Using mRNA differential display analysis, we isolated a salt-induced transcript that showed a significant sequence homology with an EREBP/AP2 DNA binding motif from oilseed rape plants. With this cDNA fragment as a probe, cDNA clone Tsi1 (for Tobacco stress-induced gene1 ) was isolated from a tobacco cDNA library. RNA gel blot analysis indicated that transcripts homologous with Tsi1 were induced not only in NaCl-treated leaves but also in leaves treated with ethephon or salicylic acid. Transient expression analysis using a Tsi1 :: smGFP fusion gene in BY-2 cells indicated that the Tsi1 protein was targeted to the nucleus. Fusion protein of Tsi1 with GAL4 DNA binding domain strongly activated transcription in yeast, and the transactivating activity was localized to the 13 C-terminal amino acids of Tsi1. Electrophoretic mobility shift assays revealed that Tsi1 could bind specifically to the GCC and the DRE/CRT sequences, although the binding activity to the former was stronger than that to the latter. Furthermore, Agrobacteriummediated transient expression and transgenic plants expressing Tsi1 demonstrated that overexpression of the Tsi1 gene induced expression of several pathogenesis-related genes under normal conditions, resulting in improved tolerance to salt and pathogens. These results suggest that Tsi1 might be involved as a positive trans -acting factor in two separate signal transduction pathways under abiotic and biotic stress.
INTRODUCTIONPlants respond to adverse environmental stress and pathogen attack by expressing specific genes and synthesizing a large number of stress proteins that have putative roles in stress adaptation and plant defense (Skriver and Mundy, 1990;Reymond and Farmer, 1998). The signals that mediate systemic responses must be transmitted rapidly throughout the plant and may involve cell-to-cell signaling. Putative systemic signals include ethylene (Ecker and Davis, 1987), salicylic acid (Durner et al., 1997), jasmonic acid (Farmer and Ryan, 1990), and abscisic acid (Zeevaart and Creelman, 1988). Communication between these plant hormones might modulate the expression of abiotic and biotic stress-responsive genes in plants. However, the interactions between these hormone-mediated signal pathways and molecular mechanisms governing their cross-regulation have remained generally unresolved.Ethylene is a phytohormone that affects virtually all stages of plant development, including seed germination, cell elongation, cell fate, fruit ripening, senescence, and abscission.Ethylene also is a key regulator that mediates a plant's response to biotic and abiotic stresses (O'Donnell et al., 1996; Lund et al., 1998). Both ethylene and salicylic acid, a plant defense signal, accumulate in plants during pathogen infection (Penninckx et al., 1996). Among the different classes of ethylene-responsive genes, the most extensively studied are those activated by ethylene in response to pathogen attack. This class includes basic chitinase,  -1,3-glucanases, defensins, and other pathogenesis-related (PR)...