Ascorbate (AsA), the most abundant water-soluble redox compound in plants and eukaryotic algae, has multiple functions. There is compelling genetic evidence that the biosynthesis of AsA proceeds via a D-mannose/L-galactose pathway and is the most significant source of AsA in plants. AsA plays important roles in antioxidative defense, particularly via the AsA/glutathione cycle. AsA peroxidase (APX) plays a central role in the cycle and is emerging as a key enzyme in cellular H(2)O(2) metabolism. Plants possess diverse APX isoenzymes in cellular compartments, including the chloroplast, cytosol, and microbody. In algae, however, the number and distribution of APX proteins are quite limited. Recent progress in molecular biological analysis of APX isoenzymes has revealed elaborate mechanisms for the tissue-dependent regulation of two chloroplastic APX isoenzymes by alternative splicing, and for redox regulation of cytosolic APX gene expression in response to light stress. Furthermore, transgenic plants overexpressing a chloroplastic APX isoenzyme enable us to evaluate the behavior of the enzyme under conditions of photo-oxidative stress. Molecular physiological analysis has revealed that cytosolic APX is part of the system modulating the cellular H(2)O(2) level in redox signaling.