Abstract
The phenylpropanoid pathway serves as a rich source of metabolites in plants and provides precursors for lignin biosynthesis. Lignin first appeared in tracheophytes and has been hypothesized to have played pivotal roles in land plant colonization. In this review, we summarize recent progress in defining the lignin biosynthetic pathway in lycophytes, monilophytes, gymnosperms, and angiosperms. In particular, we review the key structural genes involved in p-hydroxyphenyl-, guaiacyl-, and syringyl-lignin biosynthesis across plant taxa and consider and integrate new insights on major transcription factors, such as NACs and MYBs. We also review insight regarding a new transcriptional regulator, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, canonically identified as a key enzyme in the shikimate pathway. We use several case studies, including EPSP synthase, to illustrate the evolution processes of gene duplication and neo-functionalization in lignin biosynthesis. This review provides new insights into the genetic engineering of the lignin biosynthetic pathway to overcome biomass recalcitrance in bioenergy crops.
Highlights
Phylogenetic Occurrence of the Phenylpropanoid Pathway and LigninThe phenylpropanoid pathway serves as a rich source of metabolites in plants and provides precursors for lignin biosynthesis
It is hypothesized that the first land plants possessed adaptive metabolic, physiologic, and morphologic changes as a means of coping with abiotic stresses, such as UV-B irradiation and desiccation (Niklas et al, 2017)
The other eight enzymes belong to lignin-specific pathway (Figure 1), including cinnamoyl CoA reductase (CCR), cinnamyl alcohol dehydrogenase (CAD), coumarate 3hydroxylase (C3H), coumaroyl shikimate 3 -hydroxylase (C3 H), ferulate/coniferaldehyde 5-hydroxylase (F5H), caffeate/5-hydroxy-coniferaldehyde 3/5-O-methyltransferase (COMT), caffeoyl CoA 3-O-methyltransferase (CCoAOMT), hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT), and caffeoyl shikimate esterase (CSE). p-coumaroyl CoA is converted into the simplest H-lignin monomer by a reductase CCR and a dehydrogenase CAD
Summary
The phenylpropanoid pathway serves as a rich source of metabolites in plants and provides precursors for lignin biosynthesis. Lignin first appeared in tracheophytes and has been hypothesized to have played pivotal roles in land plant colonization. We summarize recent progress in defining the lignin biosynthetic pathway in lycophytes, monilophytes, gymnosperms, and angiosperms. We review the key structural genes involved in p-hydroxyphenyl-, guaiacyl-, and syringyl-lignin biosynthesis across plant taxa and consider and integrate new insights on major transcription factors, such as NACs and MYBs. We review insight regarding a new transcriptional regulator, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, canonically identified as a key enzyme in the shikimate pathway. This review provides new insights into the genetic engineering of the lignin biosynthetic pathway to overcome biomass recalcitrance in bioenergy crops
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