A Molecular Blueprint of Lignin Repression

Abstract

Although lignin is essential to ensure the correct growth and development of land plants, it may be an obstacle to the production of lignocellulosics-based biofuels, and reduces the nutritional quality of crops used for human consumption or livestock feed. The need to tailor the lignocellulosic biomass for more efficient biofuel production or for improved plant digestibility has fostered considerable advances in our understanding of the lignin biosynthetic pathway and its regulation. Most of the described regulators are transcriptional activators of lignin biosynthesis, but considerably less attention has been devoted to the repressors of this pathway. We provide a comprehensive overview of the molecular factors that negatively impact on the lignification process at both the transcriptional and post-transcriptional levels. Challenging LigninLignin is a major cell wall component that fulfills fundamental functions in plant development as well as in defense against pests and pathogens. This heteropolymer impregnates the compound middle lamella and the secondary cell walls (SCWs) of cells genetically programmed to be lignified, such as xylem tracheary elements as well as xylem and phloem fibers. Lignin biosynthesis spans the phenylpropanoid and the monolignol pathways, from phenylalanine to p-coumaryl, coniferyl, and sinapyl alcohols. Monolignol homeostasis is regulated through a balance between the molecular factors that antagonistically regulate their biosynthesis. After monolignols are excreted into the apoplast, they are oxidized by the phenol-oxidoreductase laccases and/or by class III peroxidases, and are polymerized by radical coupling. A complex regulatory network involving phytohormones, transcription factors (TFs), and post-transcriptional events guide SCW deposition and lignification [1]. This network prevents lignin deposition in tissues undergoing active division or elongation such as apical meristems, vascular cambium, and immature xylem cells, as well as in non-lignified tissues such as stem pith and flax or hemp bast fibers that harbor gelatinous walls under normal developmental conditions. Similarly, this network negatively regulates lignification in xylem tissue formed in response to mechanical stresses such as, for instance, poplar tension wood that harbors hypolignified gelatinous walls.

Grapevine VlbZIP30 improves drought resistance by directly activating VvNAC17 and promoting lignin biosynthesis through the regulation of three peroxidase genes

“…Similarly, Quan et al 19 used a cis -regulatory motif analysis and speculated that eight TF families (MYB/SANT, bHLH, AT-Hook, TCR, TBP, HD-ZIP, C2H2, and bZIP) interact with lignin biosynthesis genes in poplar. Among these families, the regulation of lignin biosynthesis by MYB and NAC TFs has been thoroughly analyzed 29 – 31 , and recently, the involvement of bHLH, WRKY, ERF, and HD-ZIP family genes in wood formation was also investigated 1 , 28 , 32 , 33 . However, to date, little is known about how bZIP TFs are involved in lignin biosynthesis.…”

Phosphoproteome analysis reveals an extensive phosphorylation of proteins associated with bast fiber growth in ramie

“…Accordingly, the findings of these previous studies provide evidence to indicate that KNOX proteins play potentially roles in the regulation of fiber growth. Notably, KNOX proteins can repress the expression of lignin biosynthetic genes, either directly by binding to the promoters of these genes or indirectly by forming heterodimers with BELL proteins, thereby negatively regulating lignin biosynthesis [ 44 ]. In the present study, we found that the ramie fiber development-regulated KNOX protein whole_GLEAN_10029667 underwent phosphorylated modification during the fiber growth stage, and identified an associated kinase (whole_GLEAN_10021119) and phosphatase (whole_GLEAN_10002382).…”

Comparative transcriptomics of tropical woody plants supports fast and furious strategy along the leaf economics spectrum in lianas

“…We have identified MYB46, MYB4/MYB308, and MYB75 reported to be involved in lignin repression downregulated in liana leaves [117] (Table 2, SI Fig. S3).…”

“…S3). MYB46 targets a set of 13 genes in lignin biosynthesis [74, 93, 117–119]. A repressor of endoreduplication called E2Fc is also known to bind to the promoter of MYB46 and suggests a potential crosstalk with the cell cycle regulator FBL-17 [93, 118].…”