New starch phenotypes produced by TILLING in barley

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.

Mutations in starch biosynthesis genes affect chloroplast development in wheat pericarp

Starch bioengineering in cereals has produced a plethora of genotypes with new nutritional and technological functionalities. Modulation of amylose content from 0 to 100% was inversely correlated with starch digestibility and promoted a lower glycemic index in food products. In wheat, starch mutants have been reported to exhibit various side effects, mainly related to the seed phenotype. However, little is known about the impact of altered amylose content and starch structure on plant metabolism. Here, three bread wheat starch mutant lines with extreme phenotypes in starch branching and amylose content were used to study plant responses to starch structural changes. Omics profiling of gene expression and metabolic patterns supported changes, confirmed by ultrastructural analysis in the chloroplast of the immature seeds. In detail, the identification of differentially expressed genes belonging to functional categories related to photosynthesis, chloroplast and thylakoid (e.g. CURT1), the alteration in the accumulation of photosynthesis-related compounds, and the chloroplast alterations (aberrant shape, grana stacking alteration, and increased number of plastoglobules) suggested that the modification of starch structure greatly affects starch turnover in the chloroplast, triggering oxidative stress (ROS accumulation) and premature tissue senescence. In conclusion, this study highlighted a correlation between starch structure and chloroplast functionality in the wheat kernel.

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.

Green revolution to genome revolution: driving better resilient crops against environmental instability

Crop improvement programmes began with traditional breeding practices since the inception of agriculture. Farmers and plant breeders continue to use these strategies for crop improvement due to their broad application in modifying crop genetic compositions. Nonetheless, conventional breeding has significant downsides in regard to effort and time. Crop productivity seems to be hitting a plateau as a consequence of environmental issues and the scarcity of agricultural land. Therefore, continuous pursuit of advancement in crop improvement is essential. Recent technical innovations have resulted in a revolutionary shift in the pattern of breeding methods, leaning further towards molecular approaches. Among the promising approaches, marker-assisted selection, QTL mapping, omics-assisted breeding, genome-wide association studies and genome editing have lately gained prominence. Several governments have progressively relaxed their restrictions relating to genome editing. The present review highlights the evolutionary and revolutionary approaches that have been utilized for crop improvement in a bid to produce climate-resilient crops observing the consequence of climate change. Additionally, it will contribute to the comprehension of plant breeding succession so far. Investing in advanced sequencing technologies and bioinformatics will deepen our understanding of genetic variations and their functional implications, contributing to breakthroughs in crop improvement and biodiversity conservation.

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.

Detecting variation in starch granule size and morphology by high-throughput microscopy and flow cytometry

Starch forms semi-crystalline, water-insoluble granules, the size and morphology of which vary according to biological origin. These traits, together with polymer composition and structure, determine the physicochemical properties of starch. However, screening methods to identify differences in starch granule size and shape are lacking. Here, we present two approaches for high-throughput starch granule extraction and size determination using flow cytometry and automated, high-throughput light microscopy. We evaluated the practicality of both methods using starch from different species and tissues and demonstrated their effectiveness by screening for induced variation in starch extracted from over 10,000 barley lines, yielding four with heritable changes in the ratio of large A-granules to small B-granules. Analysis of Arabidopsis lines altered in starch biosynthesis further demonstrates the applicability of these approaches. Identifying variation in starch granule size and shape will enable identification of trait-controlling genes for developing crops with desired properties, and could help optimise starch processing.

FIND-IT: Accelerated trait development for a green evolution

Improved agricultural and industrial production organisms are required to meet the future global food demands and minimize the effects of climate change. A new resource for crop and microbe improvement, designated FIND-IT (Fast Identification of Nucleotide variants by droplet DigITal PCR), provides ultrafast identification and isolation of predetermined, targeted genetic variants in a screening cycle of less than 10 days. Using large-scale sample pooling in combination with droplet digital PCR (ddPCR) greatly increases the size of low-mutation density and screenable variant libraries and the probability of identifying the variant of interest. The method is validated by screening variant libraries totaling 500,000 barley (Hordeum vulgare) individuals and isolating more than 125 targeted barley gene knockout lines and miRNA or promoter variants enabling functional gene analysis. FIND-IT variants are directly applicable to elite breeding pipelines and minimize time-consuming technical steps to accelerate the evolution of germplasm.

