BOTANICAL BRIEFING: The Function and Metabolism of Ascorbic Acid in Plants

Ascorbate is a major metabolite in plants. It is an antioxidantand, in association with other components of the antioxidantsystem, protects plants against oxidative damage resulting fromaerobic metabolism, photosynthesis and a range of pollutants.Recent approaches, using mutants and transgenic plants, areproviding evidence for a key role for the ascorbate–glutathionecycle in protecting plants against oxidative stress. Ascorbateis also a cofactor for some hydroxylase enzymes (e.g. prolylhydroxylase) and violaxanthin de-epoxidase. The latter enzymelinks ascorbate to the photoprotective xanthophyll cycle. Arole in regulating photosynthetic electron transport has beenproposed. The biosynthetic pathway of ascorbate in plants hasnot been identified and evidence for the proposed pathways isreviewed. Ascorbate occurs in the cell wall where it is a firstline of defence against ozone. Cell wall ascorbate and cellwall-localized ascorbate oxidase (AO) have been implicated incontrol of growth. High AO activity is associated with rapidlyexpanding cells and a model which links wall ascorbate and ascorbateoxidase to cell wall extensibility is presented. Ascorbate hasalso been implicated in regulation of cell division by influencingprogression from G1 to S phase of the cell cycle. There is aneed to increase our understanding of this enigmatic moleculesince it could be involved in a wide range of important functionsfrom antioxidant defence and photosynthesis to growth regulation. Ascorbic acid; ascorbate oxidase; cell division; cell wall; growth; oxidative stress; photosynthesis; ozone; vitamin C