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.

Keywords: secondary cell walls; cell wall biosynthesis; cellulose; xylan; lignin

Figure 1.

(a) Vessels (ve) and xylary fibres (xf) are the principal cell types in Quercus wood. (b) Deposition of secondary cell walls enables the vessels in wild‐type Arabidopsis stems to withstand the negative pressure generated during water transport. (c) A reduction in the secondary cell wall thickening in the vessels of an Arabidopsis mutant results in deformation of vessels due to their inability to withstand the negative pressure generated during water transport.

Figure 2.

(a) Phloem fibres (pf) in flax stems deposit secondary cell walls (stained green) without lignin. Xylem cells (xy) contain lignified secondary cell walls (stained red). (b) Bundles of massive extraxylary fibres (ef) containing lignified secondary cell walls (stained red) below the epidermis of a yucca leaf. (c) The seed coat (sc) of jojoba has a protective layer of sclereids containing lignified secondary cell walls (stained red). (d) Endodermal cells (en) in a smilax root deposit lignified secondary cell walls. Sections were stained with safranin and fast green. Lignified cell walls were stained red.

Figure 3.

(a) The helical pattern of secondary cell walls in the vessels of protoxylem in an Arabidopsis stem. Note that the helical secondary cell walls in a vessel (arrow) were stretched during stem elongation. (b) Helical and reticulate secondary cell wall patterns in tracheary elements transdifferentiated from zinnia mesophyll cells.

Figure 4.

(a) Three distinct layers (S1, S2 and S3) of secondary cell walls in fibre cells of an Arabidopsis stem. (b) Cellulose microfibrils in the innermost layer of a mature fibre cell from an Arabidopsis stem are arranged in a flat helix. The vertical direction of the image corresponds to the elongation axis of the cell.

Figure 5.

Structural formula of secondary cell wall components including cellulose, hemicelluloses and monolignols.

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Further Reading

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How to Cite close
Zhong, Ruiqin, and Ye, Zheng‐Hua(Mar 2009) Secondary Cell Walls. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0021256]