Lignin (Latin lignum ‘wood’) is a polyphenolic polymer deposited directly in the cell wall of specialized cells. It is not only restricted to plant woody tissues but represents an integral feature ensuring the proper cellular function of many other cell types in different tissues/organs of the plant. The appearance of lignin during plant evolution coincided with the emergence of the vascular land plants in the Devonian. Mechanically weaker than cellulose, lignin nevertheless adds a significant reinforcement to any cell wall, providing an additional tensile strength.
Despite the fact that lignin is the second most abundant terrestrial biopolymer after cellulose, our understanding of lignin formation remains fragmentary. In contrast to cellulose, which presents a defined biochemical structure independently of the type of plant cell, lignin formation is cell specific and exhibits both distinct sub-cellular localization and monomeric composition: a general lignification mechanism cannot thus be drawn for all lignified cell types and may explain why lignification is still only partly understood. Our current biochemical understanding is that lignin forms in the spaces between the cellulose microfibrils by the oxidative coupling of free lignin monomers secreted directly into the plant cell wall.
A recent article in Annals of Botany reviews current understanding of lignin biosynthesis and polymerization and notes that depending on the cell type the lignification process ranges from full autonomy to complete co-operation with surrounding partner cells. 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.
Barros, J., Serk, H., Granlund, I., & Pesquet, E. (2015) The cell biology of lignification in higher plants. Annals of Botany 115(7): 1053-1074. doi: 10.1093/aob/mcv046
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. 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. 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.