the outer plastid envelope protein that typically interacts with ARC6 to promote plastid division. We therefore reveal an important role of amyloplast structure in starch granule morphogenesis in wheat.

Soluble and insoluble α-glucan synthesis in yeast by enzyme suites derived exclusively from maize endosperm

Molecular mechanisms that distinguish the synthesis of semi-crystalline α-glucan polymers found in plant starch granules from the synthesis of water-soluble polymers by nonplant species are not well understood. To address this, starch biosynthetic enzymes from maize (Zea mays L.) endosperm were isolated in a reconstituted environment using yeast (Saccharomyces cerevisiae) as a test bed. Ninety strains were constructed containing unique combinations of 11 synthetic transcription units specifying maize starch synthase (SS), starch phosphorylase (PHO), starch branching enzyme (SBE), or isoamylase-type starch debranching enzyme (ISA). Soluble and insoluble branched α-glucans accumulated in varying proportions depending on the enzyme suite, with ISA function stimulating distribution into the insoluble form. Among the SS isoforms, SSIIa, SSIII, and SSIV individually supported the accumulation of glucan polymer. Neither SSI nor SSV alone produced polymers; however, synergistic effects demonstrated that both isoforms can stimulate α-glucan accumulation. PHO did not support α-glucan production by itself, but it had either positive or negative effects on polymer content depending on which SS or a combination thereof was present. The complete suite of maize enzymes generated insoluble particles resembling native starch granules in size, shape, and crystallinity. Ultrastructural analysis revealed a hierarchical assembly starting with subparticles of approximately 50 nm diameter that coalesce into discrete structures of approximately 200 nm diameter. These are assembled into semi-crystalline α-glucan superstructures up to 4 μm in length filling most of the yeast cytosol. ISA was not essential for the formation of such particles, but their abundance was increased dramatically by ISA presence.

Starch granules and their size distribution in wheat: Biosynthesis, physicochemical properties and their effect on flour-based food systems

Starch is a vital component of wheat grain and flour, characterized by two distinct granule types: A-type starch (AS) with granules larger than 10 µm in diameter, and B-type starch (BS) with granules measuring no more than 10 µm in diameter. This review comprehensively evaluates the isolation, purification, and biosynthesis processes of these types of granules. In addition, a comparative analysis of the structure and properties of AS and BS is presented, encompassing chemical composition, molecular, crystalline and morphological structures, gelatinization, pasting and digestive properties. The variation in size distribution of granules leads to differences in physicochemical properties of starch, influencing the formation of polymeric proteins, secondary and micro-structures of gluten, chemical and physical interactions between gluten and starch, and water absorption and water status in dough system. Thus, starch size distribution affects the quality of dough and final products. In this review, we summarize the up-to-date knowledge of AS and BS, and propose the possible strategies to enhance wheat yield and quality through coordinated breeding efforts. This review serves as a valuable reference for future advancements in wheat breeding.

Is it the end of TILLING era in plant science?

Since its introduction in 2000, the TILLING strategy has been widely used in plant research to create novel genetic diversity. TILLING is based on chemical or physical mutagenesis followed by the rapid identification of mutations within genes of interest. TILLING mutants may be used for functional analysis of genes and being nontransgenic, they may be directly used in pre-breeding programs. Nevertheless, classical mutagenesis is a random process, giving rise to mutations all over the genome. Therefore TILLING mutants carry background mutations, some of which may affect the phenotype and should be eliminated, which is often time-consuming. Recently, new strategies of targeted genome editing, including CRISPR/Cas9-based methods, have been developed and optimized for many plant species. These methods precisely target only genes of interest and produce very few off-targets. Thus, the question arises: is it the end of TILLING era in plant studies? In this review, we recap the basics of the TILLING strategy, summarize the current status of plant TILLING research and present recent TILLING achievements. Based on these reports, we conclude that TILLING still plays an important role in plant research as a valuable tool for generating genetic variation for genomics and breeding projects.

Gene expression profile of the developing endosperm in durum wheat provides insight into starch biosynthesis

BACKGROUND

Durum wheat (Triticum turgidum subsp. durum) is widely grown for pasta production, and more recently, is gaining additional interest due to its resilience to warm, dry climates and its use as an experimental model for wheat research. Like in bread wheat, the starch and protein accumulated in the endosperm during grain development are the primary contributors to the calorific value of durum grains.

RESULTS

To enable further research into endosperm development and storage reserve synthesis, we generated a high-quality transcriptomics dataset from developing endosperms of variety Kronos, to complement the extensive mutant resources available for this variety. Endosperms were dissected from grains harvested at eight timepoints during grain development (6 to 30 days post anthesis (dpa)), then RNA sequencing was used to profile the transcriptome at each stage. The largest changes in gene expression profile were observed between the earlier timepoints, prior to 15 dpa. We detected a total of 29,925 genes that were significantly differentially expressed between at least two timepoints, and clustering analysis revealed nine distinct expression patterns. We demonstrate the potential of our dataset to provide new insights into key processes that occur during endosperm development, using starch metabolism as an example.

