Polyphenol oxidases (PPOs) reveal a range of forms and occur in all plants and crops. PPOs are comprised of three enzymes (catecholase, laccase, cresolase) with very different activities and specificities. Cresolase has a dualistic form (cresolase is only in plants and tyrosinase is only in animals and microorganisms). Very often in the literature the generic word "PPO" is used inappropriately as one enzyme. This should be avoided in future studies, as clear systematics and correct nomenclature of PPOs are needed for proper research. PPOs have different substrate specificities and typical inhibitors, and they catalyze hydroxylation and oxidation processes in plants. Pigment formation in cells and cellular systems is affected by active PPOs. Catecholases, laccases and cresolases are encoded by nuclear genes of plants. Various PPO DNA sequences have been found, and PPOs occur in multiple gene families. The protective potential of PPOs in plants and enhanced herbivory resistance is debated, and the final evidence has not yet appeared. The activity of PPOs in germination is recognized, but its mechanism is still not clear. Seed testa coloration in Arabidopsis thaliana is effected by laccase and not by catecholase. The TT10 gene encoding laccase in the Arabidopsis seed testa has been isolated. Arabidopsis genome analysis led to the identification of 16 other putative laccases and their genes, named AtLAC1 to AtLAC17 according to their position in the genome. Challenging areas of research for the future are seed testa PPOs and their mobilization in endosperm and micropylar regions, and PPOs as a part of the plant defense system and immunity.
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