PPOX and infection: In the process of protecting plants from pathogen infection, the key enzymes of the phenylalanine pathway, such as phenylalanine ammonia-lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO), are involved in the biosynthesis of lignin metabolites, which provides a structural barrier against fungi (Dixon et al., 2002; Mohammadi and Kazemi, 2002).