About the Author; 1: Introduction; References; 2: Distribution, Localization, and Structure of Plant Polyphenol Oxidases (PPOs); 2.1 Distribution and Localization of Plant PPOs; 2.1.1 Structure of Plant Polyphenol Oxidase; 2.2 Crystal Structure of Plant PPOs; 2.2.1 Structure of Sweet Potato (Ipomoea batatas) Catechol Oxidases; 2.2.2 Structure of Tickseed (Coreopsis grandiflora) Aurone Synthase (Catechol Oxidase); 2.2.3 Structure of Walnut (Juglans regia) Tyrosinase; References; 3: Physicochemical Properties of Polyphenol Oxidases; 3.1 Impact of pH on PPO Activity 3.2 Effects of Temperature on PPO Activity3.3 Substrate Specificity of PPOs; 3.4 Multiplicity of Plant PPOs; 3.5 Latency and Activation of PPOs; 3.6 Regulation of Polyphenol Oxidases; References; 4: Reaction Features of Polyphenol Oxidases; 4.1 Mechanism of Enzymatic Browning; 4.1.1 PPO Reaction Mechanism in N. crassa; 4.1.2 Reaction Mechanism of Catechol Oxidase from Ipomoea batatas; 4.1.3 Reaction Mechanism of Aurone Synthase from Coreopsis grandiflora; References; 5: Function(s)/Role(s) of Polyphenol Oxidases; 5.1 Role of PPOs in Plant Defense 5.1.1 Role of PPOs in Plant Resistance Against Pathogens5.1.2 Role of PPOs in Plant Resistance Against Insect Pests; 5.1.3 PPO Regulation in Response to Stress; 5.1.4 Mechanism of PPO Action in Plant Defense; 5.2 Role of PPOs in Biosynthesis of Specialized Metabolites; 5.2.1 Betalain Biosynthesis; 5.2.2 Aurone Biosynthesis; 5.2.3 Tyrosine Metabolism; 5.2.4 Lignan Biosynthesis; 5.2.5 Other Roles of PPOs in Plants; References; 6: Polyphenol Oxidase(s): Importance in Food Industry; 6.1 Control of Browning Reaction; 6.1.1 Chemical Control; 6.1.2 Physical Control; References 7: Advances in Polyphenol Oxidase (PPO) Research7.1 Genetic and Genomic Aspects of PPOs; 7.2 MicroRNA (miRNA) Technology in PPO; 7.3 miRNAs and Potato Tuber Browning; 7.4 miRNAs in Salvia miltiorrhiza; 7.5 Mutagenesis Studies of Polyphenol Oxidases; References