Reverse genetic approaches for breeding nutrient-rich and climate-resilient cereal and food legume crops

In the last decade, advancements in genomics tools and techniques have led to the discovery of many genes. Most of these genes still need to be characterized for their associated function and therefore, such genes remain underutilized for breeding the next generation of improved crop varieties. The recent developments in different reverse genetic approaches have made it possible to identify the function of genes controlling nutritional, biochemical, and metabolic traits imparting drought, heat, cold, salinity tolerance as well as diseases and insect-pests. This article focuses on reviewing the current status and prospects of using reverse genetic approaches to breed nutrient-rich and climate resilient cereal and food legume crops.

Cereal Endosperms: Development and Storage Product Accumulation

The persistent triploid endosperms of cereal crops are the most important source of human food and animal feed. The development of cereal endosperms progresses through coenocytic nuclear division, cellularization, aleurone and starchy endosperm differentiation, and storage product accumulation. In the past few decades, the cell biological processes involved in endosperm formation in most cereals have been described. Molecular genetic studies performed in recent years led to the identification of the genes underlying endosperm differentiation, regulatory network governing storage product accumulation, and epigenetic mechanism underlying imprinted gene expression. In this article, we outline recent progress in this area and propose hypothetical models to illustrate machineries that control aleurone and starchy endosperm differentiation, sugar loading, and storage product accumulations. A future challenge in this area is to decipher the molecular mechanisms underlying coenocytic nuclear division, endosperm cellularization, and programmed cell death.

Starch structure and nutritional functionality - Past revelations and future prospects

Starch exists naturally as insoluble semi-crystalline granules assembled by amylose and amylopectin. Acknowledging the pioneers, we have reviewed the major accomplishments in the area of starch structure from the early 18th century and further established the relation of starch structure to nutritional functionality. Although a huge array of work is reported in the area, the review identified that some features of starch are still not fully understood and needs further elucidation. With the rise of diet-related diseases, it has never been more important to understand starch structure and use that knowledge to improve the nutritional value of the world's principal energy source.

Molecular Functions and Pathways of Plastidial Starch Phosphorylase (PHO1) in Starch Metabolism: Current and Future Perspectives

Starch phosphorylase is a member of the GT35-glycogen-phosphorylase superfamily. Glycogen phosphorylases have been researched in animals thoroughly when compared to plants. Genetic evidence signifies the integral role of plastidial starch phosphorylase (PHO1) in starch biosynthesis in model plants. The counterpart of PHO1 is PHO2, which specifically resides in cytosol and is reported to lack L80 peptide in the middle region of proteins as seen in animal and maltodextrin forms of phosphorylases. The function of this extra peptide varies among species and ranges from the substrate of proteasomes to modulate the degradation of PHO1 in Solanum tuberosum to a non-significant effect on biochemical activity in Oryza sativa and Hordeum vulgare. Various regulatory functions, e.g., phosphorylation, protein–protein interactions, and redox modulation, have been reported to affect the starch phosphorylase functions in higher plants. This review outlines the current findings on the regulation of starch phosphorylase genes and proteins with their possible role in the starch biosynthesis pathway. We highlight the gaps in present studies and elaborate on the molecular mechanisms of phosphorylase in starch metabolism. Moreover, we explore the possible role of PHO1 in crop improvement.

The Triple Jags of Dietary Fibers in Cereals: How Biotechnology Is for High

Cereals represent an important source of beneficial compounds for human health, such as macro- and micronutrients, vitamins, and bioactive molecules. Generally, the consumption of whole-grain products is associated with significant health benefits, due to the elevated amount of dietary fiber (DF). However, the consumption of whole-grain foods is still modest compared to more refined products. In this sense, it is worth focusing on the increase of DF fractions inside the inner compartment of the seed, the endosperm, which represents the main part of the derived flour. The main components of the grain fiber are arabinoxylan (AX), β-glucan (βG), and resistant starch (RS). These three components are differently distributed in grains, however, all of them are represented in the endosperm. AX and βG, classified as non-starch polysaccharides (NSP), are in cell walls, whereas, RS is in the endosperm, being a starch fraction. As the chemical structure of DFs influences their digestibility, the identification of key actors involved in their metabolism can pave the way to improve their function in human health. Here, we reviewed the main achievements of plant biotechnologies in DFs manipulation in cereals, highlighting new genetic targets to be exploited, and main issues to face to increase the potential of cereals in fighting malnutrition.

