Editorial: Rising stars in plant metabolism and chemodiversity 2022 - phenylpropanoid metabolism and regulation

COPYRIGHT © 2023 Zhao, Liu, Lin and Li. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. TYPE Editorial PUBLISHED 21 February 2023 DOI 10.3389/fpls.2023.1159100

Phenylpropanoids are a diverse group of specialized metabolites that contribute to the basic process of plant growth and development, as well as to the plant-environment interactions. They are biosynthesized from the shikimic acid pathway via the aromatic amino acid phenylalanine or tyrosine in certain plants (Deng and Lu, 2017). These amino acids provide phenylpropanoids a C6-C3 (a phenyl group linked to a 3-C propane side chain) skeleton, producing derivatives with one, two or more aromatic rings, each ring with a variable substitution pattern and with different modifications of the C3 side chain.
The phenylpropanoid metabolism yields a huge quantity of compounds with multiple biological activities, such as flavonoids (e.g. flavanones, flavones, flavonols, flavanols, anthocyanidins, and isoflavones), which participate in organ pigmentation, UV protection and plant-microbe interactions; lignin, which is involved in the mechanical support and waterproofing of plant cell walls; condensed tannins, which give the fruit and its products important organoleptic properties, like astringency, bitterness, and colour stability; and phytoalexins, which are active in resisting herbivores and infectious pathogens (Deng and Lu, 2017). Along with their biological functions, phenylpropanoids are economically significant metabolites. They are of interest for their numerous pharmacological and industrial applications, for example, several of which are considered high-value biochemicals used in the production of perfumes, pharmaceuticals, and biopolymers (Lin and Eudes, 2020). Moreover, the phenylpropanoid-based polymers such as lignin and suberin are considered potential targets for creating recalcitrant forms of carbon in plants, towards carbon sequestration in soil and biomass (Eckardt et al., 2023). Hence, phenylpropanoids research is beneficial to provide a promising plant-based solution for carbon neutrality. Over the past decades, elaborate molecular mechanisms for regulating the phenylpropanoid pathway at multiple levels have been extensively studied. Phenylpropanoid metabolism has been revealed to be modulated by multiple regulatory mechanisms, including transcriptional, post-transcriptional, post-translational, and epigenetic regulation, and through a variety of signaling pathways, such as phytohormone, biotic stress, and abiotic stress signaling pathways (Dong and Lin, 2021). This Research Topic sought to collect recent findings in all aspects of phenylpropanoid metabolism and regulation.
Phenylpropanoids exhibit extraordinary complexity and highlevel plasticity in different species, developmental stages and in response to environmental stimuli. Significantly, many of phenylpropanoids can be specific to only one or a few plant species; it's, therefore, necessary to have comprehensive analyses of phenylpropanoids among various species to expand our knowledge gained mainly from model plant species. Wang et al. In addition to transcriptional regulation, there is growing interest in the role of epigenetic regulation in controlling phenylpropanoid metabolism. Epigenetic regulations, such asDNA methylation and demethylation, have been implicated in anthocyanin biosynthesis and associated with pigmentation. Lu et al. (in this Research Topic) showed the methylation status of CHH on the transposon element near the XsMYB113-1 influenced its expression and dynamic epigenetic regulation of the XsMYB113-1 affected anthocyanins buildup along with color changes during yellowhorn flower development.
Environment stimuli frequently trigger phenylpropanoids biosynthesis. Therefore, lignin biosynthesis can be affected by environmental variations due to climate change. Chen et al. (in this Research Topic) studied the relationship between lignin biosynthesis and 19 environmental factors in natural birch. They discovered that the lignin content in birch wood was negatively correlated with climate temperature. They also showed that DNA methylation levels in the promoter regions of two key NAC TFs, BpNST1/2 and BpSND1, were variable in birch trees grown in different environments, and suggest that DNA hypermethylation may repress the expression of these genes and thus negatively regulate lignin biosynthesis. This study provides evidence that environmental signals can lead to epigenetic variations that cause changes in lignin biosynthesis.
What these four research articles have in common is that their research objects are all non-model plants of significant economic and ecological value. As more plant species for which full-genome sequences are becoming available, it will benefit the studies to explore the functions of phenylpropanoid metabolites in more plants and enrich the diversification of phenylpropanoid research on a multi-species level.
Phenylpropanoid metabolism is often connected to a complex interaction between cell signaling and environmental influences. The studies in the current Research Topic have greatly advanced our understanding of phenylpropanoid metabolic regulation in plants via the exploration of metabolic diversity, transcriptional regulation, epigenetic modification, and environmental interaction. In the future, the ability to bridge the fields of molecular biology, epigenetics, chemistry, evolutionary biology, and molecular ecology to resolve the complexity of phenylpropanoid metabolism cooperatively will provide high-resolution information on its metabolic regulatory networks.

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