SummaryPhotorespiration is a light-dependent source of H 2 O 2 in the peroxisomes, where concentrations of this signalling molecule are regulated by catalase. Growth of Arabidopsis knock-out mutants for CATALASE2 (cat2) in ambient air caused severely decreased rosette biomass, intracellular redox perturbation and activation of oxidative signalling pathways. These effects were absent when cat2 was grown at high CO 2 levels to inhibit photorespiration, but were re-established following a subsequent transfer to air. Growth of cat2 in air at different daylengths revealed that photoperiod is a critical determinant of the oxidative stress response. Decreased growth was observed in 8-h, 12-h and 16-h photoperiods, but lesion development was dependent on long days. Experiments at different light fluence rates showed that cell death in cat2 was linked to long days and not to total light exposure or the severity of oxidative stress. Perturbed intracellular redox state and oxidative signalling pathway induction were more prominent in short days than in long days, as evidenced by glutathione status and induction of defence genes and oxidative stress-responsive transcripts. Similar daylength-dependent effects were observed in the response of mature plants transferred from short days in high CO 2 conditions to ambient air conditions. Prior growth of plants with short days in air alleviated the cat2 cell-death phenotype in long days. Together, the data reveal the influence of photoperiodic events on redox signalling, and define distinct photoperiod-dependent strategies in the acclimation versus cell-death decision in stress conditions.

Glutathione is a major cellular thiol that is maintained in the reduced state by glutathione reductase (GR), which is encoded by two genes in Arabidopsis (Arabidopsis thaliana; GR1 and GR2). This study addressed the role of GR1 in hydrogen peroxide (H 2 O 2 ) responses through a combined genetic, transcriptomic, and redox profiling approach. To identify the potential role of changes in glutathione status in H 2 O 2 signaling, gr1 mutants, which show a constitutive increase in oxidized glutathione (GSSG), were compared with a catalase-deficient background (cat2), in which GSSG accumulation is conditionally driven by H 2 O 2 . Parallel transcriptomics analysis of gr1 and cat2 identified overlapping gene expression profiles that in both lines were dependent on growth daylength. Overlapping genes included phytohormone-associated genes, in particular implicating glutathione oxidation state in the regulation of jasmonic acid signaling. Direct analysis of H 2 O 2 -glutathione interactions in cat2 gr1 double mutants established that GR1-dependent glutathione status is required for multiple responses to increased H 2 O 2 availability, including limitation of lesion formation, accumulation of salicylic acid, induction of pathogenesis-related genes, and signaling through jasmonic acid pathways. Modulation of these responses in cat2 gr1 was linked to dramatic GSSG accumulation and modified expression of specific glutaredoxins and glutathione S-transferases, but there is little or no evidence of generalized oxidative stress or changes in thioredoxin-associated gene expression. We conclude that GR1 plays a crucial role in daylength-dependent redox signaling and that this function cannot be replaced by the second Arabidopsis GR gene or by thiol systems such as the thioredoxin system.