A Green Nanostructured Pesticide to Control Tomato Bacterial Speck Disease

Bacterial speck disease, caused by Pseudomonas syringae pv. tomato (Pst), is one of the most pervasive biological adversities in tomato cultivation, in both industrial and in table varieties. In this work synthesis, biochemical and antibacterial properties of a novel organic nanostructured pesticide composed of chitosan hydrochloride (CH) as active ingredient, cellulose nanocrystals (CNC) as nanocarriers and starch as excipient were evaluated. In order to study the possibility of delivering CH, the effects of two different types of starches, extracted from a high amylose bread wheat (high amylose starch-HA Starch) and from a control genotype (standard starch-St Starch), were investigated. Nanostructured microparticles (NMP) were obtained through the spray-drying technique, revealing a CH loading capacity proximal to 50%, with a CH release of 30% for CH-CNC-St Starch NMP and 50% for CH-CNC-HA Starch NMP after 24 h. Both NMP were able to inhibit bacterial growth in vitro when used at 1% w/v. Moreover, no negative effects on vegetative growth were recorded when NMP were foliar applied on tomato plants. Proposed nanostructured pesticides showed the capability of diminishing Pst epiphytical survival during time, decreasing disease incidence and severity (from 45% to 49%), with results comparable to one of the most used cupric salt (hydroxide), pointing out the potential use of CH-CNC-Starch NMP as a sustainable and innovative ally in Pst control strategies.

Molecular and genetic bases of heat stress responses in crop plants and breeding for increased resilience and productivity

To ensure the food security of future generations and to address the challenge of the 'no hunger zone' proposed by the FAO (Food and Agriculture Organization), crop production must be doubled by 2050, but environmental stresses are counteracting this goal. Heat stress in particular is affecting agricultural crops more frequently and more severely. Since the discovery of the physiological, molecular, and genetic bases of heat stress responses, cultivated plants have become the subject of intense research on how they may avoid or tolerate heat stress by either using natural genetic variation or creating new variation with DNA technologies, mutational breeding, or genome editing. This review reports current understanding of the genetic and molecular bases of heat stress in crops together with recent approaches to creating heat-tolerant varieties. Research is close to a breakthrough of global relevance, breeding plants fitter to face the biggest challenge of our time.

The dmc1 Mutant Allows an Insight Into the DNA Double-Strand Break Repair During Meiosis in Barley (Hordeum vulgare L.)

Meiosis is a process of essential importance for sexual reproduction, as it leads to production of gametes. The recombination event (crossing-over) generates genetic variation by introducing new combination of alleles. The first step of crossing-over is introduction of a targeted double-strand break (DSB) in DNA. DMC1 (Disrupted Meiotic cDNA1) is a recombinase that is specific only for cells undergoing meiosis and takes part in repair of such DSBs by searching and invading homologous sequences that are subsequently used as a template for the repair process. Although role of the DMC1 gene has been validated in Arabidopsis thaliana, a functional analysis of its homolog in barley, a crop species of significant importance in agriculture, has never been performed. Here, we describe the identification of barley mutants carrying substitutions in the HvDMC1 gene. We performed mutational screening using TILLING (Targeting Induced Local Lesions IN Genomes) strategy and the barley TILLING population, HorTILLUS, developed after double-treatment of spring barley cultivar 'Sebastian' with sodium azide and N-methyl-N-nitrosourea. One of the identified alleles, dmc1.c, was found independently in two different M₂ plants. The G2571A mutation identified in this allele leads to a substitution of the highly conserved amino acid (arginine-183 to lysine) in the DMC1 protein sequence. Two mutant lines carrying the same dmc1.c allele show similar disturbances during meiosis. The chromosomal aberrations included anaphase bridges and chromosome fragments in anaphase/telophase I and anaphase/telophase II, as well as micronuclei in tetrads. Moreover, atypical tetrads containing three or five cells were observed. A highly increased frequency of all chromosome aberrations during meiosis have been observed in the dmc1.c mutants compared to parental variety. The results indicated that DMC1 is required for the DSB repair, crossing-over and proper chromosome disjunction during meiosis in barley.

