Defensive Roles of Polyphenol Oxidase in Plants

Plant polyphenol oxidases (PPOs) are widely distributed and well-studied oxidative enzymes, and their effects on discoloration in damaged and diseased plant tissues have been known for many years. The discovery in C.A. Ryan’s laboratory in the mid-1990s that tomato PPO is induced by the herbivore defense signals systemin and jasmonate, together with seminal work on PPO’s possible effects on herbivorous insects by G. Felton and S. Duffey has motivated many studies of PPO in the context of plant-herbivore defense. The cloning and characterization of PPO cDNAs from multiple plant species now allows for direct testing of defensive functions of PPO using transgenic plants. These have shown that PPO can contribute to insect herbivore and pathogen resistance, although how this occurs is only now being investigated more closely. Here we review progress in the functional analysis of PPO in plant defense against pests, and describe recent results that address the mechanisms of PPO as an anti-herbivore protein. We suggest that assumptions of how PPO functions as an anti-nutritive defense against lepidopterans needs to be reexamined in light of the near anoxic conditions in lepidopteran midguts. Ultimately, the efficacy of PPO should be directly tested in a greater variety of plant-insect interactions. In addition, the identification of the endogenous PPO substrates will help to define defensive and potentially other roles for PPO in plants.