Plant peroxidases (POD) reduce hydrogen peroxide (H(2)O(2)) in the presence of an electron donor. Extracellular POD can also induce H(2)O(2) production and may perform a significant function in responses to environmental stresses via the regulation of H(2)O(2) in plants. We previously described the isolation of 10 POD cDNA clones from cell cultures of sweetpotato (Ipomoea batatas). Among them, the expression of the swpa4 gene was profoundly induced by a variety of abiotic stresses and pathogenic infections (Park et al. in Mol Gen Genome 269:542-552 2003; Jang et al. in Plant Physiol Biochem 42:451-455 2004). In the present study, transgenic tobacco (Nicotiana tabacum) plants overexpressing the swpa4 gene under the control of the CaMV 35S promoter were generated in order to assess the function of swpa4 in planta. The transgenic plants exhibited an approximately 50-fold higher POD specific activity than was observed in control plants. Both transient expression analysis with the swpa4-GFP fusion protein and POD activity assays in the apoplastic washing fluid revealed that the swpa4 protein is secreted into the apoplastic space. In addition, a significantly enhanced tolerance to a variety of abiotic and biotic stresses occurred in the transgenic plants. These plants harbored increased lignin and phenolic content, and H(2)O(2 )was also generated under normal conditions. Furthermore, they showed an increased expression level of a variety of apoplastic acidic pathogenesis-related (PR) genes following enhanced H(2)O(2) production. These results suggest that the expression of swpa4 in the apoplastic space may function as a positive defense signal in the H(2)O(2)-regulated stress response signaling pathway.