Ascorbic acid in plants: Biosynthesis and function

Ascorbic acid (vitamin C) is an abundant component of plants. It reaches a concentration of over 20 mM in chloroplasts and occurs in all cell compartm ents, including the cell wall. It has proposed functions in photosynthesis as an enzyme cofactor (including synthesis of ethylene, gibberellins and an thocyanins) and in control of cell growth. A biosynthetic pathway via GDP-m annose, GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone has been proposed only recently and is supported by molecular genetic evidence from the ascorbate-deficient vtc1 mutant of Arabidopsis thaliana. Other pathways via uronic acids could provide minor sources of ascorbate. Ascorbate, at l east in some species, is a precursor of tartrate and oxalate. It has a majo r role in photosynthesis, acting in the Mehler peroxidase reaction with asc orbate peroxidase to regulate the redox state of photosynthetic electron ca rriers and as a cofactor for violaxanthin de-epoxidase, an enzyme involved in xanthophyll cycle-mediated photoprotection. The hypersensitivity of some of the vtc mutants to ozone and W-B radiation, the rapid response of ascor bate peroxidase expression to (photo)-oxidative stress, and the properties of transgenic plants with altered ascorbate peroxidase activity all support an important antioxidative role for ascorbate. In relation to cell growth, ascorbate is a cofactor for prolyl hydroxylase that posttranslationally hy droxylates proline residues in cell wall hydroxyproline-rich glycoproteins required for cell division and expansion. Additionally, high ascorbate oxid ase activity in the cell wall is correlated with areas of rapid cell expans ion. It remains to be determined if this is a causal relationship and, if s o, what is the mechanism. Identification of the biosynthetic pathway now op ens the way to manipulating ascorbate biosynthesis in plants, and, along wi th the vtc mutants, this should contribute to a deeper understanding of the proposed functions of this multifacetted molecule.