Cell wall structural foundations: Molecular basis for improving forage digestibilities

  • Hatfield R
  • Ralph J
  • Grabber J
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Forages play an important role in the world wide animal industry. The fiber (cell wall) portion makes up 300 to 800 [micro]g [g.sup.-1] of forage dry matter and represents a major source of nutritional energy for ruminants, but, unfortunately, less than 50% of this fraction is readily digested and utilized by the animal. Significant progress has been made in the past 30 to 40 yr towards understanding cell wall structure and function and developing mechanistic models that explain limitations to structural polysaccharide degradation and utilization by ruminants. In grasses, it is now clear that wall bound ferulates play a key role in cross-linking xylans to each other and to lignin, resulting in less degradable walls. Much has been accomplished in advancing our understanding of the lignification process in plants, particularly the genes and enzymes involved in the monolignol biosynthesis. The application of molecular techniques to this area has advanced our understanding of the metabolic process while providing tools for further exploration of wall structure and function and providing direction as to possible avenues to improve forage digestibility. Full Text: Forages play an important role in the world wide animal industry. The fiber (cell wall) portion makes up 300 to 800 [micro]g [g.sup.-1] of forage dry matter and represents a major source of nutritional energy for ruminants, but, unfortunately less than 50% of this fraction is readily digested and utilized by the animal. Hence, cell walls are a controlling factor in determining the quality of forages. If a greater percentage of this potential energy was made available to the animal (i.e., increase the digestibility of the cell wall fraction), there would be a considerable positive economic impact. For example, in the U.S. dairy industry, a 10% increase in wall digestion would result in an additional $380 million in milk and meat sales while reducing manure solids by 2.3 million megagrams and grain input into the diet by 3.0 million megagrams. In this section, we will review research conducted in the past 40 to 50 yr on the chemistry and biochemistry of forage cell walls that impacts forage digestibility. The structural and functional roles of plant cell walls (including utilization of wall polysaccharides by ruminants) are controlled by the composition and organization of individual wall components. Cell walls are composed of structural polysaccharides with varying compositions and structures, hydroxycinnamic acids, lignin, protein (both metabolic and structural), ions, and water. Component interactions, especially specific types of covalent linkages, control cell wall organization and structural integrity but may also control wall expansion during growth and degradation for herbivore utilization. Milestones of Forage Cell Wall Chemistry and Biochemistry When it comes to milestones in forage cell wall chemistry and biochemistry, it is a bit difficult to pinpoint critical events. R.L. Reid (1994) in discussing milestones in forage research states, "The incidence of research milestones, considered as discrete and revolutionary events, may be about as frequent as the diminishing occurrence of physical milestones on the American landscape. More often, advances in research occur in small and incremental steps, sometimes with obscure or unknown origins, but generally as variations on a preceding theme." This is a fitting description for forage wall chemistry and biochemistry in that progress is often incremental but may at times be exponential, with breakthroughs coming in rapid succession, greatly advancing the whole field. Is the milestone the exponential events or an incremental event that may have started the whole process? Although the leaps forward seem highly significant, they did not happen independently and indeed required the incremental advancements that preceded them. Rather than trying to make a judgment as to what events have been most critical in wall chemistry and biochemistry, we will look at a few critical aspects of cell wall chemistry and biochemistry and focus on a major theme that has dominated wall research. Forage quality is dependent upon several factors and their interactions (see Fahey and Hussein in this review series). Progress has been made in removing or minimizing anti-quality factors found in some forages [e.g., formononetin in red clover (Trifolium partense L.), endophyte toxicity in fescue (Festuca arundinacea Schreb.); for other examples, see Casler and Vogel in this review series]. However, it is obvious that an overriding factor in improving forage quality requires altering the cell wall fraction of the total plant to provide more energy to the animal. Typical approaches have been to decrease the, cell wall concentration while maintaining total biomass production. This has not always been an obtainable goal (Casler and Vogel, this series). The alternative is to alter the digestibility of the structural polysaccharides