Starch granule initiation and morphogenesis-progress in Arabidopsis and cereals

Starch, the major storage carbohydrate in plants, is synthesized in plastids as semi-crystalline, insoluble granules. Many organs and cell types accumulate starch at some point during their development and maturation. The biosynthesis of the starch polymers, amylopectin and amylose, is relatively well understood and mostly conserved between organs and species. However, we are only beginning to understand the mechanism by which starch granules are initiated, and the factors that control the number of granules per plastid and the size/shape of granules. Here, we review recent progress in understanding starch granule initiation and morphogenesis. In Arabidopsis, granule initiation requires several newly discovered proteins with specific locations within the chloroplast, and also on the availability of maltooligosaccharides which act as primers for initiation. We also describe progress in understanding granule biogenesis in the endosperm of cereal grains-within which there is large interspecies variation in granule initiation patterns and morphology. Investigating whether this diversity results from differences between species in the functions of known proteins, and/or from the presence of novel, unidentified proteins, is a promising area of future research. Expanding our knowledge in these areas will lead to new strategies for improving the quality of cereal crops by modifying starch granule size and shape in vivo.

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.

Molecular characterisation of two novel starch granule proteins 1 in wild and cultivated diploid A genome wheat species

Starch synthase IIa, also known as starch granule protein 1 (SGP-1), plays a key role in amylopectin biosynthesis. The absence of SGP-1 in cereal grains is correlated to dramatic changes in the grains' starch content, structure, and composition. An extensive investigation of starch granule proteins in this study revealed a polymorphism in the electrophoretic mobility of SGP-1 between two species of wheat, Triticum urartu and T. monococcum; this protein was, however, conserved among all other Triticum species that share the A genome inherited from their progenitor T. urartu. Two different electrophoretic profiles were identified: SGP-A1 proteins of T. urartu accessions had a SDS-PAGE mobility similar to those of tetraploid and hexaploid wheat species; conversely, SGP-A1 proteins of T. monococcum ssp. monococcum and ssp. boeoticum accessions showed a different electrophoretic mobility. The entire coding region of the two genes was isolated and sequenced in an attempt to explain the polymorphism identified. Several single nucleotide polymorphisms (SNPs) responsible for amino acid changes were identified, but no indel polymorphism was observed to explain the difference in electrophoretic mobility. Amylose content did not differ significantly among T. urartu, T. monococcum ssp. boeoticum and T. monococcum ssp. monococcum, except in one accession of the ssp. boeoticum. Conversely, several interspecific differences were observed in viscosity properties (investigated as viscosity profiles using a rapid visco analyzer-RVA profiles) of these cereal grains. T. monococcum ssp. boeoticum accessions had the lowest RVA profiles, T. urartu accessions had an intermediate RVA profile, whereas T. monococcum ssp. monococcum showed the highest RVA profile. These differences could be associated with the numerous amino acid and structural changes evident among the SGP-1 proteins.

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.

Critical and speculative review of the roles of multi-protein complexes in starch biosynthesis in cereals

Starch accounts for the majority of edible carbohydrate resources generated through photosynthesis. Amylopectin is the major component of starch and is one of highest-molecular-weight biopolymers. Rapid and systematic synthesis of frequently branched hydro-insoluble amylopectin and efficient accumulation into amyloplasts of cereal endosperm is crucial. The functions of multiple starch biosynthetic enzymes, including elongation, branching, and debranching enzymes, must be temporally and spatially coordinated. Accordingly, direct evidence of protein-protein interactions of starch biosynthetic enzymes were first discovered in developing wheat endosperm in 2004, and they have since been shown in the developing seeds of other cereals. This review article describes structural characteristics of starches as well as similarities and differences in protein complex formation among different plant species and among mutant plants that are deficient in specific starch biosynthetic enzymes. In addition, evidence for protein complexes that are involved in the initiation stages of starch biosynthesis is summarized. Finally, we discuss the significance of protein complexes and describe new methods that may elucidate the mechanisms and roles of starch biosynthetic enzyme complexes.

