Background: Organ size is determined by the number and size of cells, which are controlled by cell proliferation and cell expansion. Organ size in rice plays a crucial role in determination of rice yield. Cadmium (Cd), one of the major environmental pollutants, is harmful to human health. Cd-polluted rice grains have been a major source of human Cd-intake, thus reducing Cd-uptake into rice grains isimportant. Strigolactone (SL), a hormone, has been well-studied in terms of regulating plant architecture and in its signal perception, but its biological roles in cell proliferation, cell expansion and Cd-uptake have not been uncovered.Methods: We comprehensively investigated agronomic traits and cell size between WT and so1 . We treated WT and so1 with Cd, and analyzed Cd content in rice grain using inductively coupled plasma massspectrometry (ICP-MS). We used bulk segregation analysis for SO1 mapping, and analyzed sequences surrounding SO1 in rufipogo , indica, japonica and intermedia varieties of rice. Results: We identified a spontaneous rice mutant, small organs 1 ( so1) , which exhibited small organs,such as small leaves, panicles and grains. Cytological analysis indicated that the small organs of so1 were due to both impaired cell proliferation and expansion. We found that so1 grains highly accumulated Cd after Cd treatment. In the so1 mutant, an 8bp insertion resulting in a premature stop codon was identified in LOC_Os04g46470 ( High-Tillering Dwarf1 ) which is involved in Strigolactone (SL) synthesis. Because both htd1 and so1 mutants exhibited dwarf and excessive tillers, the 8bp insertion was responsible for the so1 phenotype. SO1/HTD1 was ubiquitously expressed, most highly in stem and sheath. Very few SNP polymorphisms surrounding SO1/HTD1 were identified in japonica group, whereas SNPs were abundant in the rufipogon, indica and intermedia groups.Discussion: Our work showed the novel biological roles of SO1/HTD1 in controlling rice organ size and Cd-uptake, and indicated that the SL signaling pathway is involved in cell proliferation, cell expansion, and Cd-uptake, a role completely different from the previously reported role of HTD1. Therefore, our study is helpful for understanding the molecular functions of SO1/HTD1 and SL during rice development.