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We have isolated a severe dwarf mutant derived from a Ds ( Dissociation) insertion mutant rice ( Oryza sativa var. japonica c.v. Dongjin). This severe dwarf phenotype, has short and dark green leaves, reduced shoot growth early in the seedling stage, and later severe dwarfism with failure to initiate flowering. When treated with bioactive GA3, mutants are restored to the normal wild-type phenotype. Reverse transcription PCR analyses of 22 candidate genes related to the gibberellin ( GA) biosynthesis pathway revealed that among 22 candidate genes tested, a dwarf mutant transcript was not expressed only in one Os KS2 gene. Genetic analysis revealed that the severe dwarf phenotype was controlled by recessive mutation of a single nuclear gene. The putative Os KS2 gene was a chromosome 4-located ent-kaurene synthase ( KS), encoding the enzyme that catalyses an early step of the GA biosynthesis pathway. Sequence analysis revealed that osks2 carried a 1-bp deletion in the ORF region of Os KS2, which led to a loss-of-function mutation. The expression pattern of Os KS2 in wild-type cv Dongjin, showed that it is expressed in all organs, most prominently in the stem and floral organs. Morphological characteristics of the dwarf mutant showed dramatic modifications in internal structure and external morphology. We propose that dwarfism in this mutant is caused by a point mutation in Os KS2, which plays a significant role in growth and development of higher plants. Further investigation on Os KS2 and other Os KS-like proteins is underway and may yield better understanding of the putative role of Os KS in severe dwarf mutants. [ABSTRACT FROM AUTHOR]