J. Range Manage. 54: 420–430 July 2001 Lignin and fiber digestion KENNETH J. MOORE AND HANS-JOACHIM G. JUNG Authors are professor, Department of Agronomy, Iowa State University, Ames, Iowa 50011; and research dairy scientist, USDA, Agricultural Research Service, Plant Science Research Unit and U.S. Dairy Forage Research Center Cluster, St. Paul, Minn. 55108. Abstract Resumen Lignin is a polymer formed from monolignols derived from the La lignina es un polímero formado de monolignoles derivados phenylpropanoid pathway in vascular plants. It is deposited in de la vía fenilpropanoide de las plantas vasculares. Se deposita the cell walls of plants as part of the process of cell maturation. en las paredes celulares de las plantas como parte del proceso de Lignin is considered an anti-quality component in forages maduración de la célula. En los forrajes, la lignina se considera because of its negative impact on the nutritional availability of como un componente anti-calidad por su impacto negativo en la plant fiber. Lignin interferes with the digestion of cell-wall poly- disponibilidad nutricional de la fibra de la planta. La lignina saccharides by acting as a physical barrier to microbial enzymes. interfiere con la digestión de los polisacáridos de la pared celular Lignification therefore has a direct and often important impact al actuar como barrera física para las enzimas microbianas. Por on the digestible energy (DE) value of the forage. There are a lo tanto, la lignificación tiene un impacto directo, y a menudo number of plant-related factors that affect lignification in indi- importante, en el valor de la energía digestible (ED) del forraje. vidual plants and plant communities. Lignification is under Hay un número de factores relacionados con la planta que genetic control and there are considerable differences in lignin afectan la lignificación de las plantas individuales y de las comu- concentration and composition among species and even geno- nidades vegetales. La lignificación esta bajo control genético y types within species. Genetic differences in lignification are first hay considerables diferencias entre especies, y aun entre genoti- expressed at the cellular level and are affected by biochemical pos de la misma especie, respecto a la concentración y composi- and physiological activities of the cell. As cells differentiate, dif- ción de la lignina. Las diferencias genéticas de lignificación se ferences in lignification occur depending on the tissues and expresan primeramente a nivel celular y son afectadas por las organs being developed. Lignification tends to be most intense in actividades bioquímicas y fisiológicas de la célula. Conforme la structural tissues such as xylem and sclerenchyma. Plant organs célula se diferencia ocurren diferencias en la lignificación, containing high concentrations of these tissues, such as stems, are dependiendo de los tejidos y orgános que se estén desarrollando. less digestible than those containing lower concentrations. The La lignificación tiende a ser mas intensa en tejidos estructurales relative proportion of lignified tissues and organs typically como el xilema y esclerénquima. Los órganos de la planta que increases as plants mature so there is often a negative relation- contienen altas proporciones de estos tejidos, tales como los tal- ship between digestibility and maturity. All of these plant los, son menos digestibles que aquellos que contienen bajas con- processes respond to environmental factors that can affect the centraciones. La proporción de tejidos y órganos lignificados extent and impact of lignification. Temperature, soil moisture, típicamente aumenta conforme la planta madura, por lo que a light, and soil fertility can have either direct or indirect effects on menudo hay una relación negativa entre la digestibilidad y lignification. The most useful management practices for minimiz- madurez. Todos estos procesos de la planta responden a factores ing the negative effects of lignification are manipulation of the ambientales que pueden afectar la cantidad e impacto de la lig- plant community such that it contains more desirable species and nificación. La temperatura, humedad del suelo, luz y fertilidad harvest management to maintain plants in a vegetative stage of del suelo pueden tener también efectos directos o indirectos en la development. lignificación. Las practicas de manejo mas útiles para minimizar los efectos negativos de la lignificación son la manipulación de las comunidades vegetales para que contengan mas especies Key Words: Anti-quality, digestibility, forage quality, forage uti- deseables y el manejo de la cosecha para mantener las plantas en lization estado vegetativo. Lignin is considered an anti-quality component in forages because