How Do Lignin Composition, Structure, and Cross-Linking Affect Degradability? A Review of Cell Wall Model Studies

turity, the degradability of stems, and to a lesser degree leaves, is further depressed by lignification of primaryBecause of the complexity of plant cell wall biosynthesis, the mechwalled parenchyma and epidermal tissues (Wilson and anisms by which lignin restrict fiber degradation are poorly understood. Many aspects of grass cell wall lignification and degradation Hatfield, 1997). These reductions in degradability are are successfully modeled by dehydrogenation polymer-cell wall partly related to the increased lignin content of cell (DHP-CW) complexes formed with primary walls of corn Zea mays walls; however, variations in three-dimensional strucL. This system was used to assess how variations in lignin composition, ture and composition of lignin and its hydrophobicity, structure, and cross-linking influence the hydrolysis of cell walls by encrustation, and cross-linking to other matrix compofungal enzymes. Altering the normal guaiacyl, syringyl, and p-hydroxynents also have been implicated (Chesson, 1993; Jung phenyl makeup of lignin did not influence cell wall degradability; each and Deetz, 1993). unit of lignin depressed cell wall degradability by two units. Plants Because of the anatomical, morphological, and develwith perturbed lignin biosynthesis often incorporate unusual precuropmental complexity of cell wall and lignin formation sors into lignin and one of these, coniferaldehyde, increased lignin in various plant tissues, studies with normal, mutant, hydrophobicity and further depressed degradability by up to 30%. In other studies, lignin formed by gradual “bulk” or rapid “end-wise” or transgenic germplasm often do not show consistent polymerization of monolignols had markedly different structures but relationships between lignin-matrix characteristics and similar effects on degradability. Reductions in cell wall cross-linking, cell wall degradability at the whole plant, plant part, or via oxidative coupling of feruloylated xylans to lignin or nucleophilic even tissue level (Grabber et al., 1992; Jung and Vogel, addition of cell wall sugars to lignin quinone-methide intermediates, 1992; Jung et al., 2000). Even when plant selection or increased the initial hydrolysis of cell walls by up to 46% and the enzyme downregulation is targeted at specific lignin extent of hydrolysis by up to 28%. Overall, these studies suggest that properties or lignin-matrix interactions, compensatory reductions in lignin concentration, hydrophobicity, and cross-linking or associative changes in other cell wall characteristics will improve the enzymatic hydrolysis and utilization of structural often occur, making it difficult to identify underlying polysaccharides for nutritional and industrial purposes. In ongoing mechanisms controlling cell wall degradability. Plants work, we are developing a DHP-CW system for dicots and are investigating how cross-linking and various acylated and unusual monoligmay, for example, respond to lower lignification by innols influence the formation of lignin and the degradation of cell walls creasing the amount of cross-linking (Chabannes et al., by rumen microflora. 2001), perhaps yielding no net change in digestibility. A common but faulty assumption is that correlations between a particular lignin trait and degradability demL plays a vital role in plant growth and developonstrate a cause and effect relationship. Because of the ment by improving water conduction through xycomplexity of cell wall development, associations belem tracheary elements, enhancing the strength of fitween one lignin characteristic (e.g., lignin composition) brous tissues, and limiting the spread of pathogens in and cell wall degradability can be masked, confounded, plant tissues (Iiyama et al., 1994). Lignin restricts the or merely correlated to concurrent changes in other degradation of structural polysaccharides by hydrolytic lignin properties (e.g., lignin cross-linking) that influenzymes, thereby limiting the bioconversion of forages ence cell wall degradability. Because of these factors, and fibrous crops into animal products or into liquid the underlying mechanisms by which lignin restricts defuels and other industrial products (Brown, 1985; Jung gradability of plant cell walls are poorly understood. and Deetz, 1993). Lignified dietary fiber also plays an important role in maintaining gastrointestinal function An Overview of Cell Wall Model Systems and health in humans (Ferguson et al., 2001). Because of the developmental and morphological The enzymatic degradability of cell walls in leaves and complexity of plants, a variety of simpler model systems particularly stems of plants declines during maturation have been developed in an attempt to isolate and idenbecause of accumulation and progressive lignification tify potential interactions between lignin and matrix of primary and secondary cell walls of vascular and components and, in some cases, to assess how these sclerenchyma tissues. As plants reach physiological mainteractions influence cell wall hydrolysis by enzymes or rumen microflora. One approach involves adding, U.S. Dairy Forage Research Center, USDA-ARS, 1925 Linden Drive removing, or modifying lignins or other matrix compoWest, Madison, WI 53706. This paper was originally presented at the Lignin and Forage Digestibility Symposium, 2003 CSSA Annual nents in isolated cell walls or cellulose by chemical or Meeting, Denver, CO. Received 24 Mar. 2004. Forage & Grazing Lands. enzymatic treatments to assess their role in controlling *Corresponding author ([email protected]). Abbreviations: CAD, cinnamyl alcohol dehydrogenase; DHP, dehyPublished in Crop Sci. 45:820–831 (2005). doi:10.2135/cropsci2004.0191 drogenation polymer; DHP-CW, dehydrogenation polymer-cell wall; G, guaiacyl; H, p-hydroxyphenyl; NMR, nuclear magnetic resonance; © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA PME, pectin methyl esterase; S, syringyl. 820 Published online March 28, 2005