Ascorbate Peroxidases: Crucial Roles of Antioxidant Enzymes in Plant Stress Responses

Ascorbate peroxidases (APXs) are, in general, photosynthetic eukaryote-specific enzymes, which catalyze the reduction of H2O2 using ascorbate as an electron donor. Considering the very low affinity of ascorbate with H2O2, the acquisition of APX was certainly an important event, allowing plants to use ascorbate for H2O2 metabolism. This also provides a plausible explanation for why plants accumulate a massive amount of ascorbate because this substrate is also required for stabilizing fragile APX enzymes (particularly chloroplastic isoforms). In higher plants, APXs are distributed in the cytosol, mitochondria, chloroplasts (both stroma and thylakoid membrane), and peroxisomes to modulate organellar and cellular levels of H2O2. Despite its potential toxicity, H2O2 is a relatively stable form of a reactive oxygen species, and consequently it can act as a key signaling molecule for plant stress responses. From this point of view, APXs also have a dual role, being antioxidant enzymes and H2O2 signaling regulators, and their balance is crucial for fine-tuning stress responses. In this chapter, we describe the physiological roles of APX isoforms in plants by overviewing the findings of biochemical, physiological, and genetic studies.