Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2009
Publication Date: 3/15/2009
Citation: Hatfield, R.D., Marita, J.M., Frost, K., Grabber, J.H., Ralph, J., Lu, F., Kim, H. 2009. Grass Lignin Acylation: p-Coumaroyl Transferase Activity and Cell Wall Characteristics of C3 and C4 Grasses. Planta. 229:1253-1267. Interpretive Summary: Lignin is a component of forage cell walls that gives the plant structure and aids in water transport in the plant. However, when forages are fed to dairy cattle, the lignin is indigestible by the microbes inside a cow’s rumen (first compartment of stomach) that aid in digestion. This also limits the digestibility of carbohydrates within these cell walls. These plant cell wall characteristics that limit digestion in cattle are the same that limit the conversion of plants to ethanol via cellulosic fermentation. In grasses, the lignin is modified to contain an additional phenolic group, p-coumaric acid (pCA). Although pCA is not actually a building block and is only attached to the surface of lignin, it is thought to play a role in helping the formation of lignin. If this is true, changing the levels of pCA may alter the amount and type of lignin formed, resulting in more digestible cell walls. However, bioenergy production can also be in the form of direct combustion of the plant material, in which case increased lignin and especially more pCA would increase the net energy value. Selecting forages with appropriate cell wall characteristics to fit the end use (milk production, ethanol, total combustion energy) allows for improved energy conversion efficiency and higher productivity. Therefore, we analyzed a number of cool (C3) and warm (C4) season grasses for lignin, pCA, p-coumaroyl transferase, and cell wall carbohydrate characteristics. This work provides basic information on grass cell wall characteristics that aid in the initial selection of grass forage types to be tested and further developed to increase forage utilization in farming systems. The increased use of grass forages in farming systems would have a positive impact upon the environment by stabilizing the soil, decreasing erosion, and improving nutrient cycling options.
Technical Abstract: Grasses have always been a predominate source of nutritional energy for livestock systems around the world. Species belonging to the C3 and C4 grass types have recently been championed as feedstock sources for bioenergy production. Their ultimate use would be as a source of carbohydrate for fermentation to ethanol, conversion to oils via pyrolysis, or direct conversion to energy through combustion. In the latter case, increasing lignin content would enhance the energy density of the material, improving the net energy output. However, grasses with high lignin content have lower energy conversion efficiencies for production of bioenergy, either in the form of ethanol or to milk and meat through ruminants. Grass lignins are uniquely acylated with p-coumarates (pCA), an energy-intensive process that does not have a clear metabolic function. Acylation of lignin by pCA results from the incorporation of p-coumaroyl-monolignol conjugates into the growing lignin polymer within the cell wall matrix. The acyl-transferase is a soluble enzyme that utilizes p-coumaroyl-CoA (pCA-CoA) as the activated donor molecule and sinapyl alcohol as the preferred acceptor molecule (p-coumaroyl transferase, pCAT). A spectrum of C3 and C4 grasses were evaluated for cell wall characteristics including pCA and lignin content. Acylation of lignin by pCA ranged from 10.7 to 37.7 mg g-1 cell wall (CW) and lignin ranged from 163 to 222 mg g-1 CW. Corn and grain sorghum, both C4 plants, had the highest pCA levels. All other (C3 and C4) were more similar to each other with pCA levels of 7-15 mg g-1 CW. The pCAT activities did not follow the same pattern as the pCA incorporation into lignin. Some of the highest activities were found in C3 grasses having low pCA levels. Other important cell wall components (glucose and xylose) showed variable patterns dependent upon the individual grass species, but not consistent within the C3 and C4 groupings. Enzymatic release of total carbohydrate (cellulase/xylansase mixture) from ball-milled cell walls varied among the grasses, but was not solely related to lignin content.