Induction of PPO activity by mechanical wounding and insect oral secretions. PPO activity was assayed in wild-type leaves 72 hours after mechanical wounding (W) or wounding with addition of insect (M. sexta or S. exigua) oral secretions (W + OS). OS were diluted 1:1 in water and applied in their native state (OSn) or after heat denaturation (OSd). Data are shown for one of three independent experiments, representing the mean +/- SD of four biological replicates each including pooled leaf material from three plants. Different letters indicate significant differences in PPO fold-induction normalized to unwounded controls (C; One-Way-ANOVA (F5,18 = 12.534, P < 0.001) and post-hoc Holm-Sidak for multiple comparisons at P < 0.05).