Variation in Lignin Content and Composition (Mechanisms of Control and Implications for the Genetic Improvement of Plants)

Abstract

a complex phenolic polymer, is important for mechanical support, water transport, and defense in vaseular plants. Compressive strength and hydrophobicity of xylem cell walls are imparted by the lignin polymer, which is deposited during the terminal differentiation of tracheids and other cell types. The resistance of xylem to compressive stresses imposed by water transport and by the mass of the plants is important to growth and development. In addition, the insolubility and complexity of the lignin polymer makes it resistant to degradation by most microorganisms. Therefore, lignin serves an important function in plant defense. Variation in lignin content, composition, and location is likely to affect these essential processes. The constraints on the amount, composition, and localization of lignin for normal xylem function and plant defense are not known.Lignin composition, quantity, and distribution also affect the agroindustrial uses of plant material. Digestibility and dietary conversion of herbaceous crops are affected by differences in lignin content and composition (Akin et al., 1986. Lignin is an undesirable component in the conversion of wood into pulp and paper; remova1 of lignin is a major step in the paper making process. Furthermore, the resistance of lignin to microbial degradation enhances its persistence in soils. Lignin is, therefore, a significant component in the global carbon cycle.The mechanisms of control of lignin composition and quantity have wide implications regarding the adaptation and evolution of land plants and provide a basis for improved genetic manipulation of lignin for agroindustrial end uses. In this Update, we will focus on the levels of control of lignin variation, including (a) metabolic control, (b) regulation of individual enzymes in the biosynthetic pathway, and (c) regulation of gene expression. These levels of regulation affect variation in lignin content, quality, and distribution. Finally, the implications of these regulatory mechanisms for the genetic improvement of lignin for Raleigh, North Carolina 27695-8008 agroindustrial products will be described. Lignin structure, biosynthesis, degradation, and the regulation of lignification have been extensively reviewed (Higuchi, 1985(Higuchi, ,1990Lewis and Yamamoto, 1990;Chen, 1991;Sederoff et al., 1994). L l G N l N IS A COMPLEX A N D HIGHLY VARIABLE BIOPOLYMERLignin is a complex hydrophobic network of phenylpropanoid units that is thought to result from the oxidative polymerization of one or more of three types of hydroxycinnamyl alcohol precursors (Higuchi, 1985). These alcohols, 4-hydroxycinnamyl alcohol, coniferyl alcohol, and sinapyl alcohol, give rise to p-hydroxyphenyl, guaiacyl, and syringyl lignins, respectively (Fig. 1). The three monolignol precursors differ in the extent of methoxylation. This variety of subunit substitution patterns means that a variety of intermolecular linkages can be formed during polymerization (Freudenberg and Neish, 1968;Lewis and Yamamoto, 1990). Lignin, therefore, varies in its subun...

Roles of silica and lignin in horsetail (Equisetum hyemale), with special reference to mechanical properties

“…16 Lignin functions in binding other cell wall components such as cellulose microfibrils, and hemicelluloses and gives rigidity and/or mechanical strength to plants. 17,18 Mutants with less lignin content showed decreased mechanical strength. [19][20][21] Another function is reportedly to make xylem cell walls more hydrophobic.…”

“…[19][20][21] Another function is reportedly to make xylem cell walls more hydrophobic. The lignin content of horsetail is reportedly low (about 11-13%), 8 compared with pine trees (26-30%), 18 for example. Therefore, it has been speculated that one function of lignin for providing strength has been taken over by silica in horsetail.…”

4-Coumarate:Coenzyme A Ligase in Hybrid Poplar1

“…Several genes encoding enzymes of phenylpropanoid metabolism and lignin biosynthesis have been cloned from poplar species, including PAL (Subramaniam et al, 1993;Osakabe et al, 1995), C4H (Ge and Chiang, 1996;Kawai et al, 1996), bispecific caffeic acid-Omethyltransferase (COMT; Bugos et al, 1991;Dumas et al, 1992), and CAD (van Doorsselaere et al, 1995b). The monomer composition of angiosperm lignin, including that of poplar, varies according to cell type and stage of tissue development (Campbell and Sederoff, 1996). The lignin in poplar secondary xylem (wood) is composed primarily of guaiacyl units, presumed to be derived from coniferyl alcohol and feruloyl-CoA, and syringyl units, presumed to be derived from syringyl alcohol and sinapoyl-CoA.…”

Expression of three phenylpropanoid pathway genes in Scots pine (Pinus sylvestris L.) in open-pollinated families with differing relative wood densities during early and late wood formation

“…PAL activity increases in P. taeda cells treated with plant growth regulators (EBER- HARDT et al, 1993), as well as in elicited P. banksiana cells (CAMPBELL and ELLIS, 1992). Elevated PAL activity may be necessary in order to redirect sufficient phenylalanine towards phenylpropanoid biosynthesis because of the importance of this pathway both for normal development and plant defence (CAMPBELL and SEDEROFF, 1996). Single nucleotide polymorphisms in the PAL1 gene are associated with wood density in P. radiata (DILLON et al, 2010).…”

“…Lignin biosynthesis derives from the general phenylpropanoid pathway (DIXON et al, 2002; VAN-HOLME et al, 2010). Because of the high heterogeneity of lignin among species and within a single plant, the regulation of the lignin biosynthesis pathway may vary among cell types and species, and the activity of lignin biosynthesis entry-point enzymes are likely to affect metabolite flux in the biosynthesis pathway (CAMPBELL and SEDEROFF, 1996).…”