The Salt-Stress Signal Transduction Pathway That Activates the gpx1 Promoter Is Mediated by Intracellular H₂O₂, Different from the Pathway Induced by Extracellular H₂O

Several genes encoding putative glutathione peroxidase have been isolated from a variety of plants, all of which show the highest homology to the phospholipid hydroperoxide isoform. Several observations suggest that the proteins are involved in biotic and abiotic stress responses. Previous studies on the regulation of gpx1, the Citrus sinensis gene encoding phospholipid hydroperoxide isoform, led to the conclusion that salt-induced expression of gpx1 transcript and its encoded protein is mediated by oxidative stress. In this paper, we describe the induction of gpx1 promoter:uidA fusions in stable transformants of tobacco (Nicotiana tabacum) cultured cells and plants. We show that the induction of gpx1 by salt and oxidative stress occurs at the transcriptional level. gpx1 promoter analysis confirmed our previous assumption that the salt signal is transduced via oxidative stress. We used induction of the fusion construct to achieve better insight into, and to monitor salt-induced oxidative stress. The gpx1 promoter responded preferentially to oxidative stress in the form of hydrogen peroxide, rather than to superoxide-generating agents. Antioxidants abolished the salt-induced expression of gpx1 promoter, but were unable to eliminate the induction by H2O2. The commonly employed NADPH-oxidase inhibitor diphenyleneiodonium chloride and catalase inhibited the H2O2-induced expression of gpx1 promoter, but did not affect its induction by salt. Our results led us to conclude that salt induces oxidative stress in the form of H2O2, its production occurs in the intracellular space, and its signal transduction pathway activating the gpx1 promoter is different from the pathway induced by extracellular H2O2.