Lack of starch synthase IIIa and high expression of granule-bound starch synthase I synergistically increase the apparent amylose content in rice endosperm

Inactivation of SSIIIa enhances the RS content through altering starch structure and accumulating C18:2 in japonica rice

SSIIIa was the key gene responsible for RS formation in rice endosperm. The higher RS content in ssIIIa mutant has been proposed to be majorly due to the increased amylose-lipid complexes (RS5). However, the formation of RS5 elicited by ssIIIa mutation and the importance of RS5 for total RS content in rice are still unclear. With japonica ssIIIa loss-of-function mutants created by CRISPR/Cas9 gene editing, the effects of SSIIIa mutation on RS5 were furtherly evaluated through investigating the transcriptome and metabolites. Inactivation of SSIIIa caused significant enhancement in amylose and RS content but without depletion in starch reserves. SSIIIa mutation modulated the genes involved in carbohydrate and lipid metabolisms and the redistribution of substances, led to accumulated protein, glucose, fructose, and C18:2. Besides the increased amylose content and altered amylopectin structure, the increased C18:2 contributed greatly to the enhancement in RS content in japonica ssIIIa mutants through complexing with amylose to form RS5, while the existence of lipid counted against the enhancement of RS content in indica rice. RS5 showed discrepant contributions for the total RS in rice with different genetic background. Inactivation of SSIIIa has great potential in improving RS5 content in japonica rice without great yield loss.

Explicating genetic architecture governing nutritional quality in pigmented rice

Rice is one of the most important staple plant foods that provide a major source of calories and nutrients for tackling the global hunger index especially in developing countries. In terms of nutritional profile, pigmented rice grains are favoured for their nutritional and health benefits. The pigmented rice varieties are rich sources of flavonoids, anthocyanin and proanthocyanidin that can be readily incorporated into diets to help address various lifestyle diseases. However, the cultivation of pigmented rice is limited due to low productivity and unfavourable cooking qualities. With the advances in genome sequencing, molecular breeding, gene expression analysis and multi-omics approaches, various attempts have been made to explore the genetic architecture of rice grain pigmentation. In this review, we have compiled the current state of knowledge of the genetic architecture and nutritional value of pigmentation in rice based upon the available experimental evidence. Future research areas that can help to deepen our understanding and help in harnessing the economic and health benefits of pigmented rice are also explored.

Overexpression of the ZmSUS1 gene alters the content and composition of endosperm starch in maize (Zea mays L.)

ZmSUS1 increases the amylose content of maize by regulating the expression of Shrunken2 (Sh2) and Brittle2 (Bt2) which encode the size subunits of endosperm ADP-glucose pyrophosphorylase, and Granule bound starchsynthase1 (GBSS1) and Starch synthase1 (SS1). Cereal crops accumulate starch in seeds as an energy reserve. Sucrose Synthase (SuSy) plays an important role in grain starch synthesis. In this study, ZmSUS1 was transformed into maize inbred line KN5585, and transgenic plants were obtained. Compared with the non-transgenic negative control, the content and activity of SuSy were significantly increased, the amylose content in mature seeds of transgenic maize increased by 41.1-69.2%, the total starch content increased by 5.0-13.5%, the 100-grain weight increased by 19.0-26.2% and the average diameter of starch granules increased by 10.8-17.2%. These results indicated that overexpression of ZmSUS1 can significantly improve the traits of maize seeds and obtain new lines with high amylose content. It was also found that the overexpression of ZmSUS1 may increase the amylose content by altering the expression of endosperm ADP-glucose pyrophosphorylase (AGPase) subunits Shrunken2 (Sh2) and Brittle2 (Bt2). Moreover, the ectopic expression of ZmSUS1 also affected the expression of Granule bound starch synthase1 (GBSS1) and Starch synthase1 (SS1) which encode starch synthase. This study proved the important role of ZmSUS1 in maize starch synthesis and provided a new technology strategy for improving maize starch content and yield.

More than the main structural genes: Regulation of resistant starch formation in rice endosperm and its potential application

In the past decade, research on resistant starch has evoked interest due to the prevention and inhibition of chronic human diseases, such as diabetes, cancer, and obesity. Increasing the amylose content (AC) and resistant starch (RS) has been pivotal in improving the nutritional benefit of rice. However, the exact mechanism of RS formation is complex due to interconnected genetic factors regulating amylose-amylopectin variation. In this review, we discussed the regulatory factors influencing the RS formation centered on the transcription, post-transcriptional, and post-translational processes. Furthermore, we described the developments in RS and AC levels in rice compared with other high RS cereals. Briefly, we enumerated potential applications of high RS mutants in health, medical, and other industries. We contest that the information captured herein can be deployed for marker-assisted breeding and precision breeding techniques through genome editing to improve rice varieties with enhanced RS content.

Identification of a new allele of soluble starch synthase IIIa involved in the elongation of amylopectin long chains in a chalky rice mutant

A chalky endosperm mutant (GM03) induced from an indica rice GLA4 was used to investigate the functional gene in starch biosynthesis. Bulked segregant analysis and sanger sequencing determined that a novel mutation in soluble starch synthase IIIa (SSIIIa) is responsible for the chalky phenotype in GM03. Complementary test by transforming the active SSIIIa gene driven by its native promoter to GM03 recovered the phenotype to its wildtype. The expression of SSIIIa was significantly decreased, while SSIIIa protein was not detected in GM03. The mutation of SSIIIa led to increased expression of most of starch synthesis related genes and elevated the levels of most of proteins in GM03. The CRISPR/Cas9 technology was used for targeted disruption of SSIIIa, and the mutant lines exhibited chalky endosperm which phenocopied the GM03. Additionally, the starch fine structure in the knockout mutant lines ss3a-1 and ss3a-2 was similar with the GM03, which showed increased amylose content, higher proportions of B1 and B2 chains, much lower proportions of B3 chains and decreased degree of crystallinity, leading to altered thermal properties with lower gelatinization temperature and enthalpy. Collectively, these results suggested that SSIIIa plays an important role in starch synthesis by elongating amylopectin long chains in rice.

High temperature boosts resistant starch content by altering starch structure and lipid content in rice ssIIIa mutants

High temperature (HT) during grain filling had adverse influences on starch synthesis. In this study, the influences of HT on resistant starch (RS) formation in rice were investigated. Most genes in ssIIIa mutants especially in RS4 were upregulated under Normal Temperature (NT) while downregulated under HT when compared with those of wild parent R7954. ssIIIa mutants had higher RS content, more lipid accumulation, higher proportion of short chains of DP 9-15, and less long chains of DP ≥37. ssIIIa mutation exacerbated the influences of HT on starch metabolite and caused larger declines in the expression of BEI, BEIIa, BEIIb, and SSIVb when exposed to HT. HT reduced the contents of total starch and apparent amylose significantly in wild type but not in mutants. Meanwhile, lipids were enriched in all varieties, but the amounts of starch-lipid complexes and the RS content were only heightened in mutants under HT. HT led to greatest declines in the amount of DP 9-15 and increases in the proportion of fb3 (DP ≥37); the declines and increases were all larger in mutants, which resulted in varied starch crystallinity. The increased long-chain amylopectin and lipids may be the major contributor for the elevated RS content in mutants under HT through forming more starch-lipid complexes (RSV).

