About this Research Topic
Polyphenol oxidases (PPOs), also known as tyrosinases and catechol oxidases, are copper enzymes which catalyse hydroxylation of monophenols to ortho-diphenols and the subsequent oxidation of ortho-diphenols to quinones. Plant and fungal PPO genes are nuclear encoded and plant proteins are generally targeted to plastids while fungal proteins are mainly cytosolic. While PPOs appear to have a role in biotic and abiotic stress responses, there still remains a great deal of uncertainty about their biological function.
PPO enzymes have intriguing properties which suggest other functions than those commonly postulated in the literature. These enzymes are generally considered to have a role in damaged tissues where mechanisms including quinone toxicity, a reduction in the nutritive value of proteins and the formation of structural barriers can offer protection against pathogens. Distinct subcellular compartmentation of plant PPOs and their phenolic substrates in healthy tissues supports this hypothesis. However plant PPOs are co-localised with Photosystem II proteins within the chloroplastic thylakoid system which is a highly metabolically active site, subject to extreme fluctuations in redox state. While several authors have speculated about a chloroplastic role for PPO this has not been substantiated to date. Both plant and fungal PPOs are present in both active and latent states and the reasons for this observed latency remain to be elucidated. Another significant, but as yet unexplained, feature of PPOs can be readily observed in leaf, root tuber and fruit tissues, where a close relationship exists between PPO gene expression and starch biosynthesis. There is also evidence for a biosynthetic role for PPOs in fruit and other tissues. An aspect of PPOs which is also worth consideration is the relationship between these enzymes and haemocyanins which are involved in transporting oxygen in circulatory systems of invertebrates. The high degree of homology in their active sites and the fact that haemocyanins exhibit PPO activity under certain conditions suggest some common mechanisms and may help explain latent properties of PPO enzymes.
The increasing number of sequenced genomes and advances in protein modelling, metabolite profiling and micro-spectroscopy techniques provide scope to further elucidate the physiological role of PPOs in plant tissues. We therefore propose to assemble a series of articles, which will re-evaluate potential roles of PPO in physiological processes in healthy plant and fungal tissues. These articles will reflect new developments and ways of thinking about this complex topic.
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