Polyphenol oxidase (PPO) enzymes cause the typical browning of plant extracts, mainly fruits, and damaged tissues. This is caused by the spontaneous polymerization and cross-linking of o-quinones. PPOs also produce ROS. These two paths occur as soon as the cell compartmentalization is disrupted, and PPO is released from the thylakoid to react with phenolic substrates from the vacuole. PPOs appear frequently upon wounding, and are therefore suggested to play a defensive role. For instance, PPO activity has been associated with resistance to L. decemlineata , Melanoplus spp. (grasshoppers) , and Lepidopteran larvae. Furthermore, down-regulation of PPOs in S. lycopersicum leaves lead to hypersensitivity to the Pseudomonas syringae bacteria, while overexpression increased the disease resistance. Moreover, the overexpression decreased the growth rate 2.5-fold in S. litura, and increased the mortality up to 3.3-fold. The reason for this is expected to be the highly reactive o-quinones, which will covalently modify free amino acids, thereby reducing the nutritive value of proteins . PPOs can also be combined with specific phenolic substrates in glandular trichomes to produce a kind of “super glue” to trap smaller insects .