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Extracellular adenosine 5′-triphosphate (ATP) is a well-known signalling molecule in plants and plays important roles during plant stress response. In the present work, the treatment of tobacco cell suspension culture with exogenous hydrogen peroxide (H2O2) caused the decreases of respiratory O2 uptake, intracellular ATP production, extracellular ATP level, and reduction of cell viability. Combining this observation with the finding that the oxidative phosphorylation uncoupler or the respiratory inhibitor not only decreased intracellular ATP production but also decreased the extracellular ATP level and cell viability, it is suggested that the decrease of extracellular ATP level and cell viability under H2O2 stress could be a result of the suppressed production of intracellular ATP. Treatment with ATP-degrading enzyme, apyrase, also caused the decreases of cell viability, respiratory O2 uptake, and intracellular ATP production. More importantly, addition of exogenous ATP alleviated the H2O2-induced decreases of cell viability, respiratory O2 uptake, and production of intracellular ATP. These results indicate that extracellular ATP could be an important effector in regulating the cell viability, respiratory O2 uptake, and intracellular ATP production of tobacco cell suspension culture under H2O2 stress. [ABSTRACT FROM AUTHOR]