Molecular Aspect of Good Eating Quality Formation in Japonica Rice

The composition of amylopectin is the determinant of rice eating quality under certain threshold of protein content and the ratio of amylose and amylopectin. In molecular biology level, the fine structure of amylopectin is determined by relative activities of starch branching enzyme (SBE), granule-bound starch synthase (GBSS), and soluble starch synthase (SSS) in rice grain under the same ADP-Glucose level. But the underlying mechanism of eating quality in molecular biology level remains unclear. This paper reports the differences on major parameters such as SNP and insertion-deletion sites, RNA expressions, and enzyme activities associated with eating quality of japonica varieties. Eight japonica rice varieties with significant differences in various eating quality parameters such as palatability and protein content were used in this experiment. Association analysis between nucleotide polymorphism and eating quality showed that S12 and S13 loci in SBE1, S55 in SSS1, S58 in SSS2A were significantly associated with apparent amylose content, alkali digestion value, setback viscosity, consistency viscosity, pasting temperature, which explained most of the variation in apparent amylose content, setback viscosity, and consistency viscosity; and explained almost all variations in alkali digestion value and pasting temperature. Thirty-five SNPs and insertion-deletions from SBE1, SBE3, GBSS1, SSS1, and SSS2A differentiated high or intermediate palatability rice varieties from low palatability rice varieties. Correlation analysis between enzyme activities and eating quality properties revealed that SBE25 and SSS15/W15 were positively correlated with palatability, whereas GBSS10 and GBSS15 were negatively correlated. Gene expressions showed that SBE1 and SBE3 expressions in high palatability varieties tended to be higher than middle and low palatability varieties. Collectively, SBE1, SBE3, SSS1, and SSS2A, especially SBE1 and SBE3 could improve eating quality, but GBSS1 decreased eating quality. The results indicated the possibility of developing high palatability cultivars through modification of key genes related to japonica rice eating quality formation in starch biosynthesis.当蛋白质含量与直链淀粉、支链淀粉（amylopectin）的比例处于特定阈值区间时，支链淀粉的组成是决定稻米食用品质的核心因素。在分子生物学层面，当腺苷二磷酸葡萄糖（ADP-Glucose）水平一致时，稻米籽粒内淀粉分支酶（SBE）、颗粒结合型淀粉合酶（GBSS）与可溶性淀粉合酶（SSS）的相对活性共同决定了支链淀粉的精细结构。但目前食用品质形成的分子生物学调控机制仍未阐明。本研究针对粳稻（japonica）食用品质相关的单核苷酸多态性（SNP）、插入缺失位点（insertion-deletion sites）、RNA表达量及酶活性等核心参数的差异展开分析。本实验选取了8个适口性、蛋白质含量等食用品质参数存在显著差异的粳稻品种。核苷酸多态性与食用品质的关联分析结果显示：SBE1基因内的S12、S13位点、SSS1基因内的S55位点以及SSS2A基因内的S58位点，与表观直链淀粉含量（apparent amylose content）、碱消值（alkali digestion value）、回降黏度（setback viscosity）、稠度黏度（consistency viscosity）、糊化温度（pasting temperature）均存在显著关联；上述位点可解释表观直链淀粉含量、回降黏度及稠度黏度的绝大部分表型变异，同时几乎完全解释碱消值与糊化温度的表型变异。从SBE1、SBE3、GBSS1、SSS1及SSS2A基因中筛选得到的35个SNP与插入缺失位点，可有效区分高适口性、中适口性与低适口性粳稻品种。酶活性与食用品质性状的相关性分析表明：SBE25与SSS15/W15的活性与适口性呈正相关，而GBSS10与GBSS15的活性则与适口性呈负相关。基因表达分析显示：高适口性品种中SBE1与SBE3的表达量整体高于中、低适口性品种。综合来看，SBE1、SBE3、SSS1及SSS2A（尤其是SBE1与SBE3）可提升稻米食用品质，而GBSS1则会降低食用品质。本研究结果表明，通过修饰粳稻淀粉生物合成通路中与食用品质形成相关的关键基因，有望培育出高适口性粳稻品种。