Abstract
Polyphenol Oxidases (PPOs) are copper-containing enzymes that use molecular oxygen to catalyze the oxidation of monophenolic and ortho-diphenolic compounds (Fig. 1). Both PPOs and their phenolic substrates are widespread among higher plants and fungi. The PPO-generated oxidation products, o-qui-nones, spontaneously undergo further reactions to form compounds responsible for the commonly observed browning of injured or diseased plant tissues.1 The physiological significance of PPO and these browning processes in plants has remained enigmatic despite intense research efforts, summarized in numerous reviews.2–7 Although a wealth of studies has attempted to elucidate the role of PPO in pathogen defense,2,8–10 to date the most convincing case has been made for a function of PPO in defense against insects.7,11–14 Interestingly, PPO is important for both insect entrapment and antinutritive defensive mechanisms, suggesting that PPO is a versatile enzyme which may have other, yet to be discovered functions.
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