We have investigated the regulation of the rice (Oryza sativa) gene OsSAMDC, which encodes an S-adenosyl-L-methionine decarboxylase (SAMDC) involved in polyamine biosynthesis. Clones representing genes differentially expressed at 5 degrees C and 20 degrees C were isolated from a cDNA library prepared from the chilling-tolerant japonica-type cultivar Yukihikari. The full-length OsSAMDC cDNA consists of 1560 bp, with the longest ORF encoding a polypeptide of 398 amino acids. Southern analysis indicated that there are two types of gene for SAMDC in the Yukihikari genome. Analysis of the expression of OsSAMDC by Northern hybridization revealed relatively high levels of mRNA in the leaves, nodes and internodes. We also analyzed the response of OsSAMDC to various abiotic stress treatments and plant hormones. Upon exposure to cold stress (5 degrees C) the level of OsSAMDC transcripts in the cold-resistant Yukihikari genotype continued to increase for up to 72 h. In contrast, there was no change in OsSAMDC transcription in the susceptible indica cultivar TKM9 under the same conditions. Ethephon induced the accumulation of OsSAMDC transcripts to similar extents in both genotypes. Examination of polyamine levels in the cold-resistant Yukihikari genotype revealed that spermidine levels were elevated during the course of cold treatment. These results suggest that the induction of the OsSAMDC gene in response to cold may be used as a molecular marker for the ability of rice seedlings to withstand exposure to low temperatures.