Revisiting the advancements in plant polyphenol oxidases research

Polyphenol oxidase (PPO), a copper-containing enzyme encoded by nuclear genes, is widely found in plants, animals, fungi, and bacteria. In the presence of oxygen, PPO catalyzes the oxidation of phenolic compounds in plants, leading to the formation of quinones that subsequently polymerize to produce melanin. This process ultimately results in enzymatic browning, which adversely affects the organoleptic quality and nutritional value of fruits and vegetables. To achieve a comprehensive and systematic understanding of PPOs, this paper reviews various aspects of PPO, concluding its protein structure, physicochemical properties, physiological functions (such as stress response mechanisms), the correlation between PPO activity and enzymatic browning, as well as strategies to alleviate browning by inhibiting PPO activity. Additionally, we discuss strategies to enhance PPO activity to help plants in coping with unfavorable growth conditions. Furthermore, we provide insights into future research directions for PPO. In conclusion, this comprehensive summary of the biological functions of PPO aims to establish a theoretical foundation for future plant research on plant stress tolerance and fruit quality.