Assessing and Exploiting Functional Diversity in Germplasm Pools to Enhance Abiotic Stress Adaptation and Yield in Cereals and Food Legumes

There is a need to accelerate crop improvement by introducing alleles conferring host plant resistance, abiotic stress adaptation, and high yield potential. Elite cultivars, landraces and wild relatives harbor useful genetic variation that needs to be more easily utilized in plant breeding. We review genome-wide approaches for assessing and identifying alleles associated with desirable agronomic traits in diverse germplasm pools of cereals and legumes. Major quantitative trait loci and single nucleotide polymorphisms (SNPs) associated with desirable agronomic traits have been deployed to enhance crop productivity and resilience. These include alleles associated with variation conferring enhanced photoperiod and flowering traits. Genetic variants in the florigen pathway can provide both environmental flexibility and improved yields. SNPs associated with length of growing season and tolerance to abiotic stresses (precipitation, high temperature) are valuable resources for accelerating breeding for drought-prone environments. Both genomic selection and genome editing can also harness allelic diversity and increase productivity by improving multiple traits, including phenology, plant architecture, yield potential and adaptation to abiotic stresses. Discovering rare alleles and useful haplotypes also provides opportunities to enhance abiotic stress adaptation, while epigenetic variation has potential to enhance abiotic stress adaptation and productivity in crops. By reviewing current knowledge on specific traits and their genetic basis, we highlight recent developments in the understanding of crop functional diversity and identify potential candidate genes for future use. The storage and integration of genetic, genomic and phenotypic information will play an important role in ensuring broad and rapid application of novel genetic discoveries by the plant breeding community. Exploiting alleles for yield-related traits would allow improvement of selection efficiency and overall genetic gain of multigenic traits. An integrated approach involving multiple stakeholders specializing in management and utilization of genetic resources, crop breeding, molecular biology and genomics, agronomy, stress tolerance, and reproductive/seed biology will help to address the global challenge of ensuring food security in the face of growing resource demands and climate change induced stresses.

The analysis of the different functions of starch-phosphorylating enzymes during the development of Arabidopsis thaliana plants discloses an unexpected role for the cytosolic isoform GWD2

The genome of Arabidopsis thaliana encodes three glucan, water dikinases. Glucan, water dikinase 1 (GWD1; EC 2.7.9.4) and phosphoglucan, water dikinase (PWD; EC 2.7.9.5) are chloroplastic enzymes, while glucan, water dikinase 2 (GWD2) is cytosolic. Both GWDs and PWD catalyze the addition of phosphate groups to amylopectin chains at the surface of starch granules, changing its physicochemical properties. As a result, GWD1 and PWD have a positive effect on transitory starch degradation at night. Because of its cytosolic localization, GWD2 does not have the same effect. Single T-DNA mutants of either GWD1 or PWD or GWD2 have been analyzed during the entire life cycle of A. thaliana. We report that the three dikinases are all important for proper seed development. Seeds from gwd2 mutants are shrunken, with the epidermal cells of the seed coat irregularly shaped. Moreover, gwd2 seeds contain a lower lipid to protein ratio and are impaired in germination. Similar seed phenotypes were observed in pwd and gwd1 mutants, except for the normal morphology of epidermal cells in gwd1 seed coats. The gwd1, pwd and gwd2 mutants were also very similar in growth and flowering time when grown under continuous light and all three behaved differently from wild-type plants. Besides pinpointing a novel role of GWD2 and PWD in seed development, this analysis suggests that the phenotypic features of the dikinase mutants in A. thaliana cannot be explained solely in terms of defects in leaf starch degradation at night.

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.

Potato starch synthases: Functions and relationships

Starch, a very compact form of glucose units, is the most abundant form of storage polyglucan in nature. The starch synthesis pathway is among the central biochemical pathways, however, our understanding of this important pathway regarding genetic elements controlling this pathway, is still insufficient. Starch biosynthesis requires the action of several enzymes. Soluble starch synthases (SSs) are a group of key players in starch biosynthesis which have proven their impact on different aspects of the starch biosynthesis and functionalities. These enzymes have been studied in different plant species and organs in detail, however, there seem to be key differences among species regarding their contributions to the starch synthesis. In this review, we consider an update on various SSs with an emphasis on potato SSs as a model for storage organs. The genetics and regulatory mechanisms of potato starch synthases will be highlighted. Different aspects of various isoforms of SSs are also discussed.

