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We identified the rice floral organ development mutant, termed as open hull and male sterile 1 ( ohms1 ), from the progeny of the indica restorer line Zhonghui 8015 treated with 60Co γ-ray irradiation. The ohms1 mutant exhibited an open hull and lemma- and palea-like structure conversion between the anthers and stigma, which resulted in the ohms1 mutant spikelet showing ‘tridentate lemma’. The ohms1 mutant was entirely sterile but had 60%–70% fertile pollen. Genetic analysis and gene mapping showed that ohms1 was controlled by a single recessive gene, and the mutant gene was fine-mapped to a 42-kb interval on the short arm of chromosome 3 between markers KY2 and KY29. Sequence analysis of the four open reading frames in this region revealed that the mutant carried a single nucleotide transformation (A to G) at the last base of the fifth intron, which was likely corresponded to ohms1 phynotype, in an MIKC type MADS-box gene OsMADS1 (LOC_Os03g11614). Enzyme digestion and cDNA sequencing further indicated that the variable splicing was responsible for the deletion of the sixth exon in ohms1 , but no structural changes in the MADS domain or amino acid frame shifts appeared. Additionally, real-time fluorescent quantitative PCR analysis showed that the OsMADS1 expression level decreased significantly in the ohms1 mutant. The expression levels of rice flowering factors and floral glume development-related genes also changed significantly. These results demonstrate that OsMADS1 may play an important role in rice floral organ development, particularly in floral glume development and floret primordium differentiation. [ABSTRACT FROM AUTHOR]