Dynamic Change in Starch Biosynthetic Enzymes Complexes during Grain-Filling Stages in BEIIb Active and Deficient Rice

Starch is the predominant reserve in rice (Oryza sativa L.) endosperm, which is synthesized by the coordinated efforts of a series of starch biosynthetic-related enzymes in the form of a multiple enzyme complex. Whether the enzyme complex changes during seed development is not fully understood. Here, we investigated the dynamic change in multi-protein complexes in an indica rice variety IR36 (wild type, WT) and its BEIIb-deficient mutant (be2b) at different developmental stages. Gel permeation chromatography (GPC) and Western blotting analysis of soluble protein fractions revealed most of the enzymes except for SSIVb were eluted in smaller molecular weight fractions at the early developing stage and were transferred to higher molecular weight fractions at the later stage in both WT and be2b. Accordingly, protein interactions were enhanced during seed development as demonstrated by co-immunoprecipitation analysis, suggesting that the enzymes were recruited to form larger protein complexes during starch biosynthesis. The converse elution pattern from GPC of SSIVb may be attributed to its vital role in the initiation step of starch synthesis. The number of protein complexes was markedly decreased in be2b at all development stages. Although SSIVb could partially compensate for the role of BEIIb in protein complex formation, it was hard to form a larger protein complex containing over five proteins in be2b. In addition, other proteins such as PPDKA and PPDKB were possibly present in the multi-enzyme complexes by proteomic analyses of high molecular weight fractions separated from GPC. Two putative protein kinases were found to be potentially associated with starch biosynthetic enzymes. Collectively, our findings unraveled a dynamic change in the protein complex during seed development, and potential roles of BEIIb in starch biosynthesis via various protein complex formations, which enables a deeper understanding of the complex mechanism of starch biosynthesis in rice.

Interaction of starch branching enzyme 3 and granule-bound starch synthase 1 alleles increases amylose content and alters physico-chemical properties in japonica rice (Oryza sativa L.)

Four near-isogenic lines (NILs) with different allele combinations of the starch branching enzyme 3 (SBE3) and granule-bound starch synthase 1 (GBSS1) were developed by crossing the japonica rice cultivars "Dodamssal" and "Hwayeong." The associations between sequence variations in SBE3 and GBSS1, and starch-related traits were investigated. These sequence variations led to changes in seed morphology, starch structure, starch crystallinity, amylopectin chain length distribution, digestibility, apparent amylose content (AAC), and resistant starch content (RS). SBE3 and GBSS1 showed genetic interaction in regulating AAC and RS. Gene expression profiling of panicle tissues revealed significant differences in expression levels of GBSS1, SBE3, and other starch-related genes among the four NILs, indicating that variations in GBSS1 and SBE3 changed the expression level of starch-related genes. These variations contributed to the changes observed in AAC, RS, and physico-chemical characteristics of the rice starch from the NILs.

Improving Agricultural Traits While Maintaining High Resistant Starch Content in Rice

BACKGROUND

Resistant starch (RS) is beneficial for human health. Loss of starch branching enzyme IIb (BEIIb) increases the proportion of amylopectin long chains, which greatly elevates the RS content. Although high RS content cereals are desired, an increase in RS content is often accompanied by a decrease in seed weight. To further increase the RS content, genes encoding active-type starch synthase (SS) IIa, which elongates amylopectin branches, and high expression-type granule-bound SSI (GBSSI), which synthesizes amylose, were introduced into the be2b mutant rice. This attempt increased the RS content, but further improvement of agricultural traits was required because of a mixture of indica and japonica rice phonotype, such as different grain sizes, flowering times, and seed shattering traits. In the present study, the high RS lines were backcrossed with an elite rice cultivar, and the starch properties of the resultant high-yielding RS lines were analyzed.

RESULTS

The seed weight of high RS lines was greatly improved after backcrossing, increasing up to 190% compared with the seed weight before backcrossing. Amylopectin structure, gelatinization temperature, and RS content of high RS lines showed almost no change after backcrossing. High RS lines contained longer amylopectin branch chains than the wild type, and lines with active-type SSIIa contained a higher proportion of long amylopectin chains compared with the lines with less active-SSIIa, and thus showed higher gelatinization temperature. Although the RS content of rice varied with the cooking method, those of high RS lines remained high after backcrossing. The RS contents of cooked rice of high RS lines were high (27-35%), whereas that of the elite parental rice was considerably low (< 0.7%). The RS contents of lines with active-type SSIIa and high-level GBSSI expression in be2b or be2b ss3a background were higher than those of lines with less-active SSIIa.

CONCLUSIONS

The present study revealed that backcrossing high RS rice lines with elite rice cultivars could increase the seed weight, without compromising the RS content. It is likely that backcrossing introduced loci enhancing seed length and width as well as loci promoting early flowering for ensuring an optimum temperature during RS biosynthesis.

Diurnal changes in starch molecular structures and expression profiles of starch biosynthesis enzymes in rice developing seeds

The diurnal changes in the expression profiles of starch synthesis related enzymes (SSREs) has been previously studied in transitory starches, while its influences on storage starch molecular structures in the rice endosperm during seed development have not been elucidated. In this study, the changes in the transcript levels of starch synthesis related genes (SSRGs), the protein abundances and enzyme activities of SSREs as well as starch molecular structures in rice endosperm at 10 days after flowering (DAF) over the diurnal cycle were analyzed. It was found that the expression profiles of SSRG and the protein contents of SSREs displayed different diurnal patterns between two indica rice varieties with medium- and high-amylose content (AC), respectively. The expression levels of SSRGs were higher in the light time, and most SSREs also accumulated during this period except debranching enzymes. Amylose synthesis displayed distinct diurnal patterns in two rice varieties, which is attributed to the diurnal changes in the protein content of granule-bound starch synthase I (GBSSI), but amylopectin chain-length distributions (CLDs) remained unaltered due to its vast numbers of branches. The results provide the first step to understand the roles of each enzyme isoform involved in starch synthesis in response to diurnal regulation in rice endosperm.

