Structural And Genetic Basis Of Rice Starch Digestibility

Rice is a major crop in China and it provides the largest component of daily food crop energy for people in South China.The rice grain mainly comprises starch and smaller amounts of protein,which are easily hydrolyzed in the human digestive tract to glucose and amino acids,which are subsequentally absorbed by the body.Increasing prosperity and improved lifestyles have the unfortunate side effect of an excessive intake of carbohydrates,which is closely associated with type II diabetes,high blood pressure and heart disease,and thus significantly affects human health.Improving the nutritional value of staple foods while providing sufficient food energy and improving human health calls for breeding rice varieties with a slow digestion and/or high resistant starch content.In previous study,we have screened a series of rice germplasms by measure the rice starch digestibility and identified some rice germplasms with low starch digestibility.In the present study,4 germplasms with similar high amylose content but significantly different in rice starch digestibility were employed to do the comparison of rice grain physicochemical and starch structure profiles.Subsequently,the indica rice MRX with slow starch digestibility was used for the genetic segregating population construction and QTL mapping of starch digestibility.In addition,the high-generation backcross inbred lines(BILs)with different digestion rates were generated and employed to indentify the effects of different QTLs on rice starch digestion profiles.The main findings are as follows:1、The results showed that rice flours from MRX and ZXM exhibited lower digestion curve and even the purified starch without protein and lipid showed the same low digestion curve.The physicochemical properties were analyzed and all the varieties showed similar apparent amylose content(AAC),gel consistency(GC)and total starch content while MRX exhibited a higher protein content(PC)than others.The scanning electron microscope(SEM)observation of starch granule in vitro digestion after interval times showed that starch granules from MRX and ZXM rice exhibited a stable structure during the digestion process,which showed a strong resistant to enzyme erosion.The thermal properties of native starches from different varieties were further analyzed by DSC,and starches from MRX presented significantly higher gelatinization temperature(GT)and gelatinization enthalpy than others.The starch retrogradation properties were further analyzed and the results showed that both MRX and ZXM starches were more easily to retrogradation and exhibit strong retrogradation endothermic peaks than others.The relative molecular weight distribution of starches from different varieties was firstly characterized by GPC,and the results showed that the relative area of the amylopectin short chain chains AP1 fraction from MRX and ZXM were higher than others.The chain length distribution of debranched amylopectin fractions was analyzed by HPAEC.The amylopectin in both MRX and ZXM samples exhibited more B1 chains(DP 12-24)than others,which might be the main reason for the differences in starch digestion properties among different varieties.2、The analysis of the digestion characteristics of rice from the F3 population generated by the cross between indica rice MRX and japonica rice Nipponbare(Wxa)showed that the digestion characteristics were continuous variation in the population.It showed a typical genetic pattern of quantitative traits,and showed a skewed distribution(skewness of 0.785).Meanwhile,the SD90 of most rice lines is less than the medium(31.46%).For QTL analysis,140 polymorphic molecular markers distributed on 12 chromosomes were identified and used for population genotype detection.Combined with SD90 value and genotype data,a total of 3 SD90 QTLs(qSD90-3,qSD90-8 and qSD90-9)were detected,which were distributed on chromosomes 3,8 and 9,respectively.The phenotypic variation explained by qSD90-9 was 14.21%.In addition,we constructed six high-generation backcross lines(BILs)carrying 3 QTLs under Nipponbare background(carrying Wxa and Wxb).We found that qSD90-9 had a significant effect on reducing rice digestibility under the two backgrounds while other QTLs showed no significant effect.The analysis of starch physical and chemical properties showed that changes in amylopectin fine structure might be the key reason for the differences in starch digestibility.To sum up,the results laid the foundation for the further identification of this locus,and also provided important germplasm for the cultivation of rice lines with relatively slow starch digestion and good palatability.