Secondary Cell Walls

Abstract Secondary cell walls are the major constituent of tracheary elements and fibres in wood, which is the most abundant biomass produced by plants. They provide strong mechanical strength to tracheary elements and fibres, and ultimately to plant organs. The principal components of secondary walls are cellulose, hemicellulose and lignin. Cellulose microfibrils together with hemicelluloses form the main load‐bearing network in secondary walls, in which lignin is impregnated to form another crosslinked network to provide hydrophobicity and more rigidity. The biosynthesis of secondary walls is a highly coordinated developmental process that involves a coordinated expression of secondary wall biosynthetic genes regulated by a cascade of transcription factors. Because secondary walls in the form of wood and fibres are the most abundant, renewable plant products, understanding how they are constructed will provide novel strategies for genetic improvement of wood and fibres to better suit our needs. Key concepts Cell walls are the plant ‘exoskeleton’ that dictates the cell shape and collectively the plant form. The ability for plant cells to make secondary cell walls is considered to be one of the most important evolutionary landmarks for vascular plants. Secondary cell walls are the main constituent of tracheary elements and fibres of wood, which is the most abundant biomass produced by land plants. Secondary cell walls in tracheary elements and fibres provide mechanical strength to plant organs. Secondary cell walls in tracheary elements are deposited in specific patterns, including helical, annular, scalariform, reticulated and pitted patterns. The principal components of secondary cell walls are cellulose, hemicelluloses and lignin. Cellulose is synthesized by the cellulose synthase complexes located at the plasma membrane. Hemicelluloses, including glucomannan and xylan, are synthesized at the Golgi and then secreted via vesicles into the cell walls. Lignin is synthesized through dehydrogenative polymerization of monolignols, p ‐coumaryl alcohol, coniferyl alcohol and syringyl alcohol, which is catalysed by oxidases including peroxidases and laccases. The biosynthesis of secondary cell walls is regulated by a transcriptional network comprising of a cascade of transcription factors.