Class III Peroxidases: Functions, Localization and Redox Regulation of Isoenzymes

Class III peroxidases (POXs; EC. 1.11.1.7), are secretory, multifunctional plant enzymes that catalyze the oxidation of a variety of substrates by hydrogen peroxide (H2O2). They show a remarkable diversity of isoenzymes, are encoded by a large number of paralogous genes, and are involved in a broad range of metabolic processes throughout plant growth and development. Peroxidases isoenzymes are located in the cell wall, apoplast and vacuole, and may be either soluble or ionically and covalently cell wall bound. They are involved in cell wall cross-linking and loosening, lignification and suberization, auxin catabolism and secondary metabolism. Due to their ability to control the levels of reactive oxygen species (ROS), POXs are efficient components of the antioxidative system induced in response to environmental stress, such as pathogen attack, metal excess, salinity, drought and high light intensity. In addition to the peroxidative function, POXs can catalyze H2O2 production in the oxidative cycle. Peroxidases are responsible either for cell elongation or cell wall stiffening, affecting carbon allocation, auxin level and redox homeostasis, which implicates their key role as being in the regulation of growth and defence under stress condition. This chapter will discuss novel insights into the functions of PODs with special emphasis on their localization, substrate specificity and the regulation of redox homeostasis.