CONCLUSION

We provide a valuable resource for studying endosperm development in this increasingly important crop species.

FLOURY ENDOSPERM 6 mutations enhance the sugary phenotype caused by the loss of ISOAMYLASE1 in barley

Barley double mutants in two genes involved in starch granule morphology, HvFLO6 and HvISA1, had impaired starch accumulation and higher grain sugar levels than either single mutant. Starch is a biologically and commercially important glucose polymer synthesized by plants as semicrystalline starch granules (SGs). Because SG morphology affects starch properties, mutants with altered SG morphology may be useful in breeding crops with desirable starch properties, including potentially novel properties. In this study, we employed a simple screen for mutants with altered SG morphology in barley (Hordeum vulgare). We isolated mutants that formed compound SGs together with the normal simple SGs in the endosperm and found that they were allelic mutants of the starch biosynthesis genes ISOAMYLASE1 (HvISA1) and FLOURY ENDOSPERM 6 (HvFLO6), encoding starch debranching enzyme and CARBOHYDRATE-BINDING MODULE 48-containing protein, respectively. We generated the hvflo6 hvisa1 double mutant and showed that it had significantly reduced starch biosynthesis and developed shrunken grains. In contrast to starch, soluble α-glucan, phytoglycogen, and sugars accumulated to higher levels in the double mutant than in the single mutants. In addition, the double mutants showed defects in SG morphology in the endosperm and in the pollen. This novel genetic interaction suggests that hvflo6 acts as an enhancer of the sugary phenotype caused by hvisa1 mutation.

Characterization of Oil Body and Starch Granule Dynamics in Developing Seeds of Brassica napus

Brassica napus is the most important oilseed crop in the world, and the lipid was stored in the oil body (OB) in the form of triacylglycerol. At present, most of studies on the relationship between oil body morphology and seed oil content in B. napus was focused on mature seeds. In the present study, the OBs in different developing seeds of B. napus with relatively high oil content (HOC) of about 50% and low oil content (LOC) of about 39% were analyzed. It was revealed that the size of OBs was first increased and then decreased in both materials. And in late seed developmental stages, the average OB size of rapeseed with HOC was higher than that of LOC, while it was reversed in the early seed developmental stages. No significant difference was observed on starch granule (SG) size in HOC and LOC rapeseed. Further results indicated that the expression of genes that involved in malonyl-CoA metabolism, fatty acid carbon chain extension, lipid metabolism, and starch synthesis in the rapeseed with HOC was higher than that of rapeseed with LOC. These results give some new insight for understanding the dynamics of OBs and SGs in embryos of B. napus.

Starch parameters and short-term temperature fluctuations - Important but not yet in focus?

Plants are regularly challenged by unfavorable environmental conditions. As climate change continues, adverse situations such as drought, heat, and cold are expected to increase and become more severe. Most starchy crops are affected by such stresses. In recent years, researchers have made many new discoveries about starch metabolism in general and also on granule structure, including effects on starch following longer-term temperature stresses. However, in this study, we focus on short-term temperature stress on storage starch granule properties. Here our knowledge is less and it is likely that also short-term temperature stresses can affect various starch parameters. Therefore, we see a need for this type of analysis and discuss the matter in more detail and we conclude that a deeper knowledge particularly of starch granule parameters could allow targeted breeding of cultivars that exhibit different starch characteristics as a result of short-term stress. For these reasons, we are convinced that more comprehensive research on the effects of short-term temperature stress on starch granule characteristics is important, necessary, and timely.

Loss of PROTEIN TARGETING TO STARCH 2 has variable effects on starch synthesis across organs and species

Recent work has identified several proteins involved in starch granule initiation, the first step of starch synthesis. However, the degree of conservation in the granule initiation process remains poorly understood, especially among grass species differing in patterns of carbohydrate turnover in leaves, and granule morphology in the endosperm. We therefore compared mutant phenotypes of Hordeum vulgare (barley), Triticum turgidum (durum wheat), and Brachypodium distachyon defective in PROTEIN TARGETING TO STARCH 2 (PTST2), a key granule initiation protein. We report striking differences across species and organs. Loss of PTST2 from leaves resulted in fewer, larger starch granules per chloroplast and normal starch content in wheat, fewer granules per chloroplast and lower starch content in barley, and almost complete loss of starch in Brachypodium. The loss of starch in Brachypodium leaves was accompanied by high levels of ADP-glucose and detrimental effects on growth and physiology. Additionally, we found that loss of PTST2 increased granule initiation in Brachypodium amyloplasts, resulting in abnormal compound granule formation throughout the seed. These findings suggest that the importance of PTST2 varies greatly with the genetic and developmental background and inform the extent to which the gene can be targeted to improve starch in crops.

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.

