A single amino acid mutation of OsSBEIIb contributes to resistant starch accumulation in rice

Structural and physicochemical properties of rice starch from a variety with high resistant starch and low amylose content

Research on the physicochemical properties of rice-derived endo-sperm high resistant starch (RS) with low amylose content (AC) is limited. In this study, we evaluated the physicochemical characteristics of such a starch variety and revealed that the starch granules exhibit a smoother, more refined surface with distinct edges, increased compactness, higher order of surface, and fewer cavities compared to those of a low RS rice variety. The starch crystal was classified as an A-type, which may be connected to the high amylose-lipid complex content. The branched internal long chains (B2 + B3) were abundant, allowing for easy entanglement with other molecular chains and a compact structure. Differential scanning calorimetry revealed the need for high temperature and energy to disrupt the double helix structure within the crystallization region of starch. Furthermore, starch viscosity analysis revealed a high cold paste viscosity, consistency, and setback value, with recrystallization yielding a stable structure, increased viscosity, and enhanced hydrolysis resistance to enzymes.

Physicochemical properties and molecular mechanisms of different resistant starch subtypes in rice

Resistant starch (RS) can help prevent diabetes and decrease calorie intake and that from plants are the main source of mankind consumption. Rice is many people's staple food and that with higher RS will help health management. A significantly positive correlation exists between apparent amylose content (AAC) of rice and its RS content. In this study, 72 accessions with moderate or high AAC were selected to explore the regulatory mechanisms and physicochemical properties on different proceeding types of rice RS. RS in raw milled rice (RSm), hot cooked rice (RSc), and retrogradation rice (RSr) showed a wide variation and distinct controlling mechanisms. They were co-regulated by Waxy (Wx), soluble starch synthase (SS) IIb and SSI. Besides that, RSm was also regulated by SSIIa and SSIVb, RSc by granule-bound starch synthase (GBSS) II and RSr by GBSSII and Pullulanase (PUL). Moreover, Wx had significant interactions with SSIIa, SSI, SSIIb and SSIVb on RSm, but only the dominant interactions with SSIIb and SSI on RSc and RSr. Wx was the key factor for the formation of RS, especially the RSc and RSr. The genes had the highest expression at 17 days after flowering and were beneficial for RS formation. The longer the chain length of starch, the higher the RS3 content. RSc and RSr were likely to be contained in medium-size starch granules. The findings favor understanding the biosynthesis of different subtypes of RS.

Starch Properties of Roasting Rice from Naturally High-Resistant Starch Rice Varieties

This study investigates the effects of moisture content control on the characteristics, properties, and in vitro starch digestion of roasted rice powder made from natural high-resistant starch (RS) rice varieties. The results demonstrate that adjusting the moisture content before roasting significantly affects the RS content of the roasted rice powder. Among various moisture levels tested, the addition of 15% water (rice-to-water ratio of 85:15) before roasting resulted in the highest RS content, reaching 22.61%. Several key parameters of the rice samples before and after optimal moisture control were analyzed, including thermal stability, chain length distribution, volatile flavor composition, and scanning electron microscopy. Additionally, in vitro digestion properties were measured. The findings revealed that the volatile flavor compounds in the high-RS roasted rice significantly increased compared to non-roasted rice. Moreover, the thermal stability of the rice samples improved, and the chain length distribution exhibited significant changes. The water absorption and expansion properties were significantly lower in the high-RS roasted rice. Furthermore, the in vitro starch digestion of the roasted flour made from high-RS rice showed a significantly lower digestion rate compared to common rice, indicating a lower starch hydrolysis index in high-RS rice with the sbe-rs genotype. Overall, the roasting process of natural high-RS rice modifies its characteristics, increases the RS content, enhances the flavor, and results in a lower starch digestion rate compared to common rice. This study provides valuable data for the food industry to promote the application of high-RS rice varieties with mutations in the SBEIIb gene, such as Youtangdao2 (YTD2).

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.

Starch fine molecular structures: The basis for designer rice with slower digestibility and desirable texture properties

Development of whole rice with low glycaemic index has been achieved, however, these rices are frequently associated with a poor texture property. Recent advances in terms of understanding the importance of starch fine molecular structures on the starch digestibility/texture of cooked whole rice have shed new insights on mechanisms of starch digestibility and texture from molecular levels. With an extensive discussion on the correlative and causal relationships among starch molecular structure, texture and starch digestibility of cooked whole rice, this review identified desirable starch fine molecular structures contributing to both slow starch digestibility and preferable textures. For instance, the selection of rice variety having more amylopectin intermediate chains while less amylopectin long chains might help develop cooked whole rice with both slower starch digestibility and softer texture. The information could help rice industry transform cooked whole rice into a healthier food product with slow starch digestibility and desirable texture.

