Amylopectin branch-chain length distribution is a key determinant of rice starch functionality and eating quality, yet the genetic basis underlying specific chain fractions remains incompletely understood in diverse germplasm. In this study, amylopectin fine structure was quantified in 137 accessions of the Korean Rice Core Selection (KRICE_Core) using HPAEC-PAD, and genome-wide association studies were conducted with 2.1 million high-quality SNPs under the FarmCPU model. The short-chain distribution (SCD, DP 6-12) varied from 26.85% to 37.20%, whereas the intermediate-chain distribution (ICD, DP 12-24) ranged from 52.57% to 61.04%, and the two fractions showed a strong inverse correlation. GWAS identified two major loci on chromosomes 4 and 6, with the chromosome 6 region showing exceptionally strong association signals and co-localizing with Os06g0229800 (SSIIa). Haplotype analysis of SSIIa detected five non-synonymous SNPs defining five haplotypes. Hap1 and Hap5 were associated with increased SCD and reduced ICD, whereas indica-related haplotypes (Hap2-Hap4) showed the opposite trend, indicating that SSIIa allelic variation quantitatively shifts the balance between short and intermediate chain fractions. In a panel of 113 Korean-bred cultivars, Hap1 and Hap5 together accounted for > 99% of genotypes, suggesting strong directional selection for japonica-type SSIIa alleles during Korean breeding. These findings provide robust molecular targets for designing rice varieties with tailored starch architecture and cooking qualities.

We performed a molecular marker-based analysis of quantitative trait loci (QTLs) for the traits that determine appearance quality of the grains using 120 double haploid (DH) lines developed by anther culture from F₁ hybrid of a cross between Cheongcheong (Oryza sativa L. ssp. Indica) and Nagdong (Oryza sativa L. ssp. Japonica). The traits studied included grain length, grain width, grain thickness, length to width ration and thousand grain weight. This experiment was conducted with two replications of 2013 and 2014. A linkage map with 217 DNA markers was constructed spanning across 2,067.1 centiMorgans (cM) at an average interval of 9.5 cM between markers and covering 12 rice chromosomes. Interval mapping procedure was used to identify the QTLs controlling five grain traits and QTLs detected were further confirmed using composite interval mapping. A total of 24 QTLs affecting grain appearances were identified and mapped on all the twelve chromosomes for 2 years at grain quality. Nine of the 24 QTLs were reproducibly detected in two year trials. Major QTL for grain length was detected on chromosome 5 in 2013 with a phenotypic variation of 64% and chromosome 7 in 2014 that explained 55% of the phenotypic variation. The QTL findings in this study will in future faciltate the gene isolation and breeding application for improvement of rice grain shape and yield.

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