Ascorbate in the Apoplast: Metabolism and Functions

Ascorbic acid (AA) is one of the most important antioxidants and redox-active substances of plants found in the apoplast. In the form of ascorbate anion, it can be exported to the cell wall through anion channels, where it is further oxidized to dehydroascorbic acid (DHA) and in this form is translocated back into the cytoplasm through as yet unidentified transporters. In the apoplast AA is exposed to both enzymatic and non-enzymatic oxidation, participating in the antioxidant and pro-oxidant processes: detoxification of O₃, generation of hydroxyl radicals, reduction of phenoxyl radicals, etc. The intermediate product of the AA oxidation in apoplast, monodehydroascorbic acid (MDHA), serves as an extracellular electron acceptor for a number of redox chains localized at the plasma membrane (PM). All three forms of ascorbate, AA, DHA and MDHA, can potentially be involved in the cell signalling. Released from the cell through anionic channels during PM depolarization, AA can stimulate the production of hydroxyl radicals that activate Ca²⁺-permeable channels, and thus trigger Ca²⁺-signalling. Due to the ability to generate hydroxyl radicals and reduce phenoxyl radicals, AA affects cell wall extensibility and extension growth. By reducing Fe³⁺ to a freely soluble transport form of Fe²⁺, AA is involved in iron uptake by root cells. Apoplastic AA also plays an important role in a number of other processes and is potentially a unique signal-regulatory molecule functioning in the plant cell walls.