Ascorbic acid: metabolism and functions of a multi-facetted molecule.

Ascorbic acid (vitamin C) is the most abundant antioxidant in plants. Its biosynthetic pathway via GDP-D-mannose and L-galactose, which was proposed only recently, is now supported by molecular genetic evidence from Arabidopsis thaliana and transgenic potato plants. Except for the last step (which is located on the inner mitochondrial membrane) the pathway is cytosolic, sharing GDP-sugar intermediates with cell-wall polysaccharide and glycoprotein synthesis. Ascorbate peroxidase is emerging as a key enzyme in the fine control of H(2)O(2) concentration; its expression being controlled by redox signals and H(2)O(2). Convincing evidence of the involvement of ascorbate in cell division and growth is also accumulating. Its role as a cofactor in the synthesis of cell wall hydroxyproline-rich glycoproteins is one mechanism for this function.