The cell biology of lignification in higher plants

Abstract

Background: Lignin is a polyphenolic polymer that strengthens and waterproofs the cell wall of specialized plant cell types. Lignification is part of the normal differentiation programme and functioning of specific cell types, but can also be triggered as a response to various biotic and abiotic stresses in cells that would not otherwise be lignifying.

Scope: Cell wall lignification exhibits specific characteristics depending on the cell type being considered. These characteristics include the timing of lignification during cell differentiation, the palette of associated enzymes and substrates, the sub-cellular deposition sites, the monomeric composition and the cellular autonomy for lignin monomer production. This review provides an overview of the current understanding of lignin biosynthesis and polymerization at the cell biology level.

Conclusions: The lignification process ranges from full autonomy to complete co-operation depending on the cell type. The different roles of lignin for the function of each specific plant cell type are clearly illustrated by the multiple phenotypic defects exhibited by knock-out mutants in lignin synthesis, which may explain why no general mechanism for lignification has yet been defined. The range of phenotypic effects observed include altered xylem sap transport, loss of mechanical support, reduced seed protection and dispersion, and/or increased pest and disease susceptibility.

Keywords: Arabidopsis thaliana; Lignin; laccases; lignification; monolignols; non-cell autonomous processes; peroxidases; plant cell wall.

Fig. 1.

Lignified cell types in higher plants. The role of lignin: as a transport barrier; in water and nutrient transport; for mechanical support; for seed protection and dispersion; and as a response to biotic and abiotic factors.

Fig. 2.

General phenylpropanoid pathway showing lignin biosynthesis gene mutations in Arabidopsis thaliana (REF3, REF8, CCR1, REF1 and FAH1) and respective side pathway reactions (coloured boxes) affected by the mutational change. Dotted boxes represent lignin monomers that are incorporated in the lignin polymer. PAL, phenylalanine ammonia-lyase; TAL, tyrosine ammonia-lyase; C4H, cinnamate 4-hydroxylase; 4CL, 4-coumarate:CoA ligase; C3H, p-coumarate 3-hydroxylase; HCT, p-hydroxycinnamoyl-CoA:quinate/shikimate p-hydroxycinnamoyltransferase; CSE, caffeoyl shikimate esterase; CCoAOMT, caffeoyl-CoA O-methyltransferase; CCR, cinnamoyl-CoA reductase; CAD, cinnamyl alcohol dehydrogenase; COMT, caffeic acid O-methyltransferase; F5H, ferulate 5-hydroxylase; PMT, p-coumaroyl-CoA:monolignol transferase; CAldh, coniferaldehyde dehydrogenase; UGT, UDP-glucosyltransferase; BGLU, β-glucosidase; SGT, sinapic acid:UDP-glucosyl sinapoyltransferase; SMT, sinapoylglucose:malate sinapoyltransferase.

Fig. 3.

Cell-specific expression of lignin monomer synthesis transcripts and proteins. Stem cross-sections of (A) arabidopsis, (B) Brachypodium and (C) Populus indicating the different cell types included in the studies in (D). Blue arrow, tracheary elements (TEs); orange arrow, xylem parenchyma (XPs); red arrow, ray parenchyma (RPs); purple arrow, xylary fibres (XFs). Scale bars = 100 μm. (D) Percentage of studies supporting cell-specific expression of lignin monomer biosynthesis genes in TEs, XPs, RPs or XFs. Numbers in the first column indicate the number of individual studies, respectively, for each gene.

Fig. 4.

Sub-cellular localization of lignin monomer biosynthesis proteins. (A) Scheme illustrating the lignin monomer biosynthesis pathway and monomer transport in respect to sub-cellular localization of the lignin monomer synthesis proteins. Tyr, tyrosine; Phe, phenylalanine. (B) Percentage of studies supporting specific subcellular localization in: cytosol, endoplasmic reticulum (ER), Golgi, plastids, vesicles, plasma membrane (PM) or cell wall (CW). Numbers in the first column indicate the number of individual studies, respectively, for each protein. ATR2, arabidopsis cytochrome P450 reductase 2.

Fig. 5.

General lignin polymerization. Lignin monomers are exported across the plasma membrane either by passive diffusion (PD), by exocytosis (vesicle) or through ABC transporters and/or proton-coupled antiporter (PCA). Laccases and peroxidases activate the monomer radicals, resulting in the end-wise addition of and/or cross-reaction of the radical oligo/monomers with the extending polymer(s). Classical production of H₂O₂ and O₂ derives from a two-step enzymatic process: NADPH oxidase and superoxide dismutase (SOD). Dirigent proteins (DiPs) stereospecifically restrict the radical coupling to one type of linkage. Manganese (Mn) acts as a redox shuttle to mediate radical activation (Onnerud et al., 2002). This schematic representation does not support equal importance or intervention of different possibilities used by specific cell types to form the lignin polymer.

Fig. 6.

Different levels of co-operative and autonomous lignification depending on the cell types studied: xylem tracheary elements, xylem fibres and endodermal cells. (A) Different stages of TE formation showing post-mortem TE lignification through neighbouring parenchyma cells that provide monolignols, laccases/peroxidases (LAC/PRX) and O₂/H₂O₂ produced by NADPH oxidase and superoxide dismutase (SOD). Living neighbouring TEs may also provide monolignols and incorporate LAC/PRX in the secondary cell wall before programmed cell death (PCD). (B) Differentiation and partial co-operative lignification of xylem fibres showing that monolignols, LAC/PRX and reactive oxygen species (ROS) are produced by both xylem fibres and neighbouring parenchyma cells. Living unlignified fibres may also provide monolignols. (C) Autonomous lignification in endodermal cells during Casparian strip formation, showing the formation of the Casparian strip domain by CASP proteins (Casparian strip membrane domain proteins) and the localization of ESB1 (enhanced suberin 1) in the Casparian strip zone. Later on, H₂O₂ (produced by NADPH oxidase and SOD), monolignols (exported by ABCG transporters) and PRX are supplied to the Casparian strip zone.