Vitamin C in Plants: Novel Concepts, New Perspectives, and Outstanding Issues

The concept that vitamin C (l-ascorbic acid) is at the heart of the peroxide processing and redox signaling hub in plants is well established, but our knowledge of the precise mechanisms involved remains patchy at best. Ascorbate participates in the multifaceted signaling pathways initiated by both reactive oxygen species (ROS) and reactive nitrogen species. Crucially, the apoplastic ascorbate/dehydroascorbate (DHA) ratio that is regulated by ascorbate oxidase (AO) sculpts the apoplastic ROS (apoROS) signal that controls polarized cell growth, biotic and abiotic defences, and cell to cell signaling, as well as exerting control over the light-dependent regulation of photosynthesis. Here we re-evaluate the roles of ascorbate in photosynthesis and other processes, addressing the question of how much we really know about the regulation of ascorbate homeostasis and its functions in plants, or how AO is regulated to modulate apoROS signals. The role of microRNAs in the regulation of AO activity in relation to stress perception and signaling must be resolved. Similarly, the molecular characterization of ascorbate transporters and mechanistic links between photosynthetic and respiratory electron transport and ascorbate synthesis/homeostasis are a prerequisite to understanding ascorbate homeostasis and function. Similarly, there is little in vivo evidence for ascorbate functions as an enzyme cofactor.