Reactive oxygen species (ROS), such as H₂O₂, are impor-tant plant cell signaling molecules involved in responses to biotic and abiotic stresses and in developmental and physiological processes. Despite the well-known physio-logical functions of ethylene production and stress signaling via ROS during stresses, whether ethylene acts alone or in conjunction with ROS has not yet been fully elucidated. Therefore, we investigated the relationship between ethylene production and ROS accumulation during the response to abiotic stress. We used three independent transgenic tobacco lines, CAS-AS-2, -3 and -4, in which an antisense transcript of the senescence-related ACC synthase (ACS) gene from carnation flower (CARACC, GenBank accession No. M66619) was expressed heterologously. Biphasic ethylene biosynthesis was reduced significantly in these transgenic plants, with or without H₂O₂ treatment. These plants exhibited significantly reduced H₂O₂-induced gene-specific expression of ACS members, which were regulated in a time-dependent manner. The higher levels of NtACS1 expression in wild-type plants led to a second peak in ethylene production, which resulted in a more severe level of necrosis and cell death, as determined by trypan blue staining. In the transgenic lines, upregulated transcription of CAB, POR1 and RbcS resulted in increased photosynthetic performance following salt stress. This stress tolerance of H₂O₂-treated transgenic plants resulted from reduced ethylene biosynthesis, which decreased ROS accumulation via increased gene expression and activity of ROS-detoxifying enzymes, including MnSOD, CuZnSOD, and catalase. Therefore, it is suggested that ethylene plays a potentially critical role as an amplifier for ROS accumulation, implying a synergistic effect between biosynthesis of ROS and ethylene.

Keywords: abiotic stress, biphasic, ethylene, reactive oxygen species, tolerance