The role of L-ascorbic acid recycling in responding to environmental stress and in promoting plant growth.

L-Ascorbic acid (Asc) is the most abundant water-soluble antioxidant in plants. It serves as a cofactor for enzymes involved in photosynthesis, hormone biosynthesis, and the regeneration of antioxidants such as α-tocopherol. Once used, Asc can be recycled by several different mechanisms. The short-lived monodehydroascorbate (MDHA) radical, produced following Asc oxidation, can be recycled following reduction by ferredoxin or monodehydroascorbate reductase (MDAR). MDHA can also undergo disproportionation into dehydroascorbate (DHA) and Asc. DHA can be recycled into Asc by dehydroascorbate reductase (DHAR) before it undergoes irrevocable hydrolysis. Through its recycling, Asc content and its redox state are maintained, which is critical under conditions of high demand, for example during high light or other stress conditions that increase reactive oxygen species (ROS) production. This review provides an overview of research in the last decade revealing the role that Asc recycling plays during germination, growth, and reproduction, as well as in response to environmental stress. These findings highlight the importance of DHAR- and MDAR-mediated mechanisms of Asc recycling in maintaining ROS at non-damaging levels while modulating ROS signalling function.