Plants are always exposed to the menace of oxidative stress and protect themselves by activating a variety of defense responses. However, molecular mechanisms for oxidative stress-induced gene expression are largely unknown. We here studied the roles of the oxidative stress-responsive putative voltage-dependent Ca2+ permeable channels, NtTPC1A and NtTPC1B, and cell cycle in H2O2-induced expression of antioxidant enzymes, glutathione peroxidase (GPX) and ascorbate peroxidase (APX) in tobacco BY-2 cells. H2O2-induced [Ca2+](cyt) rise and expression of GPX and APX were inhibited by the cosuppression of NtTPC1A/B as well as Al ion, a specific blocker for NtTPC1s, and enhanced by overexpression of AtTPC1, suggesting that NtTPC1s are the major Ca2+-permeable channels activated by H2O2 and that Ca2+ influx via NtTPC1s is involved in induction of H2O2-triggered gene expression. Oxidative stress-induced signal transduction mechanisms were highly dependent on the phases of the cell cycle; H2O2-induced [Ca2+](cyt) rise and expression of GPX and APX as well as the level of NtTPC1s transcripts correlated with each other and were maximal at G1 phase. In contrast, the cell cycledependence of hypoosmotic shock-induced [Ca2+](cyt) rise that is known to be independent of NtTPC Is was almost reverse and maximal at S phase. These results suggest that the cell cycle-dependent regulation of oxidative stress-induced [Ca2+](cyt) rise and expression of NtTPC1s contribute to the cell cycle dependence of H2O2-induced expression of peroxidases. Various Ca2+-mediated signal transduction pathways are differentially regulated by the cell cycle. (c) 2005 Elsevier Inc. All rights reserved.