Resistant starch formation in rice: Genetic regulation and beyond

Resistant starch (RS), a healthy dietary fiber, is a particular type of starch that has attracted much research attention in recent years. RS has important roles in reducing glycemic index, postprandial blood glucose levels, and serum cholesterol levels, thereby improving and preventing many diseases, such as diabetes, obesity, and cardiovascular disease. The formation of RS is influenced by intrinsic properties of starch (e.g., starch granule structure, starch crystal structure, and amylose-to-amylopectin ratio) and non-starch components (e.g., proteins, lipids, and sugars), as well as storage and processing conditions. Recent studies have revealed that several starch-synthesis-related genes (SSRGs) are crucial for the formation of RS during seed development. Several transcription factors and mRNA splicing factors have been shown to affect the expression or splicing of SSRGs that regulate RS content, suggesting their potential roles in RS formation. This review focuses mainly on recent research progress on the genetic regulation of RS content and discusses the emerging genetic and molecular mechanisms of RS formation in rice.

A single base change at exon of Wx-A1 caused gene inactivation and starch properties modified in a wheat EMS mutant line

BACKGROUND

Wheat is an essential source of starch. The GBSS or waxy genes are responsible for synthesizing amylose in cereals. The present study identified a novel Wx-A1 null mutant line from an ethyl methanesulfonate (EMS)-mutagenized population of common wheat cv. SM126 using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and agarose gel analyses.

RESULTS

The alignment of the Wx-A1 gene sequences from the mutant and parental SM126 lines showed only one single nucleotide polymorphism causing the appearance of a premature stop codon and Wx-A1 inactivation. The lack of Wx-A1 protein resulted in decreased amylose, total starch and resistant starch. The starch morphology assessment revealed that starch from mutant seeds was more wrinkled, increasing its susceptibility to digestion. Regarding the starch thermodynamic properties, the gelatinization temperature was remarkably reduced in the mutant compared to parental line SM126. The digestibility of native, gelatinized, and retrograded starches was analyzed for mutant M4-627 and the parental SM126 line. In the M4-627 line, rapidly digestible starch contents were increased, whereas resistant starch was decreased in the three types of starch.

CONCLUSION

Waxy protein is essential for starch synthesis. The thermodynamic characteristics were decreased in the Wx-A1 mutant line. The digestibility properties of starch were also affected. Therefore, the partial waxy mutant M3-627 might play a significant role in food improvement. Furthermore, it might also be used to produce high-quality noodles. © 2021 Society of Chemical Industry.

Starch biosynthetic protein complex formation in rice <i>ss2a be2b (</i>+<i>)</i> double mutant differs from their parental single mutants

Amylopectin, which consists of highly branched glucose polymers, is a major component of starch. Biochemical processes that regulate the elongation of glucose polymers and the generation and removal of glucose branches are essential for determining the properties of starch. Starch synthases (SSs) and branching enzyme (BE) mainly form complexes consisting of SSI, SSIIa, and BEIIb during endosperm development. Loss of BEIIb in rice is complemented by BEIIa, but the compensatory effects differ depending on the presence or absence of inactive BEIIb. To better understand these compensatory mechanisms, ss2a be2b (+) double mutant, which possessed truncated inactive SSIIa and inactive BEIIb, were analyzed. Soluble proteins separated by gel filtration chromatography showed that SSIIa and BEIIb proteins in the wild-type exhibited a broad range of elution patterns and only small amounts were detected in high molecular mass fractions. In contrast, most of truncated inactive SSIIa and inactive BEIIb from ss2a be2b (+) were found in high molecular mass fractions, and the SSI-SSIIa-BEIIb trimeric protein complex found in the wild-type was likely absent in ss2a be2b (+). Those SSIIa and BEIIb proteins in high molecular mass fractions in ss2a be2b (+) were also identified by mass spectrometry. Parental ss2a single mutant had negligible amounts of SSIIa suggesting that the truncated inactive SSIIa was recruited to high-molecular mass complexes in the presence of inactive BEIIb in ss2a be2b (+) double mutant. In addition, SSIVb might be involved in the formation of alternative protein complexes with < 300 kDa in ss2a be2b (+).

Relative importance of branching enzyme isoforms in determining starch fine structure and physicochemical properties of indica rice

Down-regulation of starch branching enzymes alters fine structure and starch properties, especially the B-type crystalline pattern and extremely high amylose content identified in the BEIIb-deficiency mutant in the indica rice. The relative importance of the starch branching enzymes in determining the molecular fine structure and starch functional properties were uncovered in this study. An indica rice, Guangluai 4 with high amylose content (AC) and high gelatinization temperature (GT) was used to generate the clustered regularly interspaced short palindromic repeats (CRISPR)/associated protein-9 (Cas9) knockout lines. Five mutant lines were identified including be1-1, be1-2, be2a-1, be2a-2 and be2b-1, and analysis of western blot showed the CRISPR/Cas9 system was successful in inducing mutations in the targeted genes. AC of be2b-1 (34.1%) was greater than that of wild type (WT) (27.4%) and other mutants. Mutations of either BEI or BEIIa did not alter the starch crystallite pattern (A-type). The BEIIb deficiency caused an opaque endosperm phenotype, changed the crystallite pattern from A- to B-type, and dramatically increased the degree of ordered structure, the relative proportion of amylose chains and intermediate to long amylopectin chains, average chain length of amylopectin molecules as well as GT. The BEIIa deficiency had no effect on the proportion of amylose chains, the length of amylopectin intermediate-long chains, conclusion temperature and enthalpy of gelatinization. Down-regulation of BEI increased the proportion of shortest amylopectin chains (fa) but decreased the proportion of long amylopectin chains (fb2 and fb3), leading to a lower GT. It is concluded that the relative importance in determining starch fine structures and functionality was in the order of BEIIb > BEI > BEIIa. Our results provide new information for utilizations of BE-deficient mutants in rice quality breeding.

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.

