Physiological Role of Ascorbic Acid Recycling Enzymes in Plants

Abstract

Ascorbic acid (AsA) is oxidized to monodehydroascorbate (MDHA), which dissociates to form dehydroascorbate (DHA) instead of detoxifying reactive oxygen species (ROS). MDHA and DHA are directly reduced to AsA by two reductases, monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), respectively. They contribute to maintaining AsA contents and its redox status, which are dependent on the rate of its biosynthesis and recycling. The primary functions of MDHAR and DHAR appear to recycle AsA in the AsA-glutathione (GSH) cycles for diminishing ROS produced during photosynthesis in leaves. In fruits, MDHAR and DHAR might function in a complementary manner to maintain the AsA redox status during fruit development and ripening. Also, MDHAR and DHAR function as part of the AsA-GSH cycles in the different plant cellular compartments, like chloroplasts, mitochondria, and peroxisomes. Taking into account the physiological functions of AsA in plants, MDHAR and DHAR as AsA regenerators are paid much attention in engineering of stress tolerance and nutrient values. Transgenic plants overexpressing MDHAR and DHAR exhibit an increase in AsA contents and enhanced stress tolerance. This chapter focuses on the primary structures and gene expressions of plant MDHAR and DHAR isozymes as well as their contributions to the AsA contents in the leaves and fruits of plants. Also, this chapter provides the information about the roles of MDHAR and DHAR in the chloroplast, the cytosol, the guard cells, and stress tolerance in plants.