Dark-induced decrease in ascorbate levels in Arabidopsis leaves occurs independently of ascorbate peroxidase and oxidase, recycling enzymes, and senescence signaling

Ascorbate is a key antioxidant that protects plant cells from oxidative damage. While plants actively synthesize ascorbate during the day, its degradation becomes prominent under prolonged dark conditions. Since ascorbate degradation begins with its oxidized form, dehydroascorbate (DHA), this process inherently requires ascorbate oxidation. However, the molecular mechanisms underlying dark-induced ascorbate oxidation and subsequent degradation remain unclear. In this study, we investigated the role of intracellular and extracellular ascorbate redox regulation in controlling this process. Using Arabidopsis knockout mutants for key enzymes involved in ascorbate oxidation and recycling, including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), and ascorbate oxidase (AO), as well as NADPH oxidases (rbohD and rbohF), we found that none of these enzymes significantly influenced the dark-induced decrease in ascorbate levels. Notably, ascorbate levels decreased similarly in newly generated multiple mutants, including a quintuple mutant (∆dhar pad2 mdar5), which has severely impaired ascorbate recycling capacity, and the ao2 rbohD double mutant, which is strongly expected to exhibit a highly altered apoplastic redox state. Furthermore, we examined the potential involvement of senescence signaling, including ORESARA1 and ethylene signaling components, but found no evidence for their contribution. These findings indicate that the dark-induced decrease in ascorbate levels is not governed by conventional pathways for ascorbate oxidation and recycling or senescence signaling processes, suggesting an unidentified regulatory mechanism.