Active-type starch synthase (SS) IIa from indica rice partially complements the sugary-1 phenotype in japonica rice endosperm

Introduction of higher SSIIa activity to mild-type isa1 mutant by crossing results in restoration of crystallinity, starch granule structure, and production of plump seeds. Isoamylase 1 (ISA1) removes improper α-1, 6 glycosidic branches of amylopectin generated by starch branching enzymes and is essential for the formation of proper amylopectin structure. Rice isa1 (sug-1) mutants in japonica cultivar with less-active starch synthase IIa (SSIIa) and low granule-bound SSI (GBSSI) expression display wrinkled seed phenotype by accumulating water-soluble phytoglycogen instead of insoluble amylopectin. Expression of active SSIIa in transgenic rice produced with a severe-type isa1 mutant accumulated some insoluble glucan with weak B-type crystallinity at the periphery of seeds but their seeds remained wrinkled. To see whether introduction of high levels of SSIIa and/or GBSSI can restore the grain filling of the mild-type sug-1 mutant (EM653), new rice lines (SS2a gbss1^L isa1, ss2a^L GBSS1 isa1, and SS2a GBSS1 isa1) were generated by crossing japonica isa1 mutant (ss2a^L gbss1^L isa1) with wild type indica rice (SS2a GBSS1 ISA1). The results showed that SS2a gbss1^L isa1 and SS2a GBSS1 isa1 lines generated chalky plump seeds accumulating insoluble amylopectin-like glucans with an increase in DP 13-35, while ss2a^L GBSS1 isa1 generated wrinkly seeds and accumulated soluble glucans enriched with DP < 13. Scanning electron microscopic observation of cross-section of the seeds showed that SS2a gbss1^L isa1 and SS2a GBSS1 isa1 produced wild type-like polygonal starch granules. These starches showed the A-type crystallinity comparable to the wild type, while the japonica isa1 mutant and the transgenic rice do not show any or little crystallinity, respectively. These results indicate that introduction of higher SSIIa activity can mostly complements the mild-type sug-1 phenotype.

Functional Interactions between Enzymes Involved in Amylose and Amylopectin Biosynthesis in Rice Based on Mathematical Models

Starch biosynthesis is controlled by multiple enzymes, including granule-bound starch synthase I (GBSSI), soluble starch synthases (SSs), branching enzymes (BEs), and debranching enzymes (DBEs). Although the role of individual isoforms has been primarily elucidated, the precise information about how they work together in the synthesis of specific amylose and amylopectin chains is still unclear. In this study, starch molecular chain-length distributions (CLDs) of five rice varieties with different amylose contents were measured by fluorophore-assisted carbohydrate electrophoresis and size-exclusion chromatography and fitted with two mathematical models, and the protein abundance of 11 starch synthesis-related enzymes was measured by western blotting. The correlation between model fitting parameters of amylose and amylopectin CLDs demonstrated that amylose and amylopectin syntheses are closely dependent. GBSSI could interact with BEI, BEIIb, SSIIa, SSIVb, ISA1, PUL, and PHO1 to synthesize the amylopectin intermediate and long chains as well as amylose chains. In addition, the interaction among SSIVb and SSI, SSIIa, BEI, BEIIb, ISA1, and PUL possibly suggests that SSIVb assists them to synthesize the amylopectin chains. The results can help understand the mechanisms about the functional interaction of different enzyme isoforms in starch biosynthesis.

Metabolic factors restricting sink strength in superior and inferior spikelets in high-yielding rice cultivars

Many high-yielding rice cultivars with large sink size (total number of spikelet per unit area × mean grain weight) have been developed, but some japonica cultivars developed in Japan often fail to attain the expected high yield due to low sink strength of spikelets. Although there is natural variation in sink strength of spikelets among high-yielding cultivars, metabolic factors involved in the natural variation and relationships of sink strength in spikelets with final percentage of filled spikelets are not fully understood. In the present study, we examined cultivar differences in sink strength for superior and inferior spikelets (i.e. earlier fertilizing spikelets with faster growth and later fertilizing ones with slower growth, respectively) in a panicle, using each spikelet at 10 d after the onset of development (10 DAD) when starch accumulation in endosperm was actively proceeding. Nine high-yielding cultivars were used: five japonica-dominant and four indica-dominant cultivars. Cultivar differences were observed in starch contents at 10 DAD in each spikelet type, and indica cultivars had higher starch contents than japonica cultivars in both superior and inferior spikelets. In addition, starch contents at 10 DAD were closely related to percentage of filled grains at maturity in both spikelet types. The activities of sucrose synthase (SUS) and uridine diphosphoglucose pyrophosphorylase (UGP), and the protein levels of phosphorylase 1 (Pho1), were higher in indica than japonica cultivars, and were positively correlated with starch contents at 10 DAD for both superior and inferior spikelets; although metabolic states, revealed from relations between intermediate metabolites and starch contents, differed among spikelet types. Consequently, it was considered that SUS and UGP at the step from sucrose cleavage to adenosine diphosphoglucose synthesis, and Pho1 at the starch biosynthesis step, were key metabolic factors involved in cultivar differences of sink strength (ability to synthesize starch).

Breeding of High Cooking and Eating Quality in Rice by Marker-Assisted Backcrossing (MABc) Using KASP Markers

The primary goals of rice breeding programs are grain quality and yield potential improvement. With the high demand for rice varieties of premium cooking and eating quality, we developed low-amylose content breeding lines crossed with Samgwang and Milkyqueen through the marker-assisted backcross (MABc) breeding program. Trait markers of the SSIIIa gene referring to low-amylose content were identified through an SNP mapping activity, and the markers were applied to select favorable lines for a foreground selection. To rapidly recover the genetic background of Samgwang (recurrent parent genome, RPG), 386 genome-wide markers were used to select BC₁F₁ and BC₂F₁ individuals. Seven BC₂F₁ lines with targeted traits were selected, and the genetic background recovery range varied within 97.4–99.1% of RPG. The amylose content of the selected BC₂F₂ grains ranged from 12.4–16.8%. We demonstrated the MABc using a trait and genome-wide markers, allowing us to efficiently select lines of a target trait and reduce the breeding cycle effectively. In addition, the BC₂F₂ lines confirmed by molecular markers in this study can be utilized as parental lines for subsequent breeding programs of high-quality rice for cooking and eating.

Generation and Starch Characterization of Non-Transgenic BEI and BEIIb Double Mutant Rice (Oryza sativa) with Ultra-High Level of Resistant Starch

BACKGROUND

Cereals high in resistant starch (RS) are gaining popularity, as their intake is thought to help manage diabetes and prediabetes. Number of patients suffering from diabetes is also increasing in Asian countries where people consume rice as a staple food, hence generation of practically growable high RS rice line has been anticipated. It is known that suppression of starch branching enzyme (BE) IIb increases RS content in cereals. To further increase RS content and for more practical use, we generated a non-transgenic be1 be2b double mutant rice (Oryza sativa) line, which completely lacked both proteins, by crossing a be1 mutant with a be2b mutant.

RESULTS

The be1 be2b mutant showed a decrease in intermediate amylopectin chains and an increase in long amylopectin chains compared with be2b. The amylose content of be1 be2b mutant (51.7%) was the highest among all pre-existing non-transgenic rice lines. To understand the effects of chewing cooked rice and cooking rice flour on RS content, RS content of mashed and un-mashed cooked rice as well as raw and gelatinized rice flour were measured using be1 be2b and its parent mutant lines. The RS contents of mashed cooked rice and raw rice flour of be1 be2b mutant (28.4% and 35.1%, respectively) were 3-fold higher than those of be2b mutant. Gel-filtration analyses of starch treated with digestive enzymes showed that the RS in be1 be2b mutant was composed of the degradation products of amylose and long amylopectin chains. Seed weight of be1 be2b mutant was approximately 60% of the wild type and rather heavier than that of be2b mutant.

