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This pathway forms a metabolic crossroad with multiple branching points leading to the formation of different complex phenolics ( Barros et al., 2015 Tohge et al., 2017). In plants, phenylalanine and tyrosine establish the starting point of the C 6C 3 phenylpropanoid pathway. It produces simple phenolic and aromatic amino acids. The majority of known phenolic metabolites derive from the shikimate pathway present in plants, prokaryotes, fungi and some protists. Smaller phenolics such as salicylic acid, tannins, (neo)lignans or phytoalexins act as chemical or olfactory signals to coordinate responses to environmental factors and biotic interactions ( Treutter, 2006).
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Lignin and other structural phenolic polymers accumulate in cuticle, seed coat, and vascular system to enable plant vertical growth, resistance to desiccation and herbivores, as well as long distance water transport ( Barros et al., 2015). Phenolic pigments, like melanins and flavonoids, are antioxidants that protect all major prokaryotic and eukaryotic taxa against UV radiation and reactive oxygen species and function as visual signals to pollinators or seed dispersers in plants ( Cheynier et al., 2013 Carletti et al., 2014). Phenolics provide solutions to many of the difficulties posed by terrestrial habitats, and their chemical diversification is closely associated with the transition to life on land ( Stafford, 2000). Phenolic compounds form a large and heterogeneous group of primary and secondary metabolites that contain at least one hydroxylated aromatic ring. Altogether, this review provides a basis to better understand the functional overlaps and specificities between and within the three major families of phenoloxidases, their evolutionary trajectories, and their importance for plant primary and secondary metabolism. With these approaches, we generated a framework to explain the reported functional differences between paralogs, while also hinting at the likely diversity of yet undescribed LAC functions. Using the example of laccases (LACs), we also performed a meta-analysis of enzyme kinetics, a comprehensive phylogenetic analysis and machine-learning based protein structure modelling to link functions, evolution, and structures in this group of phenoloxidases. In this review, we provide an overview of the differences and similarities between phenoloxidases in their protein structure, reaction mechanism, substrate specificity, and functional roles. In contrast to other biopolymers, whose polymerisation is catalysed by homologous gene families, polyphenolic metabolism depends on phenoloxidases, a group of heterogeneous oxidases that share little beyond the eponymous common substrate. The metabolism of polyphenolic polymers is essential to the development and response to environmental changes of organisms from all kingdoms of life, but shows particular diversity in plants. Arrhenius Laboratories, Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.