No abstract

Several quantitative trait loci (QTLs) associated with the apparent quality of brown rice were identified. QTL analysis was carried out using F 2 and F 3 populations derived from a cross between two japonica varieties, Hana-echizen (high quality of brown rice) and Niigatawase (low quality with numerous white-back and basalwhite kernels). F 2 individuals were grown in paddy fields in 2003, and F 3 lines were grown in paddy fields and in a greenhouse to expose them to high temperature stress during the ripening period in 2004. Apparent quality of brown rice was evaluated based on the percentage of white-back and basal-white kernels. Two putative QTLs associated with white-back kernels in the F 2 population grown under low temperature conditions in paddy fields in 2003 were identified on chromosomes 3 and 6. The closest SSR markers were RM4512 and RM3034, respectively. One putative QTL associated with basal-white kernels in the F 2 population was identified on chromosome 6. The closest marker was RM3034. Two putative QTLs associated with whiteback kernels in the F 3 population grown under high temperature conditions in paddy fields in 2004 were identified on chromosomes 4 and 6. The closest SSR markers were RM3288 and RM3034, respectively. One putative QTL associated with white-back plus basal-white kernels in the F 3 population grown under high temperature stress in the greenhouse was identified on chromosome 6. The closest marker was RM3034. The QTLs identified near RM3034 on the short arm of chromosome 6 contributed most to the apparent quality of brown rice. The QTLs identified near RM4512 and RM3288 which also affected the apparent quality of brown rice, were detected in either the F 2 or F 3 population. The QTLs identified in the present study should be useful for marker-assisted selection to breed varieties with a high apparent quality of brown rice, especially varieties with tolerance to kernel damage due to high temperature stress during the ripening period.

Starch is a biologically and commercially important polymer of glucose. Starch is organized into starch grains (SGs) inside amyloplasts. The SG size differs depending on the plant species and is one of the most important factors for industrial applications of starch. There is limited information on genetic factors regulating SG sizes. In this study, we report the rice (Oryza sativa) mutant substandard starch grain6 (ssg6), which develops enlarged SGs in endosperm. Enlarged SGs are observed starting at 3 d after flowering. During endosperm development, a number of smaller SGs appear and coexist with enlarged SGs in the same cells. The ssg6 mutation also affects SG morphologies in pollen. The SSG6 gene was identified by map-based cloning and microarray analysis. SSG6 encodes a protein homologous to aminotransferase. SSG6 differs from other rice homologs in that it has a transmembrane domain. SSG6-green fluorescent protein is localized in the amyloplast membrane surrounding SGs in rice endosperm, pollen, and pericarp. The results of this study suggest that SSG6 is a novel protein that controls SG size. SSG6 will be a useful molecular tool for future starch breeding and applications.

Decline in the apparent quality of rice (Oryza sativa L.) grain due to high temperatures during ripening recently became a major concern in many areas in Japan. The occurrence of white-back kernels (WBK) is one of the main problems of heat-induced quality decline. We identified QTLs associated with the occurrence of WBK using recombinant inbred lines (RILs) and verified their effects using near-isogenic lines (NILs). The QTL analysis used F7 and F8 RILs derived from ‘Hana-echizen’ (HE), which is tolerant to high temperature, × ‘Niigata-wase’ (NW), which is sensitive to high temperature. Four QTLs were identified on chromosomes 3, 4, 6, and 9 (qWB3, qWB4, qWB6 and qWB9). To verify the effects of qWB6 and qWB9, we developed two NILs in which qWB6 or both were introduced from HE into the NW background. The HE allele at qWB6 significantly decreased WBK under multiple environments. The combination of qWB6 and qWB9 in an F2 population derived from a cross between a NIL and NW showed that the NW allele at qWB9 significantly decreased WBK if the qWB6 allele was HE. These results will be of value in marker-assisted selection for the breeding of rice with tolerance to heat-induced quality decline.

To establish a marker-assisted selection system for the eating quality of cooked rice, we performed quantitative trait locus (QTL) analysis using 188 recombinant inbred lines (RILs) derived from crosses of two japonica cultivars, 'Sakihikari' and 'Nipponbare'. The former has excellent eating quality with strong stickiness whereas the latter is less sticky. We evaluated the stickiness of cooked rice using a sensory test, as well as amylose content (AC), amylographic characteristics (AM), and days-to-heading of RILs, which affect stickiness, in 2005, 2006 and 2007, and used them for QTL analysis. Six QTLs for stickiness were identified on chromosomes 1, 3 (two QTLs), 6, 7 and 8. Of these, qST3-1 on the short arm of chromosome 3 was detected throughout the three-year experiments but the others were detected in one year only. We also mapped two QTLs for AC, 19 QTLs for AM, and three QTLs for days-to-heading. Stickiness of RILs was significantly correlated with AM, and several QTLs for AM were detected in the same QTL regions for stickiness. These results thus suggested that AM was important for the stickiness of RILs. The obtained QTL information is useful to identify DNA markers tightly linked to genes controlling eating quality.