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.

Enhancing the functional properties of rice starch through biopolymer blending for industrial applications: A review

Rice starch has been used in various agri-food products due to its hypoallergenic properties. However, rice starch has poor solubility, lower resistant starch content with reduced retrogradation and poor functional properties. Hence, its industrial applications are rather limited. The lack of comprehensive information and a holistic understanding of the interaction between rice starch and endo/exogenous constituents to improve physico-chemical properties is a prerequisite in designing industrial products with enhanced functional attributes. In this comprehensive review, we highlight the potentials of physically mixing of biopolymers in upgrading the functional characteristics of rice starch as a raw material for industrial applications. Specifically, this review tackles rice starch modifications by adding natural/synthetic polymers and plasticizers, leading to functional blends or composites in developing sustainable packaging materials, pharma- and nutraceutical products. Moreover, a brief discussion on rice starch chemical and genetic modifications to alter starch quality for the deployment of rice starch industrial application is also highlighted.

Identifying genes for resistant starch, slowly digestible starch, and rapidly digestible starch in rice using genome-wide association studies

BACKGROUND

The digestibility of starch is important for the nutritive value of staple food. Although several genes are responsible for resistant starch (RS) and slowly digestible starch (SDS), gaps persist concerning the molecular basis of RS and SDS formation due to the complex genetic mechanisms of starch digestibility.

OBJECTIVES

The objective of this study was to identify new genes for starch digestibility in rice and interprete the genetic mechanisms of RS and SDS by GWAS.

METHODS

Genome-wide association studies were conducted by associating the RS and SDS phenotypes of 104 re-sequenced rice lines to an SNP dataset of 2,288,867 sites using a compressed mixed linear model. Candidate genes were identified according to the position of the SNPs based on data from the MSU Rice Genome Annotation Project.

RESULTS

Seven quantitative trait loci (QTLs) were detected to be associated with the RS content, among which the SNP 6 m1765761 was located on Waxy. Starch branching enzymes IIa (BEIIa) close to QTL qRS-I4 was detected and further identified as a specific candidate gene for RS in INDICA. Two QTLs were associated with SDS, and the LOC_Os09g09360 encoding lipase was identified as a causal gene for SDS.

CONCLUSIONS

GWAS is a valid strategy to genetically dissect the formation of starch digestion properties in rice. RS formation in grains is dependent on the rice type; lipid might also contribute to starch digestibility and should be an alternative factor to improve rice starch digestibility.

Evaluation of the Quality of a High-Resistant Starch and Low-Glutelin Rice (Oryza sativa L.) Generated through CRISPR/Cas9-Mediated Targeted Mutagenesis

A high-resistant starch (RS) and low-glutelin diet is beneficial for the health of patients with diabetes and kidney diseases. Rice is an important food crop worldwide. Previous studies have demonstrated that downregulating the expression of rice starch branching enzyme IIb (SBEIIb) affected the composition and the structure of starch. However, there has been no report about generating the loss-of-function mutants of SBEIIb using low-glutelin rice cultivars as recipients. In this study, we adopted a CRISPR/Cas9 system to induce site-specific mutations at the SBEIIb locus in an elite low-glutelin japonica rice cultivar derived from Low Glutelin Content-1 (LGC-1) and successfully obtained two independent transgene-free sbeIIb/Lgc1 mutant lines. In the mutant lines, the apparent amylose content (AAC) was increased by approximately 1.8-fold and the RS content reached approximately 6%. The glutelin content was approximately 2%, maintaining the low-glutelin trait of the recipient cultivar. The formation mechanism of RS was explored by analyzing the fine structures and the properties of starch. According to the X-ray diffraction pattern and the increased lipid content, the high RS content of the sbeIIb/Lgc1 lines was attributed to the increased content of amylose-lipid complex. Further analyses of the nutritional quality revealed that the soluble sugar and lipid contents, especially sucrose and unsaturated fatty acids, increased in the sbeIIb/Lgc1 lines significantly. This research is expected to facilitate the cultivation and the application of functional rice suitable for patients with diabetes and kidney diseases.

