Activation of auxin signalling counteracts photorespiratory H2O2-dependent cell death.

The high metabolic flux through photorespiration constitutes a significant part of the carbon cycle. Although the major enzymatic steps of the photorespiratory pathway are well characterized, little information is available on the functional significance of photorespiration beyond carbon recycling. Particularly important in this respect is the peroxisomal catalase activity which removes photorespiratory H2O2 generated during the oxidation of glycolate to glyoxylate, thus maintaining the cellular redox homeostasis governing the perception, integration and execution of stress responses. By performing a chemical screen, we identified 34 small molecules that alleviate the negative effects of photorespiration in Arabidopsis thaliana mutants lacking photorespiratory catalase (cat2). The chlorophyll fluorescence parameter photosystem II maximum efficiency (Fv′/Fm′) was used as a high-throughput readout. The most potent chemical that could rescue the photorespiratory phenotype of cat2 is a pro-auxin that contains a synthetic auxin-like substructure belonging to the phenoxy herbicide family, which can be released in planta. The naturally occurring indole-3-acetic acid (IAA) and other chemically distinct synthetic auxins also inhibited the photorespiratory-dependent cell death in cat2 mutants, implying a role for auxin signalling in stress tolerance.