Physiochemical properties of rice with contrasting resistant starch content

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.

Discrepancies in resistant starch and starch physicochemical properties between rice mutants similar in high amylose content

The content of resistant starch (RS) was considered positively correlated with the apparent amylose content (AAC). Here, we analyzed two Indica rice mutants, RS111 and Zhedagaozhi 1B, similar in high AAC and found that their RS content differed remarkably. RS111 had higher RS3 content but lower RS2 content than Zhedagaozhi 1B; correspondingly, cooked RS111 showed slower digestibility. RS111 had smaller irregular and oval starch granules when compared with Zhedagaozhi 1B and the wild type. Zhedagaozhi 1B showed a B-type starch pattern, different from RS111 and the wild type, which showed A-type starch. Meantime, RS111 had more fa and fb1 but less fb3 than Zhedagaozhi 1B. Both mutants showed decreased viscosity and swelling power when compared with the parents. RS111 had the lowest viscosity, and Zhedagaozhi 1B had the smallest swelling power. The different fine structures of amylopectin between RS111 and Zhedagaozhi 1B led to different starch types, gelatinization properties, paste viscosity, and digestibility. In addition to enhancing amylose content, modifications on amylopectin structure showed great potent in breeding rice with different RS2 and RS3 content, which could meet the increasing needs for various rice germplasms.

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).

Influence of Resistant Starch in Whole Rice on Human Gut Microbiota─From Correlation Implications to Possible Causal Mechanisms

Rice is the main staple food for a large population around the world, while it generally has a high glycemic index and low resistant starch (RS) content. Although many strategies have been applied to develop healthier rice products with increased RS contents, their actual effects on gut microbiota and human health remain elusive. In this review, currently available production methods of rice RS are briefly summarized, followed by a critical discussion on their interactions with gut microbiota and subsequent effects on human health, from correlation implications to causal mechanisms. Different contents, types, and structures of RS have been produced by strategies such as genetic manipulation and controlling cooking conditions. The difference can largely determine effects of rice RS on gut microbiota composition and metabolites by specific RS-gut microbiota interactions. This review can thus help the rice industry develop rice products with desirable RS contents and structures to generally improve human health.

Variation of resistant starch content in different processing types and their starch granules properties in rice

Ninety-nine lines from recombinant inbred lines were selected to investigate the effects of starch synthesis-related genes on resistant starch (RS) content in different proceeding types. RS in raw milled rice (RSm), hot cooked rice (RSc), and retrogradation rice (RSr) showed a wide variation among the lines, especially RSm arrived at 10.61%. Divergent variability of RSm, RSc and RSr indicated that there were different regulation mechanisms for them. Waxy wildtype allele (Wx^a) could elevate RSm, RSc and RSr, but Soluble starch synthase IIa (SSIIa) only played a vital role in regulating RSm. Wx^a-indica SSIIa could increase RSm, and Wx^a-japonica SSIIa (SSIIaj) could elevate RSc and RSr. The mean diameter of Wx^a-SSIIaj was significantly bigger than others. The bigger starch granules, the higher RSc and RSr. Starch granules morphology with high-RSm would have a higher percentage in polyhedral and angular shape. The results provide new information for rice breeding with high-RS content.