Lignin and Biomass: A Negative Correlation for Wood Formation and Lignin Content in Trees

Abstract

Studies in populations of forest tree hybrids have shown a negative correlation of biomass growth (usually measured as wood volume) and lignin content (Kirst et al., 2004; Novaes et al., 2009). The control of growth and lignin appears to be highly regulated, implying that selection for improved growth rate could effect a reduction in lignin content. Trees with increased biomass and reduced lignin would provide a yield advantage for pulp and paper production, as well as for production of biofuels. The purpose of this article is to briefly review the evidence for the correlation between lignin content and biomass growth and to discuss this correlation in a metabolic context. We review evidence that the regulation of the balance between lignin biosynthesis and biomass growth is mediated, at least in part, at the level of transcription, and expand previous results to show specific regulation within gene families of monolignol biosynthesis. Finally, we suggest a model of physiological control for the regulation of the relationship of wood formation and lignin content in trees. Wood, used by human societies for millennia, remains one of the world’s most abundant raw materials for industrial products and renewable energy. Wood is the secondary xylem of vascular plants, a tissue formed from the terminal differentiation of the inner side of the cambial meristem for vertical and horizontal transport of water, nutrients, and extractives. The secondary cell wall structure and composition of wood are the primary determinants of its physical and chemical properties, and of its energy content. Wood is typically composed of about 25% lignin, and 70% cellulosic carbohydrates, with roughly 45% cellulose and 25% hemicelluloses (Sjostrom, 1993). Lignin is a complex phenolic polymer that provides an embedding material for the cellulosic polymers of the secondary cell walls. It is also the major polymer in the middle lamellae between adjacent cell walls (Plomion et al., 2001; Boerjan et al., 2003). Lignin provides the hydrophobic surface that allows plants to transport water to heights greater than 100 m (Carder, 1995; Koch et al., 2004) and contributes to the mechanical strength that can support trees weighing more than 2,000 metric tons (Fry and White, 1938). Lignin’s physical and chemical properties also serve as a barrier against the invasion of pests and pathogens (Vance et al., 1980; Bhuiyan et al., 2009). For the forest products industries, lignin is the major barrier to efficient extraction of cellulose fibers for pulp and paper production. For the bioenergy industries, lignin is a barrier to saccharification for production of liquid biofuel (Li et al., 2003; Chen and Dixon, 2007). While lignin is fundamental to growth and adaptation of herbaceous and woody plants (Sarkanen and Ludwig, 1971; Boerjan et al., 2003; Ralph et al., 2007; Heitner et al., 2010; Vanholme et al., 2010), it is not clear how much lignin is needed or how much its composition may vary. Within a plant, lignin content can vary greatly in different tissues; for example, lignin is very low in young shoots and high in wood. In different tree species the lignin content of wood can vary from 15% to 40% (Sarkanen and Ludwig, 1971). Within a species, average lignin content of wood is much less variable, often ranging only a few percent (Einspahr et al., 1964; van Buijtenen et al., 1968). Lignin content is also increased or decreased in wood formed under gravitropic stimulation or mechanical stress (known as reaction wood; Timell, 1969, 1986; Andersson-Gunneras et al., 2006). In softwoods (gymnosperms), compression wood may be up to 40% lignin, and in hardwoods (angiosperms), tension wood fibers have a specialized gelatinous cell layer that is almost devoid of lignin (Timell, 1969). Lignin composition is also highly variable. There are two main types of subunits in lignin. One derived from coniferyl alcohol monomers forms guaiacyl (G) units, resulting in a lignin typical of softwoods. A second monomer sinapyl alcohol forms syringyl (S) units after polymerization. A mixed type of lignin with both G and S units is typical of hardwoods, and is characterized by the ratio (S/G ratio) of subunits. The S/G ratio varies greatly among hardwoods (Sarkanen and Ludwig, 1971). In herbaceous and field crops, significant reductions in lignin have been achieved by traditional breeding, spontaneous mutation, and by transgenesis. In general, reduced lignin is associated with depressed yields (Pedersen et al., 2005). However, in many cases, lignin can be reduced without reducing yield or fitness. Genetic background and diverse environments have a significant role. Because of its roles in water transport, mechanical support, and biodefense, lignin is intimately associated with the evolution of vascular plants (Robinson, 1990). Early large plants may have had higher lignin contents than we find today. Ferns and lycopods, the predominant plants of the middle to late Paleozoic (350–