Ascorbate peroxidases (APX), localized in the cytosol, peroxisomes, mitochondria and chloroplasts of plant cells, catalyze the reduction of H(2)O(2) to water by using ascorbic acid (ASA) as specific electron donor. The chloroplastic isoenzymes of APX are involved in the water-water cycle, which contributes to the photophosphorylation coupled to the photosynthetic electron transport. In order to better clarify the contribution of thylakoidal APX (tAPX) to the reactive oxygen species (ROS) scavenging activity, as well as to the fine modulation of ROS for signaling, we produced Arabidopsis lines overexpressing tAPX. These lines show an increased resistance to treatment with the O(2)(-) generating herbicide Paraquat (Pq). However, when challenged with photoinhibitory treatments at high light or low temperature, or with iron (Fe) or copper (Cu) overload, the tAPX-overexpressing lines show no increased resistance with respect to controls, indicating that in such experimental conditions, tAPX overexpression does not reinforce plant defenses against the oxidative stresses tested. Interestingly, the nitric oxide (NO)-donor sodium nitroprusside (SNP) represses accumulation of tAPX transcript; SNP also partially inhibits tAPX enzymatic activity. After treatment with SNP, the tAPX-overexpressing lines show reduced symptoms of damage with respect to control plants treated with SNP. These transgenic lines confirm that H(2)O(2) acts in partnership with NO in causing cell death and highlight the important role of tAPX in the fine modulation of H(2)O(2) for signaling.