CONCLUSIONS

The endosperm starch in be1 be2b double mutant rice were enriched with long amylopectin chains. This led to a great increase in RS content in cooked rice grains and rice flour in be1 be2b compared with be2b single mutant. be1 be2b generated in this study must serve as a good material for an ultra-high RS rice cultivar.

Effects of soluble starch synthase IIa allelic variation on rice grain quality with different Waxy backgrounds

BACKGROUND

Soluble starch synthase IIa (ALK, SSII-3) is the major gene regulating gelatinization temperature (GT) and SSII-3 M1 is an effective marker for identifying SSIIa alleles. However, the haplotypes of SSIIa alleles amplified by SSII-3M1 and their allelic effect sunder different Waxy (Wx) background in non-glutinous rice remain unclear.

RESULTS

By integrating the genetic background analysis and by the identification of the genotypes of Wx and SSIIa, we found that the SSIIa alleles amplified by SSII-3 M1 were haplotype 1 (G/G/GC, indica-type) and 4 (A/G/TT, japonica-type), which had a significant effect on pasting temperature (PaT), hot paste viscosity (HPV), and the alkali spreading value (ASV). There were significant effects of SSIIa alleles on HPV, cool paste viscosity (CPV), and consistency value with different Wx backgrounds. The apparent amylose content (AAC) of samples significantly affected the accuracy of GT, which was represented by the manually determined pasting temperature (PTm).

CONCLUSIONS

The SSIIa alleles amplified by SSII-3 M1 are indica type and japonica type. Different SSIIa haplotypes significantly affect HPV, CPV, PaT, and ASV. GT, PaT, and PTm are mainly affected by SSIIa alleles. The classification of all samples with different haplotypes of SSIIa indicates that their AAC (Wx genotypes) is essentially. The effects of SSIIa alleles are themselves affected by different Wx alleles. © 2020 Society of Chemical Industry.

Effects of soluble starch synthase genes on eating and cooking quality in semi waxy japonica rice with Wxmp

The purpose of this study is to reveal the genetic mechanism of the variation of amylose content among different semi waxy or glutinous japonica rice in the background of Wxmp gene. Sixty-four semi waxy lines derived from the hybrid progenies of Wujing 13 and Milky Princess (Kantou 194) with polymorphism in soluble starch synthase gene SSIIa (SSII-3) and SSIIIa (SSIII-2) but no polymorphism in other starch synthase related genes were used as test materials. The genotypes of SSIIa and SSIIIa allele were identified by molecular markers, and the allelic effects of SSIIa and SSIIIa gene on amylose content (AC), gel consistency (GC), gelatinization temperature (GT) and rapid visco analyzer (RVA) profile characteristics were analyzed. The significant effects of SSIIa and SSIIIa alleles and the interactive effects between two genes on AC, GT, GC and RVA profile characteristics were found. The SSIIa and SSIIIa alleles from Wujing13 shown positive effects on AC with an average increase of 1.87 and 1.23% in 2 years respectively. There was no significant effect on GT for SSIIa or SSIIIa allele but remarkable influence on GT when the co-existence of the two genes. The genotype SSIIampSSIIIamp shown 1.34 °C higher GT than genotype SSIIawjSSIIIawj (mp and wj indicated that the gene was derived from Milky Princess and Wujing 13 respectively, the same as in the below). Different genes and alleles resulted in significant different GC. The genetic effect of SSIIawj and SSIIIamp on GC was 8.74 and 9.62 mm respectively. The GC of SSIIawjSSIIIamp was 10.64 and 16.95 mm higher than that of SSIIampSSIIIawj and SSIIawjSSIIIawj, respectively. The allele SSIIawj could increase the peak viscosity (PKV), hot paste viscosity (HPV), cool paste viscosity (CPV) and breakdown viscosity (BDV), while decrease the consistency viscosity (CSV) and setback viscosity (SBV). However for the allele SSIIIawj the opposite was true. The genotype SSIIawjSSIIIamp had the largest PKV, HPV and CPV, the genotype SSIIawjSSIIIawj had the largest BDV and CSV, but the genotype SSIIawjSSIIIamp had the least SBV. According to the comprehensive effect of each trait, the genotype SSIIawjSSIIIamp was the best. The allelic variation and interaction effect of SSIIa and SSIIIa genes have important reference value for improving cooking and eating quality of semi waxy japonica rice.

SSIIIa-RNAi suppression associated changes in rice grain quality and starch biosynthesis metabolism in response to high temperature

High temperature (HT) is a main environmental restraint that affects rice yield and grain quality. In this study, SSIIIa-RNAi and its wild-type (WT) were used to investigate the effect of HT exposure on the isozyme-specific variation of several key starch biosynthesis enzymes in developing endosperms and its relation to starch properties. SSIIIa-RNAi had minimal impact on grain chalky occurrence under normal temperature growth, but it could up-grade the susceptibility of grain chalky occurrence to HT exposure, due to the relatively sensitive response of AGPase and SSI to HT exposure. Different from WT, SSIIIa-RNAi had the relatively enriched proportion of chains with DP 13-16 under HT, and HT-induced decline in the proportion of DP < 12 became much larger for SSIIIa-RNAi relative to WT. SSIIIa-RNAi significantly enhanced the expression of SSI isozyme and total SS activity, whereas SSI-RNAi deficiency had little impact on the expression of SSIIIa isozyme. In this regard, the compensatory increase in SSI isozyme as a result of SSIIIa deficiency occurred only in a one-way manner. SSIIIa-RNAi caused a striking elevation in BEIIa expression, and the effect of SSIIIa deficiency on the chain length distribution in relation to HT exposure was closely associated with the participation of BEIIa, SSI, and their interaction in amylopectin biosynthesis.

