In our previous work [1] we investigated the role of tomato GDP-mannose pyrophosphorylase (EC 2.7.7.22) in plants by overexpressing its gene in tobacco leaves and showed its function in AsA metabolism and detoxification of reactive oxygen species under temperature stresses. In this study, we use the antisense technique to block the endogenous GMPase gene expression in tobacco in order to further investigate its function. Northern and western blot analysis confirmed that the expression of endogenous tobacco GMPase mRNA and protein was inhibited by this antisense expression. Consequently, the activity of GMPase and the content of AsA in the leaves of antisense transgenic plants were markedly decreased. This was also the case for the activities of both chloroplastic SOD (superoxide dismutase EC 1.15.1.1), APX (ascorbate peroxidase EC 1.11.1.7) and the content of AsA in leaves of the transgenic plants. On the contrary, the contents of H(2)O(2) and O(2) (-•) were increased. Meanwhile, the net photosynthetic rate (Pn) and the maximal photochemical efficiency of PSII (Fv/Fm) also declined in the leaves of antisense plants. Under high or low temperature stresses, the seed germination rate of the antisense transgenic plants was significantly decreased in comparison with that of the wild-type tobacco. Interestingly, the antisense plants had smaller leaves and an earlier onset of flowering. In conclusion, the depletion of GMPase decreased the content of AsA, resulting in the plants susceptible to the oxidative damage caused by temperature stresses and subjected to developmental alternations.