Leaf size is closely related to photosynthesis and greatly affects rice productivity. Therefore, the search for quantitative trait Locus (QTL) that regulates leaf size is important for improving productivity through rice breeding. The F₉ recombinant inbred line (RIL) population from the cross between ‘Unbong 40’ (large leaves) and ‘Odae’ (ordinary size leaves) was grown in a test field, and the size traits of the flag leaves and 2nd leaves, along with stem and panicle length, were measured. Through QTL analysis of these traits, we detected 20 QTLs on chromosomes 1, 3, 6, 9, 11, and 12. In particular, the flag leaf length QTL qFLL3, the flag leaf area QTL qFLA3, the 2nd leaf length QTL q2LL3-2, and the 2nd leaf area QTL q2LA3 were clustered in the 149.0-161.6 cM region of chromosome 3, indicating that one QTL gene in this region may have pleiotropic effects that regulate the flag leaf length, flag leaf area, 2nd leaf length, and 2nd leaf area. In addition, the flag leaf width QTL qFLW9 and 2nd leaf width QTL q2LW9 were clustered in the 2.1-22.86 cM region on chromosome 9, which suggests that one QTL gene in this region may have pleiotropic effects that regulate both the flag leaf width and 2nd leaf width. These results serve as a valuable reference for breeding programs aiming to increase photosynthesis and yield by identifying QTLs for leaf size traits in Korean japonica rice.

A dwarf mutant rice line was selected from an Ac/Ds insertion mutant population and named dwf1. The phenotype of F₁ and F₂ plants derived from a cross between dwf1 and Dongjin indicated that a single recessive gene is responsible for the mutant phenotype, and we named this gene dwf1. Resequencing of the dwf1 line and Dongjin (wild type) revealed 42,386 homozygous single nucleotide polymorphisms (SNPs) between dwf1 line and Dongjin. MutMap analysis was performed by sequencing a DNA pool prepared from 100 mutant type plants in the dwf1/Dongjin F₂ population, and it was found that the dwf1 gene was located in the 23 ~ 30 Mbp region on chromosome 4. In this region, we found a non-synonymous SNP in the Os04g0469800 gene, which was reported as D11 gene encoding a cytochrome P450 family protein involved in the biosynthesis of brassinosteroids (BRs). This SNP was regarded as the causative SNP for the dwf1 phenotype, and the dwf1 gene is a novel allele of D11. We performed mapping of the dwf1 gene with five SNP markers on chromosome 4 with 190 dwf1/Dongjin F₂ plants. The phenotype of F₂ plants was completely co-segregated with genotypes of the J10402 marker, which was developed based on the non-synonymous SNP in the D11 gene. These results will contribute to the study of the molecular biological functions of the D11 gene and BRs.