Since we reviewed this area in 2001 ( Turner et al., 2001), the field has grown enormously. Mechanically, the SCWs can be compared with reinforced concrete, with the cellulose microfibrils analogous to steel rods that are embedded in an amorphous matrix of lignin. Typical SCWs are composed of cellulose (40–80%), hemicellulose (10–40%), lignin (5–25%), and cell wall proteins. As the major component of wood and also grass biomass, these lignified cell walls are the major source of plant biomass, and it is these lignified SCWs that are the focus of this review. Most SCWs, however, are associated with woody tissue. The SCWs of some cell types, such as collenchyma, are cellulose rich and contain little or no lignin. While primary cell walls are formed by all plant cells, SCWs are produced only by some specialized cell types that are normally required either to support the plant or for water transport. Once cells reach their final size and shape, many will produce a thicker secondary cell wall (SCW). Actively growing cells are surrounded by a thin primary cell wall that can be altered to allow for expansion. While knowledge of individual components in the pathway has improved dramatically, how they function together to make the final polymers and how these individual polymers are incorporated into the wall remain less well understood.īiofuels, cellulose, lignin, lignin engineering, regulation, SCW, xylan, xylan engineering IntroductionĮvery plant cell is contained within the cell walls. In this review, we will attempt to highlight the most recent progress in understanding the biosynthetic pathways for secondary cell wall components, how these pathways are regulated, and how this knowledge may be exploited to improve cell wall properties that facilitate breakdown without compromising plant growth and productivity. SCWs are largely composed of three main polymers: cellulose, hemicellulose, and lignin. Partly as a consequence of this renewed economic importance, excellent progress has been made in understanding how cell wall components are synthesized. As the main constituent of plant biomass, secondary cell walls are central to attempts to generate second-generation biofuels. Secondary cell walls (SCWs) are produced by specialized plant cell types, and are particularly important in those cells providing mechanical support or involved in water transport.
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