Secondary Cell Walls
Two types of cell wall are produced by plants, which differ in both composition and function. The primary cell wall surrounds the dividing and growing cells and provides mechanical strength while allowing cells to grow and divide. The secondary walls are much thicker and stronger than the primary walls. However, both primary and secondary walls contain pectin, cellulose and hemicellulose, although in different proportions.
The secondary cell wall are the primary constituent of fibers and tracheary elements in wood, which is basically the most abundant biomass produced by plants.
Secondary walls provide strong mechanical strength not only to tracheary elements but to fibers as well and ultimately to various organs of plant. Their major components are lignin, cellulose and hemicellulose. The lignin is impregnated to cellulose microfibrils and hemicellulose to form another cross linked network to provide more rigidity and hydrophobicity. Both cellulose microfibrils and hemicellulose form the secondary wall’s main load-bearing network.
Cell Wall Biosynthesis
The biosynthesis of secondary walls is a highly coordinated developmental process wherein an involved coordinated expression of secondary wall biosynthetic genes is being regulated by a cascade of transcription factors. Due to the fact that secondary walls, in the form of fibers and wood, are the most abundant renewable plant products, it is vital to understand how such materials are being constructed for the purpose of genetic improvement to better suit the needs of humans.
Except for some reproductive cells, all plant cells are surrounded by primary cell walls. However, secondary cell walls are only present in specialized cells known as sclerenchyma that include tracheids and vessels –which are tracheary elements; as well as sclereids and fibers. They are most abundantly present in fibers of wood and tracheary elements, which provide mechanical strength in order to withstand the negative pressure that is being generated inside tracheary elements during transpiration. The major significance of seconday wall in the function of tracheary elements is shown by genetic mutants, showing that the deformation of vessels can result when there is defect in secondary cell wall deposition. This can further lead to wilting phenotypes and impaired water transport.