Re-programming of gene expression in the CS8 rice line over-expressing ADPglucose pyrophosphorylase induces a suppressor of starch biosynthesis

The CS8 transgenic rice (Oryza sativa L.) lines expressing an up-regulated glgC gene produced higher levels of ADPglucose (ADPglc), the substrate for starch synthases. However, the increase in grain weight was much less than the increase in ADPglc levels suggesting one or more downstream rate-limiting steps. Endosperm starch levels were not further enhanced in double transgenic plants expressing both glgC and the maize brittle-1 gene, the latter responsible for transport of ADPglc into the amyloplast. These studies demonstrate that critical processes within the amyloplast stroma restrict maximum carbon flow into starch. RNA-seq analysis showed extensive re-programming of gene expression in the CS8 with 2073 genes up-regulated and 140 down-regulated. One conspicuous gene, up-regulated ~15-fold, coded for a biochemically uncharacterized starch binding domain-containing protein (SBDCP1) possessing a plastid transit peptide. Confocal microscopy and transmission electron microscopy analysis confirmed that SBDCP1 was located in the amyloplasts. Reciprocal immunoprecipitation and pull-down assays indicated an interaction between SBDCP1 and starch synthase IIIa (SSIIIa), which was down-regulated at the protein level in the CS8 line. Furthermore, binding by SBDCP1 inhibited SSIIIa starch polymerization activity in a non-competitive manner. Surprisingly, artificial microRNA gene suppression of SBDCP1 restored protein expression levels of SSIIIa in the CS8 line resulting in starch with lower amylose content and increased amylopectin chains with a higher degree of polymerization. Collectively, our results support the involvement of additional non-enzymatic factors such as SBDCP in starch biosynthesis.

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

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

Carbohydrate reserves and seed development: an overview

Seeds are one of the most important food sources, providing humans and animals with essential nutrients. These nutrients include carbohydrates, lipids, proteins, vitamins and minerals. Carbohydrates are one of the main energy sources for both plant and animal cells and play a fundamental role in seed development, human nutrition and the food industry. Many studies have focused on the molecular pathways that control carbohydrate flow during seed development in monocot and dicot species. For this reason, an overview of seed biodiversity focused on the multiple metabolic and physiological mechanisms that govern seed carbohydrate storage function in the plant kingdom is required. A large number of mutants affecting carbohydrate metabolism, which display defective seed development, are currently available for many plant species. The physiological, biochemical and biomolecular study of such mutants has led researchers to understand better how metabolism of carbohydrates works in plants and the critical role that these carbohydrates, and especially starch, play during seed development. In this review, we summarize and analyze the newest findings related to carbohydrate metabolism's effects on seed development, pointing out key regulatory genes and enzymes that influence seed sugar import and metabolism. Our review also aims to provide guidelines for future research in the field and in this way to assist seed quality optimization by targeted genetic engineering and classical breeding programs.

Starch Synthase IIa-Deficient Mutant Rice Line Produces Endosperm Starch With Lower Gelatinization Temperature Than Japonica Rice Cultivars

The gelatinization temperature of endosperm starch in most japonica rice cultivars is significantly lower than that in most indica rice cultivars. This is because three single nucleotide polymorphisms in the Starch synthase (SS) IIa gene in japonica rice cultivars (SSIIa^J ) significantly reduce SSIIa activity, resulting in an increase in amylopectin short chains with degree of polymerization (DP) ≤ 12 compared to indica rice cultivars (SSIIa^I ). SSIIa forms a trimeric complex with SSI and starch branching enzyme (BE) IIb in maize and japonica rice, which is likely important for the biosynthesis of short and intermediate amylopectin chains (DP ≤ 24) within the amylopectin cluster. It was unknown whether the complete absence of SSIIa further increases amylopectin short chains and reduces gelatinization temperature and/or forms altered protein complexes due to the lack of a suitable mutant. Here, we identify the SSIIa-deficient mutant rice line EM204 (ss2a) from a screen of ca. 1,500 plants of the rice cultivar Kinmaze (japonica) that were subjected to N-methyl-N-nitrosourea mutagenesis. The SSIIa gene in EM204 was mutated at the boundary between intron 5 and exon 6, which generated a guanine to adenine mutation and resulted in deletion of exon 6 in the mRNA transcript. SSIIa activity and SSIIa protein in developing endosperm of EM204 were not detected by native-PAGE/SS activity staining and native-PAGE/immunoblotting, respectively. SSIIa protein was completely absent in mature seeds. Gel filtration chromatography of soluble protein extracted from developing seeds showed that the SSI elution pattern in EM204 was altered and more SSI was eluted around 300 kDa, which corresponds with the molecular weight of trimeric complexes in wild type. The apparent amylose content of EM204 rice grains was higher than that in its parent Kinmaze. EM204 also had higher content of amylopectin short chains (DP ≤ 12) than Kinmaze, which reduced the gelatinization temperature of EM204 starch by 5.6°C compared to Kinmaze. These results indicate that EM204 starch will be suitable for making foods and food additives that easily gelatinize and slowly retrograde.

Starch Synthase IIa-Deficient Mutant Rice Line Produces Endosperm Starch With Lower Gelatinization Temperature Than Japonica Rice Cultivars

The gelatinization temperature of endosperm starch in most japonica rice cultivars is significantly lower than that in most indica rice cultivars. This is because three single nucleotide polymorphisms in the Starch synthase (SS) IIa gene in japonica rice cultivars (SSIIaJ ) significantly reduce SSIIa activity, resulting in an increase in amylopectin short chains with degree of polymerization (DP) ≤ 12 compared to indica rice cultivars (SSIIaI ). SSIIa forms a trimeric complex with SSI and starch branching enzyme (BE) IIb in maize and japonica rice, which is likely important for the biosynthesis of short and intermediate amylopectin chains (DP ≤ 24) within the amylopectin cluster. It was unknown whether the complete absence of SSIIa further increases amylopectin short chains and reduces gelatinization temperature and/or forms altered protein complexes due to the lack of a suitable mutant. Here, we identify the SSIIa-deficient mutant rice line EM204 (ss2a) from a screen of ca. 1,500 plants of the rice cultivar Kinmaze (japonica) that were subjected to N-methyl-N-nitrosourea mutagenesis. The SSIIa gene in EM204 was mutated at the boundary between intron 5 and exon 6, which generated a guanine to adenine mutation and resulted in deletion of exon 6 in the mRNA transcript. SSIIa activity and SSIIa protein in developing endosperm of EM204 were not detected by native-PAGE/SS activity staining and native-PAGE/immunoblotting, respectively. SSIIa protein was completely absent in mature seeds. Gel filtration chromatography of soluble protein extracted from developing seeds showed that the SSI elution pattern in EM204 was altered and more SSI was eluted around 300 kDa, which corresponds with the molecular weight of trimeric complexes in wild type. The apparent amylose content of EM204 rice grains was higher than that in its parent Kinmaze. EM204 also had higher content of amylopectin short chains (DP ≤ 12) than Kinmaze, which reduced the gelatinization temperature of EM204 starch by 5.6°C compared to Kinmaze. These results indicate that EM204 starch will be suitable for making foods and food additives that easily gelatinize and slowly retrograde.

