Recent findings have suggested that reactive oxygen species (ROS) are important signaling molecules for regulating plant responses to abiotic and biotic stress and that there exist source- and kind-specific pathways for ROS signaling. In plant cells, a major source of ROS is chloroplasts, in which thylakoid membrane-bound ascorbate peroxidase (tAPX) plays a role in the regulation of H(2)O(2) levels. Here, to clarify the signaling function of H(2)O(2) derived from the chloroplast, we created a conditional system for producing H(2)O(2) in the organelle by chemical-dependent tAPX silencing using estrogen-inducible RNAi. When the expression of tAPX was silenced in leaves, levels of oxidized protein in chloroplasts increased in the absence of stress. Microarray analysis revealed that tAPX silencing affects the expression of a large set of genes, some of which are involved in the response to chilling and pathogens. In response to tAPX silencing, the transcript levels of C-repeat/DRE binding factor (CBF1), a central regulator for cold acclimation, was suppressed, resulting in a high sensitivity of tAPX-silenced plants to cold. Furthermore, tAPX silencing enhanced the levels of salicylic acid (SA) and the response to SA. Interestingly, we found that tAPX silencing-responsive genes were up- or down-regulated by high light (HL) and that tAPX silencing had a negative effect on expression of ROS-responsive genes under HL, suggesting synergistic and antagonistic roles of chloroplastic H(2)O(2) in HL response. These findings provide a new insight into the role of H(2)O(2)-triggered retrograde signaling from chloroplasts in the response to stress in planta.