that make up forage walls, providing a more efficiently utilized nutrient source. Searching for the molecular mechanisms controlling wall degradation has been the main focus of chemical and biochemical studies over the past 40 yr. Cell Wall Components Carbohydrates Structural polysaccharides can have quite complex molecular structures, yet if removed from the wall they are readily degraded to their component monosaccharides by microbial enzymes. However, when left within the wall matrix polysaccharide degradation is restricted and in some cases, such as the xylans in legumes, nearly escape ruminal degradation (Buxton and Brasche, 1991; Hatfield and Weimer, 1995). An exception is the pectic polysaccharides that are rapidly and extensively degraded from wall matrices, including heavily lignified stems (Chesson and Monro, 1982; Dehority et al., 1962; Hatfield and Weimer, 1995). Degradation is not only rapid but does not result in a decrease in ruminal pH that is often seen with rapidly degraded starch (Hatfield and Weimer, 1995). Pectic polysaccharides form a significant portion of alfalfa (Medicago sativa L.) cell walls (100-300 [micro]g [g.sup.-1]) (Hatfield, 1992) but from a nutritional point of view have been lumped in with the soluble fraction of forages. This is unfortunate because it has resulted in this wall fraction being ignored as a possible means of improving wall digestibility. Unlike cytoplasmic carbohydrates that are subject to metabolic turnover, pectic polysaccharides remain constant once incorporated into the wall. A small sampling of alfalfa and red clover cultivars revealed a reasonable level of variability for total pectin in both stems and leaves, suggesting genetic selection for increased pectins should be feasible (Hatfield and Smith, 1995). If one selects for additional pectic polysaccharides, there may be a better chance of maintaining high total biomass production. Unfortunately, this selection strategy will only be fruitful for legume forages, since grasses typically have only 10 [micro]g [g.sup.-1] or less of their dry matter composed of pectic polysaccharides. Proteins Proteins generally make up less than 50 [micro]g [g.sup.-1] of the wall depending on the tissue type and maturity. As with polysaccharides, proteins outside the wall matrix are susceptible to degradation, but as a part of the total wall structure they may be completely resistant to degradation and pass intact through the digestive tract. The role of proteins bridges both metabolic and structural functions. Key enzymes are critical in the lignification process. For example, one-electron oxidation of not only monolignols but also ferulates (formation of radicals) is controlled within the wall by peroxidases and/or oxidases (O'Malley et al., 1993). For processes in which there is a need to tightly control radical formation (i.e., cross-coupling to form diferulates or coupling ferulates to monolignols), peroxidases offer plants considerable control. It is most likely that peroxidases are incorporated into the wall matrix and positioned within general areas critical for cross-linking. No reaction will occur until hydrogen peroxide (co-substrate) is supplied to initiate the one-electron oxidation of the phenol substrate, thus controlling the cross-coupling reaction until the appropriate time in wall development. Structural proteins, found in both monocot and dicot walls, appear to play critical roles in cross-linking wall components, particularly in primary walls. It is also possible that metabolic proteins become cross-linked to lignin resulting in a more structural function once wall development has reached a specific stage (Evans and Himmelsbach, 1991). Lignin and Degradation Although the wall is the sum of its parts and the interaction of individual components dictates structure and function, we can identify key elements having a pivotal role in regulating wall degradation. The culprit is lignin, with some assistance from molecules that cross-link lignin to other wall components. This is not to say that other components do not influence wall degradation. Indeed, the interactions among polysaccharides and proteins can impact degradation rates and extents of cell wall degradation (Hatfield, 1993). In the early 1900s, Waksman and Cordon (1938) recognized the inhibitory effect of lignin on cellulose degradation by soil microbes. Research establishing the negative correlation of lignin content in forage walls with in vitro wall degradation was not published until some 30 to 40 yr later (Johnson et al., 1962; Kamstra et al., 1958; Van Soest et al., 1966). Since that time, researchers have pursued the underlying molecular mechanism restricting wall polysaccharide degradation. We will develop the lignin and cross-linking story in more detail. What is Lignin? Lignin is defined as "polymeric natural products arising from an enzyme-initiated dehydrogenative polymerization of three primary precursors" (Sarkanen and Ludwig, 1971). Per

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  • R. D. Hatfield

  • J. Ralph

  • J. H. Grabber

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