Induction of PPO activity by S. exigua and M. sexta feeding. PPO activity was assayed in wild-type (blue) and OPR3-RNAi plants (green) before (C), 48 and 72 hours after insect feeding. (A) PPO induction by S. exigua. (B) PPO induction by M. sexta. Data were normalized to PPO levels in unwounded wild-type controls and represent the mean +/- SD of 2 to 3 independent experiments each with four leaf samples. Significant differences between wild-type and OPR3-RNAi plants are indicated (t-test; ** P < 0.01, *** P < 0.001).