Rice gene Oryza sativa Drought Stress Response-1 (OsDSR-1) was one of the genes identified to be responsive to drought stress in the panicle of rice at booting and heading stages by both microarray and quantitative real-time PCR analyses. OsDSR-1 encodes a putative calcium-binding protein, and its overexpression in Arabidopsis rendered transgenic plants to produce much shorter lateral roots (LRs) than wild-type (WT) plants in the medium supplemented with abscisic acid (ABA), suggesting that OsDSR-1 may act as a positive regulator during the process of ABA inhibition of LR development. No significant difference was observed in the total LR length between WT and transgenic plants in the media with the increase of only osmotic stress caused by NaCl, LiCl, and mannitol, while transgenic Arabidopsis seedlings appeared to produce larger root systems with longer total LR lengths under highpotassium conditions than WT seedlings. Further analysis showed that external $Ca^{2+}$ was required for the production of larger root systems, indicating that the promotion by OsDSR-1 of the LR development of transgenic Arabidopsis seemed to occur in a $Ca^{2+}$-dependent manner under high-potassium conditions. We propose that OsDSR-1 may function as a calcium sensor of the signal transduction pathway controlling the LR development under high-potassium conditions.