Glutathione oxidation in response to intracellular H₂O₂: Key but overlapping roles for dehydroascorbate reductases

Glutathione is a pivotal molecule in oxidative stress, during which it is potentially oxidized by several pathways linked to H₂O₂ detoxification. We have investigated the response and functional importance of 3 potential routes for glutathione oxidation pathways mediated by glutathione S-transferases (GST), glutaredoxin-dependent peroxiredoxins (PRXII), and dehydroascorbate reductases (DHAR) in Arabidopsis during oxidative stress. Loss-of-function gstU8, gstU24, gstF8, prxIIE and prxIIF mutants as well as double gstU8 gstU24, gstU8 gstF8, gstU24 gstF8, prxIIE prxIIF mutants were obtained. No mutant lines showed marked changes in their phenotype and glutathione profiles in comparison to the wild-type plants in either optimal conditions or oxidative stress triggered by catalase inhibition. By contrast, multiple loss of DHAR functions markedly decreased glutathione oxidation triggered by catalase deficiency. To assess whether this effect was mediated directly by loss of DHAR enzyme activity, or more indirectly by upregulation of other enzymes involved in glutathione and ascorbate recycling, we measured expression of glutathione reductase (GR) and expression and activity of monodehydroascorbate reductases (MDHAR). No evidence was obtained that either GRs or MDHARs were upregulated in plants lacking DHAR function. Hence, interplay between different DHARs appears to be necessary to couple ascorbate and glutathione pools and to allow glutathione-related signaling during enhanced H₂O₂ metabolism.