Auxin and H2O2 play vital roles in plant development and environmental responses; however, it is unclear whether and how H2O2 modulates auxin levels. Here, we investigate this question using cat2-1 mutant, which exhibits reduced catalase activity and accumulates high levels of H2O2 under photorespiratory conditions. At a light intensity of 150 μmol m−2 s−1, the mutant exhibited up-curled leaves that have increased H2O2 contents and decreased auxin levels. At low light intensities (30 μmol m−2 s−1), the leaves of the mutant were normal, but exhibited reduced H2O2 contents and elevated auxin levels. These findings suggest that H2O2 modulates auxin levels. When auxin was directly applied to cat2-1 leaves, the up-curled leaves curled downwards. In addition, transformation of cat2-1 plants with pCAT2:iaaM, which increases auxin levels, rescued the hyponastic leaf phenotype. Using qRT-PCR, we demonstrated that the transcription of auxin synthesis-related genes and of genes that regulate leaf curvature is suppressed in cat2-1. Furthermore, application of glutathione rescued the up-curled leaves of cat2-1 and increased auxin levels, but did not change H2O2 levels. Thus, the hyponastic leaves of cat2-1 reveal crosstalk between H2O2 and auxin signalling that is mediated by changes in glutathione redox status.