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

Ascorbate is a major metabolite in plants. It is an antioxidant and, in association with other components of the antioxidant system, protects plants against oxidative damage resulting from aerobic metabolism, photosynthesis and a range of pollutants. Recent approaches, using mutants and transgenic plants, are providing evidence for a key role for the ascorbate-glutathione cycle in protecting plants against oxidative stress. Ascorbate is also a cofactor for some hydroxylase enzymes (e.g. prolyl hydroxylase) and violaxanthin de-epoxidase. The latter enzyme links ascorbate to the photoprotective xanthophyll cycle. A role in regulating photosynthetic electron transport has been proposed. The biosynthetic pathway of ascorbate in plants has not been identified and evidence for the proposed pathways is reviewed. Ascorbate occurs in the cell wall where it is a first line of defence against ozone. Cell wall ascorbate and cell wall-localized ascorbate oxidase (AO) have been implicated in control of growth. High AO activity is associated with rapidly expanding cells and a model which links wall ascorbate and ascorbate oxidase to cell wall extensibility is presented. Ascorbate has also been implicated in regulation of cell division by influencing progression from G1 to S phase of the cell cycle. There is a need to increase our understanding of this enigmatic molecule since it could be involved in a wide range of important functions from antioxidant defence and photosynthesis to growth regulation.