THE FUNCTION AND METABOLISM OF ASCORBIC-ACID IN PLANTS

Ascorbate is a major metabolite in plants. It is an antioxidant and, i n association with other components of the antioxidant system, protect s plants against oxidative damage resulting from aerobic metabolism, p hotosynthesis and a range of pollutants. Recent approaches, using muta nts and transgenic plants, are providing evidence for a key role for t he 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 r egulating photosynthetic electron transport has been proposed. The bio synthetic pathway of ascorbate in plants has not been identified and e vidence for the proposed pathways is reviewed. Ascorbate occurs in the cell wall where it is a first line of defence against ozone. Cell wal l ascorbate and cell wall-localized ascorbate oxidase (AO) have been i mplicated in control of growth. High AC activity is associated with ra pidly expanding cells and a model which links wall ascorbate and ascor bate oxidase to cell wall extensibility is presented. Ascorbate has al so been implicated in regulation of cell division by influencing progr ession from G1 to S phase of the cell cycle. There is a need to increa se our understanding of this enigmatic molecule since it could be invo lved in a wide range of important functions from antioxidant defence a nd photosynthesis to growth regulation. (C) 1996 Annals of Botany Comp any