ABSTRACT
L-ascorbic acid is the most abundant hydrophilic antioxidant of higher plant cells, reaching levels of up to 10% of the soluble carbohydrate content of cells (Smirnoff and Pallanca, 1995). Much of our understanding of the biological functions of L-AA is based upon the well-known antioxidant character of this molecule, and L-AA occupies a central role in the protection of plant cells against abiotic and biotic stresses. However, L-AA also serves as a substrate for oxalate and tartrate biosynthesis. An indication of the importance of L-AA to cells is provided by its high intracellular concentrations and the recent isolation of an L-AA-deficient Arabidopsis thaliana mutant that is hypersensitive to ozone and to SO2 (Conklin et al., 1996, 1997). Yet, despite the central role of L-AA in the modulation of processes as diverse as (photo)oxidative defense, senescence, and cell growth, the biosynthetic route for L-AA has only recently been defined in plant tissues (Wheeler et al, 1998). This chapter will therefore look at the current understanding of plant L-AA biosynthesis and metabolism, as well as the biological functions of L-AA in relation to oxidative stress.
