Semi-dwarf traits have been widely introgressed into cereal crops to improve lodging resistance. In sorghum (Sorghum bicolor L. Moench), four major unlinked dwarfing genes, Dw1-Dw4, have been introduced to reduce plant height, and among them, Dw3 and Dw1 have been cloned. Dw3 encodes a gene involved in auxin transport, whereas, Dw1 was recently isolated and identified as a gene encoding a protein of unknown function. In this study, we show that DW1 is a novel component of brassinosteroid (BR) signaling. Sorghum possessing the mutated allele of Dw1 (dw1), showed similar phenotypes to rice BR-deficient mutants, such as reduced lamina joint bending, attenuated skotomorphogenesis, and insensitivity against feedback regulation of BR-related genes. Furthermore, DW1 interacted with a negative regulator of BR signaling, BRASSINOSTEROID INSENSITIVE 2 (BIN2), and inhibited its nuclear localization, indicating that DW1 positively regulates BR signaling by inhibiting the function of BIN2. In contrast to rice and wheat breeding which used gibberellin (GA) deficiency to reduce plant height, sorghum breeding modified auxin and BR signaling. This difference may result from GA deficiency in rice and wheat does not cause deleterious side effects on plant morphology, whereas in sorghum it leads to abnormal culm bending.In crop breeding, breeders have significantly changed plant stature during the selection of improved grain crops. One famous example is the introduction of a semi-dwarf trait into rice and wheat in the 1960s. Compared to normal plants, semi-dwarf plants have lower center of gravity, which increases lodging resistance and thus enables plant to sustain high grain yield. This phenomenon was later referred as the 'green revolution' . The mechanism of semi-dwarfism that contributed to the 'green revolution' was the introduction of mutated alleles of gibberellin (GA) 20-oxidase (semidwarf 1; sd1) in rice 1, 2 and DELLA (Rht) in wheat 3 , encoding a GA biosynthesis enzyme and a dominant repressor of GA signal transduction, respectively. Furthermore, it has also been reported that the semi-dwarfism of barley, caused by the introgression of semi-dwarf 1 (sdw1/denso) into cultivars grown in Europe, probably depends on a defect in an ortholog of rice SD1 4-6 . Such wide usage of GA-related mutations to produce semi-dwarf plants has been possible due to a unique feature of GA. That is, GA deficiency specifically causes a decrease in plant height without deleterious side effects on other morphologies or physiologies, whereas dwarfism caused by other mechanisms often induces undesired phenotypes, such as abnormal leaf structure, abnormal internode elongation, and stunted seeds [7][8][9][10] . Therefore, it is rare that mutations involved in mechanisms other than GA deficiency were used to improve lodging resistance in crop breeding. One rare exception is a semi-dwarf barley mutant containing semi-brachytic 1 (uzu), a weak allele of the brassinosteroid (BR) receptor, which is grown in a limited region of East Asia, including areas...