SummaryIn plants, hydrogen peroxide (H2O2) plays a major signaling role in triggering both a defense response and cell death. Increased cellular H2O2 levels and subsequent redox imbalances are managed at the production and scavenging levels. Because catalases are the major H2O2 scavengers that remove the bulk of cellular H2O2, altering their levels allows in planta modulation of H2O2 concentrations. Reduced peroxisomal catalase activity increased sensitivity toward both ozone and photorespiratory H2O2‐induced cell death in transgenic catalase‐deficient Arabidopsis thaliana. These plants were used as a model system to build a comprehensive inventory of transcriptomic variations, which were triggered by photorespiratory H2O2 induced by high‐light (HL) irradiance. In addition to an H2O2‐dependent and ‐independent type of transcriptional response during light stress, microarray analysis on both control and transgenic catalase‐deficient plants, exposed to 0, 3, 8, and 23 h of HL, revealed several specific regulatory patterns of gene expression. Thus, photorespiratory H2O2 has a direct impact on transcriptional programs in plants.