Recent advances in polyphenol oxidase-mediated plant stress responses

Plant polyphenol oxidases (PPOs) are ubiquitous copper metalloenzymes with a biochemistry that has been known for more than a century. By the 1990s, biologists began to recognize the importance of PPOs in plant response to the infestation of herbivores and pathogens; ideas concerning a defensive role for PPOs arose to address observed evidence, and several testable hypotheses were suggested. Two pivotal discoveries in tomato (Lycopersicon esculentum Miller) plants, an inverse correlation between PPO levels and insect growth and PPO induction by defence signals, have driven many studies of PPO defence functions in the context of abiotic and biotic stresses. During the past three decades, extensive molecular research in transgenic and non-transgenic systems has partly revealed the sophisticated mechanisms underlying PPO defence against herbivores and pathogens. These understandings, rather than theoretical predictions, have driven the development of new hypotheses and advanced PPO-related studies. Here, we review progress in PPO family features, expression regulation and the defensive role of PPOs in plants. We propose assumptions of an extended range of co- and post-transcriptional processes to the regulation of unexplored PPO expression. In addition, the identification of endogenous PPO substrates and downstream targets of PPO action will be useful for elucidating PPO defensive roles. The potential effects of PPO-mediated oxidative defences on herbivore performance ultimately needs to be further investigated. Therefore, expanding multidisciplinary approaches to unexplored dimensions of PPO defence function should be a future priority.

Automated summary

Plant polyphenol oxidases (PPOs) are important enzymes in plant defense against herbivores and pathogens, with extensive research uncovering their sophisticated mechanisms over the past few decades. Understanding the regulation and defensive roles of PPOs has driven the development of new hypotheses and advanced studies in this field. Further exploration of unexplored dimensions of PPO defense functions, such as co- and post-transcriptional processes, should be a future research priority.