The down-regulation of the genes encoding Isoamylase 1 alters the starch composition of the durum wheat grain

In rice, maize and barley, the lack of Isoamylase 1 activity materially affects the composition of endosperm starch. Here, the effect of this deficiency in durum wheat has been characterized, using transgenic lines in which Isa1 was knocked down via RNAi. Transcriptional profiling confirmed the partial down-regulation of Isa1 and revealed a pleiotropic effect on the level of transcription of genes encoding other isoamylases, pullulanase and sucrose synthase. The polysaccharide content of the transgenic endosperms was different from that of the wild type in a number of ways, including a reduction in the content of starch and a moderate enhancement of both phytoglycogen and β-glucan. Some alterations were also induced in the distribution of amylopectin chain length and amylopectin fine structure. The amylopectin present in the transgenic endosperms was more readily hydrolyzable after a treatment with hydrochloric acid, which disrupted its semi-crystalline structure. The conclusion was that in durum wheat, Isoamylase 1 is important for both the synthesis of amylopectin and for determining its internal structure.

Mutagenesis and TILLING to Dissect Gene Function in Plants

Mutagenesis can be random or targeted and occur by nature or artificially by humans. However, the bulk of mutagenesis employed in plants are random and caused by physical agents such as x-ray and gamma-ray or chemicals such as ethyl-methane sulfonate (EMS). Researchers are interested in first identifying these mutations and/or polymorphisms in the genome followed by investigating their effects in the plant function as well as their application in crop improvement. The high-throughput technique called TILLING (Targeting Induced Local Lesion IN Genomes) has been already established and become popular for identifying candidate mutant individuals harboring mutations in the gene of interest. TILLING is a non-transgenic and reverse genetics method of identifying a single nucleotide changes. The procedure of TILLING comprises traditional mutagenesis using optimum type and concentration of mutagen, development of a non-chimeric population, DNA extraction and pooling, mutation detection as well as validation of results. In general, TILLING has proved to be robust in identifying useful mutant lines in diverse economically important crops of the world. The main goal of the current mini-review is to show the significance role played by mutagenesis and TILLING in the discovery of DNA lesions which are to be used in the improvement of crops for the trait of interest.

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.

Identification and Phylogenetic Analysis of a Novel Starch Synthase in Maize

Starch is an important reserve of carbon and energy in plants, providing the majority of calories in the human diet and animal feed. Its synthesis is orchestrated by several key enzymes, and the amount and structure of starch, affecting crop yield and quality, are determined mainly by starch synthase (SS) activity. To date, five SS isoforms, including SSI-IV and Granule Bound Starch Synthase (GBSS) have been identified and their physiological functions have been well characterized. Here, we report the identification of a new SS isoform in maize, designated SSV. By searching sequenced genomes, SSV has been found in all green plants with conserved sequences and gene structures. Our phylogenetic analysis based on 780 base pairs has suggested that SSIV and SSV resulted from a gene duplication event, which may have occurred before the algae formation. An expression profile analysis of SSV in maize has indicated that ZmSSV is mainly transcribed in the kernel and ear leaf during the grain filling stage, which is partly similar to other SS isoforms. Therefore, it is likely that SSV may play an important role in starch biosynthesis. Subsequent analysis of SSV function may facilitate understanding the mechanism of starch granules formation, number and structure.

The future of starch bioengineering: GM microorganisms or GM plants?

Plant starches regularly require extensive modification to permit subsequent applications. Such processing is usually done by the use of chemical and/or physical treatments. The use of recombinant enzymes produced by large-scale fermentation of GM microorganisms is increasingly used in starch processing and modification, sometimes as an alternative to chemical or physical treatments. However, as a means to impart the modifications as early as possible in the starch production chain, similar recombinant enzymes may also be expressed in planta in the developing starch storage organ such as in roots, tubers and cereal grains to provide a GM crop as an alternative to the use of enzymes from GM microorganisms. We here discuss these techniques in relation to important structural features and modifications of starches such as: starch phosphorylation, starch hydrolysis, chain transfer/branching and novel concepts of hybrid starch-based polysaccharides. In planta starch bioengineering is generally challenged by yield penalties and inefficient production of the desired product. However, in some situations, GM crops for starch bioengineering without deleterious effects have been achieved.