Among the 15 members of the poplar PPO gene family, levels of transcriptional expression of PtrPPO1, PtrPPO3, and PtrPPO11 were significantly induced by mechanical damage, the exogenous application of MeJA, or the infection of Melampsora laricipopulina, while others are developmentally regulated [48,57]. This evidence concerns the gene PPOX and infection.