Wheat genomic study for genetic improvement of traits in China

Bread wheat (Triticum aestivum L.) is a major crop that feeds 40% of the world's population. Over the past several decades, advances in genomics have led to tremendous achievements in understanding the origin and domestication of wheat, and the genetic basis of agronomically important traits, which promote the breeding of elite varieties. In this review, we focus on progress that has been made in genomic research and genetic improvement of traits such as grain yield, end-use traits, flowering regulation, nutrient use efficiency, and biotic and abiotic stress responses, and various breeding strategies that contributed mainly by Chinese scientists. Functional genomic research in wheat is entering a new era with the availability of multiple reference wheat genome assemblies and the development of cutting-edge technologies such as precise genome editing tools, high-throughput phenotyping platforms, sequencing-based cloning strategies, high-efficiency genetic transformation systems, and speed-breeding facilities. These insights will further extend our understanding of the molecular mechanisms and regulatory networks underlying agronomic traits and facilitate the breeding process, ultimately contributing to more sustainable agriculture in China and throughout the world.

Starch synthases SSIIa and GBSSI control starch structure but do not determine starch granule morphology in the absence of SSIIIa and SSIVb

High levels of two major starch synthases, SSIIa and GBSSI, in ss3a ss4b double mutant rice alter the starch structure but fail to recover the polygonal starch granule morphology. The endosperm starch granule is polygonal in wild-type rice but spherical in double mutant japonica rice lacking genes encoding two of the five major Starch synthase (SS) isozymes expressed in endosperm, SSIIIa and SSIVb. Japonica rice naturally has low levels of SSIIa and Granule-bound SSI (GBSSI). Therefore, introduction of active SSIIa allele and/or high-expressing GBSSI allele from indica rice into the japonica rice mutant lacking SS isozymes can help elucidate the compensatory roles of SS isozymes in starch biosynthesis. In this study, we crossed the ss3a ss4a double mutant japonica rice with the indica rice to generate three new rice lines with high and/or low SSIIa and GBSSI levels, and examined their starch structure, physicochemical properties, and levels of other starch biosynthetic enzymes. Lines with high SSIIa levels showed more SSI and SSIIa bound to starch granule, reduced levels of short amylopectin chains (7 ≤ DP ≤ 12), increased levels of amylopectin chains with DP > 13, and consequently higher gelatinization temperature. Lines with high GBSSI levels showed an increase in amylose content. The ADP-glucose content of the crude extract was high in lines with low or high SSIIa and low GBSSI levels, but was low in lines with high GBSSI. Addition of high SSIIa and GBSSI altered the starch structure and physicochemical properties but did not affect the starch granule morphology, confirming that SSIIIa and SSIVb are key enzymes affecting starch granule morphology in rice. The relationship among SS isozymes and its effect on the amount of substrate (ADP-glucose) is discussed.

Starch biosynthesis in cereal endosperms: An updated review over the last decade

Starch is a vital energy source for living organisms and is a key raw material and additive in the food and non-food industries. Starch has received continuous attention in multiple research fields. The endosperm of cereals (e.g., rice, corn, wheat, and barley) is the most important site for the synthesis of storage starch. Around 2010, several excellent reviews summarized key progress in various fields of starch research, serving as important references for subsequent research. In the past 10 years, many achievements have been made in the study of starch synthesis and regulation in cereals. The present review provides an update on research progress in starch synthesis of cereal endosperms over the past decade, focusing on new enzymes and non-enzymatic proteins involved in starch synthesis, regulatory networks of starch synthesis, and the use of elite alleles of starch synthesis-related genes in cereal breeding programs. We also provide perspectives on future research directions that will further our understanding of cereal starch biosynthesis and regulation to support the rational design of ideal quality grain.

Starch granule initiation in Arabidopsis thaliana chloroplasts

The initiation of starch granule formation and the mechanism controlling the number of granules per plastid have been some of the most elusive aspects of starch metabolism. This review covers the advances made in the study of these processes. The analyses presented herein depict a scenario in which starch synthase isoform 4 (SS4) provides the elongating activity necessary for the initiation of starch granule formation. However, this protein does not act alone; other polypeptides are required for the initiation of an appropriate number of starch granules per chloroplast. The functions of this group of polypeptides include providing suitable substrates (maltooligosaccharides) to SS4, the localization of the starch initiation machinery to the thylakoid membranes, and facilitating the correct folding of SS4. The number of starch granules per chloroplast is tightly regulated and depends on the developmental stage of the leaves and their metabolic status. Plastidial phosphorylase (PHS1) and other enzymes play an essential role in this process since they are necessary for the synthesis of the substrates used by the initiation machinery. The mechanism of starch granule formation initiation in Arabidopsis seems to be generalizable to other plants and also to the synthesis of long-term storage starch. The latter, however, shows specific features due to the presence of more isoforms, the absence of constantly recurring starch synthesis and degradation, and the metabolic characteristics of the storage sink organs.