Balancing the double-edged sword effect of increased resistant starch content and its impact on rice texture: its genetics and molecular physiological mechanisms

Resistant starch (RS) is the portion of starch that escapes gastrointestinal digestion and acts as a substrate for fermentation of probiotic bacteria in the gut. Aside from enhancing gut health, RS contributes to a lower glycemic index. A genome-wide association study coupled with targeted gene association studies was conducted utilizing a diverse panel of 281 resequenced Indica rice lines comprising of ~2.2 million single nucleotide polymorphisms. Low-to-intermediate RS phenotypic variations were identified in the rice diversity panel, resulting in novel associations of RS to several genes associated with amylopectin biosynthesis and degradation. Selected rice lines encoding superior alleles of SSIIa with medium RS and inferior alleles with low RS groups were subjected to detailed transcriptomic, metabolomic, non-starch dietary fibre (DF), starch structural and textural attributes. The gene regulatory networks highlighted the importance of a protein phosphatase alongside multiple genes of starch metabolism. Metabolomics analyses resulted in the identification of several metabolite hubs (carboxylic acid, sugars and polyamines) in the medium RS group. Among DF, mannose and galactose from the water-insoluble fraction were found to be highly associated with low and medium RS lines, respectively. Starch structural analyses revealed that a moderate increase in RS is also linked to an elevation of amylose 1 and amylose 2 fractions. Although rice lines with medium RS content negatively affected textural and viscosity properties in comparison to low RS, the textural property of medium RS lines was in the same acceptable range as IR64, a rice mega variety popular in Asia.

Breeding for three-line japonica hybrid rice combinations with high resistant starch content using molecular marker-assisted selection

Resistant starch (RS) is beneficial for human health, and especially for diabetics. Considering the high cost and low productivity of the Jiangtangdao 1 rice variety with high RS content, breeding high RS rice varieties exhibiting high productivity is essential. A molecular marker-assisted selection strategy was applied to increase RS content in a three-line hybrid rice variety. The functional rice variety Jiangtangdao 1, which contains sbe3-rs (on chr2) that controls the RS content, was used as the high RS content donor parent. Subsequently, male sterile maintainer and restorer lines containing homozygous sbe3-rs were bred using molecular marker-assisted selection combined with traditional breeding methods. The male sterile line was crossed with the restorer lines to identify the optimal hybrid combination with a high RS content. We obtained four combinations for which the yields were >50% higher than those of the control Jiangtangdao 1. In addition, there was no significant difference in the RS content between the combinations and Jiangtangdao 1. The hybrid rice plants with high RS content exhibited favorable agronomic traits and therefore have broad prospects for commercial application.

Improving nutritional quality of rice for human health

This review surveys rice nutritional value, mainly focusing on breeding achievements via adoption of both genetic engineering and non-transgenic strategies to improve key nutrients associated with human health. Rice (Oryza sativa) is an essential component of the diets and livelihoods of over 3.5 billion people. Polished rice is mostly consumed as staple food, fulfilling daily energy demands and part of the protein requirement. Brown rice is comparatively more nutritious, containing more lipids, minerals, vitamins, dietary fiber, micronutrients, and bioactive compounds. In this article, we review the nutritional facts about rice including the level of γ-aminobutyric acid, resistant starch, lysine, iron, zinc, β-carotene, folate, anthocyanin, various carotenoids, and flavonoids, focusing on their synthesis and metabolism and the advances in their biofortification via adoption of both conventional and genetic engineering strategies. We conclude that besides representing a staple food, rice has the potential to become a source of various essential nutrients or bioactive compounds through appropriate genetic improvements to benefit human health and prevent certain chronic diseases. Finally, we discuss the available, non-genetically engineering strategies for the nutritional improvement of rice, including their main strengths and constraints.

Dark/Light Treatments Followed by γ-Irradiation Increase the Frequency of Leaf-Color Mutants in Cymbidium

Radiation randomly induces chromosomal mutations in plants. However, it was recently found that the frequency of flower-color mutants could be specifically increased by upregulating anthocyanin pathway gene expression before radiation treatments. The mechanisms of chlorophyll biosynthesis and degradation are active areas of plant study because chlorophyll metabolism is closely connected to photosynthesis. In this study, we determined the dark/light treatment conditions that resulted in upregulation of the expression levels of six chlorophyll pathway genes, uroporphyrinogen III synthase (HEMD), uroporphyrinogen III decarboxylase (HEME2), NADPH-protochlorophyllide oxidoreductase (POR) A (PORA), chlorophyll synthase (CHLG), chlorophyllase (CLH2), and red chlorophyll catabolite reductase (RCCR), and measured their effects on the γ-irradiation-induced frequencies of leaf-color mutants in two Cymbidium cultivars. To degrade chlorophyll in rhizomes, 60–75 days of dark treatment were required. To upregulate the expressions of chlorophyll pathway genes, 10 days of light treatment appeared to be optimal. Dark/light treatments followed by γ-irradiation increased chlorophyll-related leaf mutants by 1.4- to 2.0-fold compared with γ-ray treatment alone. Dark/light treatments combined with γ-irradiation increased the frequency of leaf-color mutants in Cymbidium, which supports the wider implementation of a plant breeding methodology that increases the mutation frequency of a target trait by controlling the expression of target trait-related genes.

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.