Monodehydroascorbate reductases in plants: integrating ascorbate recycling, redox signaling, and stress adaptation.

Reactive oxygen species (ROS) are central regulators of plant growth, development, and environmental adaptation, with hydrogen peroxide (H2O2) acting as a key signaling molecule. The ascorbate-glutathione cycle is the major antioxidant pathway that maintains H2O2 homeostasis, and monodehydroascorbate reductase (MDHAR) plays a critical role by regenerating reduced ascorbate (ASC) from monodehydroascorbate (MDHA). Although traditionally viewed as an ASC-recycling enzyme, increasing evidence indicates that MDHAR has broader functions in redox regulation. Recent studies have revealed that MDHAR contributes to stress responses, developmental regulation, and ROS signaling through mechanisms that cannot be fully explained by ASC recycling alone. In this review, we summarize recent advances in the structural characteristics, regulation, subcellular specialization, and evolution of plant MDHARs. We also discuss emerging non-canonical functions of MDHAR, alternative pathways for MDHA reduction, and the mechanistic basis for the contrasting effects of MDHAR manipulation on ASC accumulation. Finally, we propose that MDHAR functions as an integrative hub linking ascorbate metabolism with cellular redox networks and highlight key challenges and future opportunities for exploiting MDHAR to improve crop stress tolerance, productivity, and nutritional quality.