Deciphering Starch Quality of Rice Kernels Using Metabolite Profiling and Pedigree Network Analysis

The physiological properties of rice grains are immediately obvious to consumers. High-coverage metabolomic characterization of the rice diversity research set predicted a negative correlation between fatty acid and lipid levels and amylose/total starch ratio （amylose ratio）, but the reason for this is unclear. To obtain new insight into the relationships among the visual phenotypes of rice kernels, starch granule structures, amylose ratios, and metabolite changes, we investigated the metabolite changes of five Japonica cultivars with various amylose ratios and two knockout mutants （e 1, a Starch synthase Ilia （SSIIla）-deficient mutant and the SSIIla/starch branching enzyme （BE） double-knockout mutant 4019） by using mass spectrometry-based metabolomics techniques. Scanning electron microscopy clearly showed that the two mutants had unusual starch granule structures. The metabolomic compositions of two cultivars with high amylose ratios （Hoshiyutaka and Yumetoiro） exhibited similar patterns, while that of the double-knockout mutant, which has an extremely high amylose ratio, differed. Rice pedigree network analysis of the cultivars and the mutants provided insight into the association between metabolic-trait properties and their underlying genetic basis in rice breeding in Japan. Multidimensional scaling analysis revealed that the Hoshiyutaka and Yumetoiro cultivars were Indica-like, yet they are classified as Japonica subpopulations. Exploring metabolomic traits is a powerful way to follow rice genetic traces and breeding history.

Effects of CO_2 enrichment and spikelet removal on rice quality under open-air field conditions

The increase of atmospheric carbon dioxide（CO_2） concentration adversely affect several quality traits of rice grains, but the biochemical mechanism remains unclear. The objectives of this study were to determine how changes in the source-sink relationship affected rice quality. Source-sink manipulation was achieved by free-air CO_2 enrichment from tillering to maturity and partial removal of spikelet at anthesis using a japonica rice cultivar Wuyunjing 23. Enrichment with CO_2 decreased the head rice percentage and protein concentration of milled rice, but increased the grain chalkiness. In contrast, spikelet removal resulted in a dramatic increase in the head rice percentage and protein concentration, and much less grain chalkiness. Neither CO_2 enrichment nor spikelet removal affected the starch content, but the distribution of starch granule size showed distinct treatment effects. O n average, spikelet removal decreased the percentage of starch granules of diameter 〉10 and 5–10 μm by 23.6 and 5.6%, respectively, and increased those with a diameter of 2–5 and 〈2 μm by 4.6 and 3.3%, respectively. In contrast, CO_2 elevation showed an opposite response： increasing the proportion of large starch granules（〉5 μm） and decreasing that of 〈5 μm. The starch pasting properties were affected by spikelet removal much more than by CO_2 elevation. These results indicated that the protein concentration and starch granule size played a role in chalkiness formation under these experimental conditions.

Generation of selectable marker-free soft transgenic rice with transparent kernels by downregulation of SSSⅡ-2

The amylose content(AC) of rice endosperm starch varies from 0 to 35%,and is associated with rice cooking and eating quality.Soft rice has low AC,generally between 6% and 15%,and its eating quality is high whether it is consumed hot or cold.However,the appearance quality of current soft rice cultivars needs to be improved,especially opaque endosperm.Conventional genetic engineering has improved some agronomic traits of soft rice varieties,but not endosperm appearance.In the present study,a RNAi construct of the soluble starch synthase Ⅱ-2(SSSⅡ-2) and the hygromycin phosphotransferase(HPT) gene were introduced into an elite japonica rice variety,Kangtiaowuyunjing(KWY8) by co-transformation.Several selectable marker-free(SMF) transgenic lines were obtained,and SSSⅡ-2 expression was significantly downregulated in selected transgenic lines,resulting in lower AC of the endosperm.The physicochemical properties of the transgenic rice kernels,including gel consistency(GC) and rapid visco analyzer(RVA) profile,differed significantly from those of wild-type rice and were similar to those of a soft rice variety,Nanjing 46(NJ46).These findings indicate that the cooking,eating,and processing qualities of transgenic rice are comparable to those of NJ46.However,the transgenic rice endosperm retained a transparent appearance under low-moisture conditions.Thus,SMF SSSⅡ-2 RNAi rice provides a resource for breeding soft rice with transparent endosperm.

Differential expressions among five Waxy alleles and their effects on the eating and cooking qualities in specialty rice cultivars

Eating and cooking qualities（ECQs） of rice are important attributes due to its major influence on consumer acceptability. To better understand the molecular mechanism of the variation in ECQs, we investigated and compared the expressions among different alleles of the Waxy（Wx） gene and its effect on ECQs in specialty rice cultivars. The results showed that the accumulation of amylose was positively and significantly correlated to the level of mature Wx m RNA and granule-bound starch synthase I（GBSS I） in developing rice grain at 12 days after flowering. The amount of GBSS I and its activity together are the main factors controlling amylose synthesis. Differences in ECQs among five Wx allele types were investigated in samples from 15 rice varieties. The apparent amylose content（AAC） and gel consistency（GC） were similar in each type of Wx allele. The AAC followed the order, Wx^a type〉Wx^in type〉Wx^b type〉Wx^mq type〉wx. Contrary to this, the GC showed an opposite trend compared to AAC. There was a wide variation in rapid visco analyzer（RVA） profile among five Wx allele types, while varieties sharing a specified Wx allele had basically the similar RVA profile, although there was a slight difference in some RVA parameters, peak, hot paste and cool paste viscosities.