Analyses of starch biosynthetic protein complexes and starch properties from developing mutant rice seeds with minimal starch synthase activities

BACKGROUND

Starch is the major component of cereal grains and is composed of essentially linear amylose and highly branched amylopectin. The properties and composition of starch determine the use and value of grains and their products. Starch synthase (SS) I, SSIIa, and SSIIIa play central roles in amylopectin biosynthesis. These three SS isozymes also affect seed development, as complete loss of both SSI and SSIIIa under reduced SSIIa activity in rice lead to sterility, whereas presence of minimal SSI or SSIIIa activity is sufficient for generating fertile seeds. SSs, branching enzymes, and/or debranching enzymes form protein complexes in cereal. However, the relationship between starch properties and the formation of protein complexes remain largely unknown. To better understand this phenomenon, properties of starch and protein complex formation were analyzed using developing mutant rice seeds (ss1 ^L /ss2a ^L /ss3a) in which all three major SS activities were reduced.

RESULTS

The SS activity of ss1 ^L /ss2a ^L /ss3a was 25%-30% that of the wild-type. However, the grain weight of ss1 ^L /ss2a ^L /ss3a was 89% of the wild-type, 55% of which was starch, showing considerable starch synthesis. The reduction of soluble SS activity in ss1 ^L /ss2a ^L /ss3a resulted in increased levels of ADP-glucose pyrophosphorylase and granule-bound starch synthase I, which are responsible for substrate synthesis and amylose synthesis, respectively. Together, these features led to an increase in apparent amylose content (34%) in ss1 ^L /ss2a ^L /ss3a compared with wild-type (20%). Gel filtration chromatography of the soluble proteins in ss1 ^L /ss2a ^L /ss3a showed that the majority of the starch biosynthetic enzymes maintained the similar elution patterns as wild-type, except that the amounts of high-molecular-weight SSI (> 300 kDa) were reduced and SSIIa of approximately 200-300 kDa were present instead of those > 440 kDa, which predominate in wild-type. Immuno-precipitation analyses suggested that the interaction between the starch biosynthetic enzymes maybe reduced or weaker than in wild-type.

CONCLUSIONS

Although major SS isozymes were simultaneously reduced in ss1 ^L /ss2a ^L /ss3a rice, active protein complexes were formed with a slightly altered pattern, suggesting that the assembly of protein complexes may be complemented among the SS isozymes. In addition, ss1 ^L /ss2a ^L /ss3a maintained the ability to synthesize starch and accumulated less amylopectin and more amylose in starch.

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.

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.

Highly phosphorylated functionalized rice starch produced by transgenic rice expressing the potato GWD1 gene

Starch phosphorylation occurs naturally during starch metabolism in the plant and is catalysed by glucan water dikinases (GWD1) and phosphoglucan water dikinase/glucan water dikinase 3 (PWD/GWD3). We generated six stable individual transgenic lines by over-expressing the potato GWD1 in rice. Transgenic rice grain starch had 9-fold higher 6-phospho (6-P) monoesters and double amounts of 3-phospho (3-P) monoesters, respectively, compared to control grain. The shape and topography of the transgenic starch granules were moderately altered including surface pores and less well defined edges. The gelatinization temperatures of both rice flour and extracted starch were significantly lower than those of the control and hence negatively correlated with the starch phosphate content. The 6-P content was positively correlated with amylose content and relatively long amylopectin chains with DP25-36, and the 3-P content was positively correlated with short chains of DP6-12. The starch pasting temperature, peak viscosity and the breakdown were lower but the setback was higher for transgenic rice flour. The 6-P content was negatively correlated with texture adhesiveness but positively correlated with the cohesiveness of rice flour gels. Our data demonstrate a way forward to employ a starch bioengineering approach for clean modification of starch, opening up completely new applications for rice starch.

Characterization of the endosperm starch and the pleiotropic effects of biosynthetic enzymes on their properties in novel mutant rice lines with high resistant starch and amylose content

Resistant starch (RS) is beneficial to human health. In order to reduce the current prevalence of diabetes and obesity, several transgenic and mutant crops containing high RS content are being developed. RS content of steamed rice with starch-branching enzyme (BE)IIb-deficient mutant endosperms is considerably high. To understand the mechanisms of RS synthesis and to increase RS content, we developed novel mutant rice lines by introducing the gene encoding starch synthase (SS)IIa and/or granule-bound starch synthase (GBSS)I from an indica rice cultivar into a japonica rice-based BEIIb-deficient mutant line, be2b. Introduction of SSIIa from an indica rice cultivar produced higher levels of amylopectin chains with degree of polymerization (DP) 11-18 than those in be2b; the extent of the change was slight due to the shortage of donor chains for SSIIa (DP 6-12) owing to BEIIb deficiency. The introduction of GBSSI from an indica rice cultivar significantly increased amylose content (by approximately 10%) in the endosperm starch. RS content of the new mutant lines was the same as or slightly higher than that of the be2b parent line. The relationship linking starch structure, RS content, and starch biosynthetic enzymes in the new mutant lines has also been discussed.

Progress in High-Amylose Cereal Crops through Inactivation of Starch Branching Enzymes

High-amylose cereal starches provide many health benefits for humans. The inhibition or mutation of starch branching enzyme (SBE) genes is an effective method to develop high-amylose cereal crops. This review summarizes the development of high-amylose cereal crops through the inactivation of one or more SBE isoforms or combination with other genes. This review also reveals the causes of increase in amylose content in high-amylose crops. A series of changes, including amylopectin structure, crystalline structure, thermal properties, and hydrolysis properties, occurs as amylose content increases. The different morphological starch granules nominated as heterogeneous starch granules or differently stained starch granules are detected in high-amylose cereal crops. Detailed studies on four heterogeneous starch granules in high-amylose rice, which is developed by antisense RNA inhibition of SBEI/IIb, indicate that granules with different morphologies possess various molecular structures and physicochemical and functional properties. This variation diversifies their applications in food and non-food industries. However, current knowledge regarding how these heterogeneous starch granules form and why they exhibit regional distribution in endosperm remain largely unknown.

Critical roles of soluble starch synthase SSIIIa and granule-bound starch synthase Waxy in synthesizing resistant starch in rice

Changes in human lifestyle and food consumption have resulted in a large increase in the incidence of type-2 diabetes, obesity, and colon disease, especially in Asia. These conditions are a growing threat to human health, but consumption of foods high in resistant starch (RS) can potentially reduce their incidence. Strategies to increase RS in rice are limited by a lack of knowledge of its molecular basis. Through map-based cloning of a RS locus in indica rice, we have identified a defective soluble starch synthase gene (SSIIIa) responsible for RS production and further showed that RS production is dependent on the high expression of the Waxy^a (Wx^a ) allele, which is prevalent in indica varieties. The resulting RS has modified granule structure; high amylose, lipid, and amylose-lipid complex; and altered physicochemical properties. This discovery provides an opportunity to increase RS content of cooked rice, especially in the indica varieties, which predominates in southern Asia.