of its negative impact on the nutritional availability of plant fiber. It differs from most other classes of antiquality com- secondary metabolite. Its evolution in plants is primarily related ponents in forages in that it is a structural compound rather than a to plant structure and function and not as a defense mechanism against other organisms. As a component of the cell wall, lignin plays an important role in morphogenesis. Cell walls form the Journal Paper N. J-18367 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa, Project No. 2899, and supported by Hatch Act structural framework of the plant architecture that provides and State of Iowa.. mechanical support for plant organs (Varner and Lin 1989). Cell Manuscript accepted 27 Nov. 00. walls also are involved in water balance, ion exchange, cell 420 JOURNAL OF RANGE MANAGEMENT 54(4), July 2001 recognition, and protection from biotic Lignin Biochemistry converts 5-hydroxyferulic acid to sinapic stresses (Boudet 1998, Varner and Lin acid is reduced in activity. As more lignin 1989, Vian 1982). mutants and transgenic plants involving Lignin is an integral component of plant While lignin has been recognized as a the lignin pathway have been character- cell walls. It is the last major biopolymer distinct chemical entity of plant cell walls ized, it has become apparent that this path- to have evolved within the plant kingdom for over 100 years (Sjostrom 1981), there way is actually more of a web than linear and is generally regarded as the second is still considerable scientific argument (Sewalt et al. 1997b). Because of this web most abundant compound, after cellulose, concerning its structure, biosynthesis, and structure and the ability of plants to incor- in the biosphere (Boudet 1998, Monties measurement. These uncertainties arise porate non-typical phenylpropanoid pre- 1991). The most important function of from the complexity of lignin synthesis cursor molecules, the results of biotechno- lignin in plants is as a structural compo- and the resultant complexity of lignin mol- logical manipulation of the pathway have nent to lend strength and rigidity to the ecules. While generalized structures for yielded unexpected results and unique cell wall. It is also important in limiting lignin have been drawn, it is not yet possi- lignin structures (Boudet 1998, Ralph et water loss by reducing permeability of the ble to definitely determine the complete al. 1998). Apparently lignin plays such an cell wall, and in impeding disease organ- structure of any isolated lignin molecule, important role in plant development that isms (Dean and Eriksson 1992, Zeikus let alone the structure of lignin in the plant alternative routes and precursors can be 1980). All of these attributes are desirable cell wall. Because there is no standard used to provide the amount of lignin nec- from the perspective of plant function and lignin structure for reference, measure- essary for normal development. When survival, but limit the nutritional value of ment of lignin concentration is empirical lignin concentration has been significantly the plant for herbivores. and very dependent on methodology. All reduced through biotechnology, non- Fiber is a nutritional entity which is of these ambiguities have made it very dif- viable plants result (Jung and Ni 1998). defined as much by its biological proper- ficult to clarify the roles of lignin in plant Deposition of lignin in the cell wall of ties as its chemical composition (Van growth and development, and the mecha- grasses appears to involve ferulate esters Soest et al. 1991). With regard to forages, nism by which lignin limits cell wall of arabinoxylans as nucleation sites. In it has been traditionally defined as the digestibility. annual ryegrass (Lolium multiflorum complex of dietary nutrients that are rela- Lignin can be characterized as a polymer Lam.), lignin cross-links to ferulates con- tively resistant to digestion and are slowly formed from monolignols derived from the sisted only of structures that would form if and only partially degraded by herbivores phenylpropanoid pathway in vascular monolignols reacted with the ferulates (Chesson and Forsberg 1988, Van Soest plants. Recent reviews provide excellent directly rather than polymeric lignin react- 1982). By this definition, fiber is com- coverage of the process and biochemistry ing with ferulate esters (Ralph et al. 1995). posed of structural polysaccharides, wall of lignification (Baucher et al. 1998, This ferulate mediated cross-linking struc- proteins, and lignin. The main antiquality Boudet 1998). Lignin is deposited in the ture is illustrated in Fig. 2. Unlike ferulic role of lignin in forages is in limiting cell walls of plants as part of the process of acid, p-coumarate