| Summary: | Tiller number is an important trait associated with yield in rice. Tiller number in Korean <i>japonica</i> rice was analyzed under greenhouse conditions in 160 recombinant inbred lines (RILs) derived from a cross between the temperate <i>japonica</i> varieties Odae and Unbong40 to identify quantitative trait loci (QTLs). A genetic map comprising 239 kompetitive allele-specific PCR (KASP) and 57 cleaved amplified polymorphic sequence markers was constructed. <i>qTN3</i>, a major QTL for tiller number, was identified at 132.4 cm on chromosome 3. This QTL was also detected under field conditions in a backcross population; thus, <i>qTN3</i> was stable across generations and environments. <i>qTN3</i> co-located with QTLs associated with panicle number per plant and culm diameter, indicating it had pleiotropic effects. The <i>qTN3</i> regions of Odae and Unbong40 differed in a known functional variant (4 bp TGTG insertion/deletion) in the 5ʹ UTR of <i>OsTB1</i>, a gene underlying variation in tiller number and culm strength. Investigation of variation in genotype and tiller number revealed that varieties with the insertion genotype had lower tiller numbers than those with the reference genotype. A high-resolution melting marker was developed to enable efficient marker-assisted selection. The QTL <i>qTN3</i> will therefore be useful in breeding programs developing <i>japonica</i> varieties with optimal tiller numbers for increased yield.
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