Formation of starch in plant cells

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

Changes in the activities of starch metabolism enzymes in rice grains in response to elevated CO2 concentration

The global atmospheric CO(2) concentration is currently (2012) 393.1 μmol mol(-1), an increase of approximately 42 % over pre-industrial levels. In order to understand the responses of metabolic enzymes to elevated CO(2) concentrations, an experiment was conducted using the Free Air CO(2) Enrichment (FACE )system. Two conventional japonica rice varieties (Oryza sativa L. ssp. japonica) grown in North China, Songjing 9 and Daohuaxiang 2, were used in this study. The activities of ADPG pyrophosphorylase, soluble and granule-bound starch synthases, and soluble and granule-bound starch branching enzymes were measured in rice grains, and the effects of elevated CO(2) on the amylose and protein contents of the grains were analyzed. The results showed that elevated CO(2) levels significantly increased the activity of ADPG pyrophosphorylase at day 8, 24, and 40 after flower, with maximum increases of 56.67 % for Songjing 9 and 21.31 % for Daohuaxiang 2. Similarly, the activities of starch synthesis enzymes increased significantly from the day 24 after flower to the day 40 after flower, with maximum increases of 36.81 % for Songjing 9 and 66.67 % for Daohuaxiang 2 in soluble starch synthase (SSS), and 25.00 % for Songjing 9 and 36.44 % for Daohuaxiang 2 in granule-bound starch synthase (GBSS), respectively. The elevated CO(2) concentration significantly increased the activity of soluble starch branching enzyme (SSBE) at day 16, 32, and 40 after flower, and also significantly increased the activity of granule-bound starch branching enzyme (GBSBE) at day 8, 32, and 40 after flower. The elevated CO(2) concentration increased the peak values of enzyme activity, and the timing of the activity peaks for SSS and GBSBE were earlier in Songjing 9 than in Daohuaxiang 2. There were obvious differences in developmental stages between the two varieties of rice, which indicated that the elevated CO(2) concentration increased enzyme activity expression and starch synthesis, affecting the final contents of starch and protein in the rice grains. Our results will provide a foundation for understanding the physiological mechanisms of rice yield under elevated atmospheric CO(2) concentrations.

Promoter regulatory domain identification of cassava starch synthase IIb gene in transgenic tobacco

Soluble starch synthase is a key enzyme in the starch biosynthesis pathway, and its enzyme activity significantly influences starch components in cassava storage root. However, studies on the regulation mechanism of soluble starch synthase gene are rare. In this study, we cloned the 5' flanking sequence of the MeSSIIb gene and predicted the distribution of cis-elements. The region from -453 to -1 was considered the primary core promoter by the quantitative detection of GUS activity in transgenic tobacco plants containing 5' truncated promoters fused with the GUS gene. Analysis results clarified that the region from -531 to -454 significantly repressed promoter activity. The region from -453 to -388 was a repressive domain of ethylene, and some unknown drought responsive cis-elements were located in the region from -387 to -1. These findings will provide useful information on the functional assay and transcriptional regulation mechanisms of the MeSSIIb gene.

Underlying Mechanisms of Zymographic Diversity in Starch Synthase I and Pullulanase in Rice-Developing Endosperm

Amylopectin is synthesized by the coordinated actions of many (iso)enzymes, including ADP-glucose pyrophosphorylase (AGPase), starch synthases (SSs), branching enzymes (BEs), and debranching enzymes (DBEs). Here, two polymorphic forms of starch synthase I (SSI) and pullulanase (PUL) in rice-developing seeds, designated as SSI-1/SSI-2 and PUL-1/PUL-2, were discovered for the first time by zymographic analysis. The SSI and PUL polymorphisms were strongly associated with the SSI microsatellite marker (p = 3.6 × 10(-37)) and PUL insertion/deletion (InDel) markers (p < 3.6 × 10(-51)). Western blotting and mass spectrometric analysis confirmed that the polymorphic bands were truly the SSI and PUL enzymes. Only one non-synonymous variation in SSI DNA sequence (the SNP A/G) causing the change of the amino acid K438 to E438 was observed, which coincided well with the polymorphic forms of SSI. Nine non-synonymous variations were found between PUL-1 and PUL-2. Two non-synonymous variations of PUL (F316L and D770E) were identified by mass spectrometric analysis, but all of the variations did not change the structure of PUL. The co-immunoprecipitation results revealed the differences in protein-protein interaction patterns, i.e., strong or weaker signals of SSI-BEI and SSI-BEIIb, between the two forms of SSI. The results will enhance our understanding of SSI and PUL properties and provide helpful information to understand their functions in starch biosynthesis in rice endosperm.

Amyloplast Membrane Protein SUBSTANDARD STARCH GRAIN6 Controls Starch Grain Size in Rice Endosperm

Starch is a biologically and commercially important polymer of glucose. Starch is organized into starch grains (SGs) inside amyloplasts. The SG size differs depending on the plant species and is one of the most important factors for industrial applications of starch. There is limited information on genetic factors regulating SG sizes. In this study, we report the rice (Oryza sativa) mutant substandard starch grain6 (ssg6), which develops enlarged SGs in endosperm. Enlarged SGs are observed starting at 3 d after flowering. During endosperm development, a number of smaller SGs appear and coexist with enlarged SGs in the same cells. The ssg6 mutation also affects SG morphologies in pollen. The SSG6 gene was identified by map-based cloning and microarray analysis. SSG6 encodes a protein homologous to aminotransferase. SSG6 differs from other rice homologs in that it has a transmembrane domain. SSG6-green fluorescent protein is localized in the amyloplast membrane surrounding SGs in rice endosperm, pollen, and pericarp. The results of this study suggest that SSG6 is a novel protein that controls SG size. SSG6 will be a useful molecular tool for future starch breeding and applications.

Screening Method for Novel Rice Starch Mutant Lines Prepared by Introducing Gene Encoding Starch Synthase IIa and Granule-bound Starch Synthase I from Indica Cultivar into a Branching Enzyme IIb-Deficient Mutant Line

The structure and properties of starch reserves in rice seeds are strongly affected by deficiencies in specific starch biosynthetic enzymes, which are highly expressed in storage tissues. Rice lines with unique seed starches should be utilized for food and industrial applications in the near future. We are currently developing novel rice mutant lines with distinct starch properties by introducing specific genes from different cultivars into mutant lines and by producing multiple combinations of single mutant lines. Obtaining the homozygous genotypes of the target genes is necessary during the screening process of these materials. In this study, we developed an effective, accurate screening method for identifying rice lines with novel starch composition. Specifically, we produced a novel mutant line in which we introduced genes encoding starch synthase IIa (SSIIa) and granule-bound starch synthase I (GBSSI) from indica cultivar into a starch branching enzyme IIb (BEIIb)-deficient mutant line.