Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 23, 2008
Publication Date: October 20, 2008
Citation: Hatfield, R.D., Marita, J.M., Frost, K. 2008. Characterization of p-coumarate accumulation, p-coumaroyl transferase, and cell wall changes during the development of corn stems. Journal of the Science of Food and Agriculture. 88:2529-2537. Interpretive Summary: Corn silage is a popular feed for dairy cows. A major factor in corn growth is how much the stem increases in its overall length. The formation and elongation of new cells along the stem account for increases in the overall height of the plant. To achieve this structural feat, cells develop cell walls that can become quite thick, especially in the rind (outer) portion of the stem. Cells in the rind are primarily water-transporting cells (vascular cells) and fiber cells that provide structural strength. They act together to form a structure much like a steel pipe that provides strength to the stem to support the leaves. Cell walls are made up of different building blocks depending upon the stage of development, much the same way the walls of a building differ depending upon the function of the walls. This work was conducted to determine how and in what proportion the different building blocks are used to make the rind portion of the stem. Lignin is one of the building blocks that may help glue cell walls together, but it also inhibits digestion of the carbohydrates in the cell wall, limiting their use in feed. The cell wall component p-coumaric acid (pCA) is attached to lignin, but does not cross-link any of the building blocks within the wall. Its function has not been clearly defined, but it may be important in the formation of lignin. Understanding how and when specific building blocks such as lignin and pCA are used during cell wall synthesis provides a blueprint for how to change cell walls for better digestibility without sacrificing strength and standability. Information from studies like this can be used by plant breeders to select for specific cell wall traits to improve digestibility or by molecular geneticist to alter specific genes that will alter cell wall formation and, as a result, improve digestibility. It is estimated that increasing the digestibility of forage cell walls by as little as 10% would have large economic returns. For dairy cows in the U.S. this would mean increased milk and meat production worth $350 million and 2.8 million tons less manure solids and 2 million tons less in grain supplements.
Technical Abstract: Corn (Zea mays L.) stem/stalk development is an orchestrated change from cell initiation to fully mature cell types. During cell wall maturation, the lignin is acylated with p-coumarates (pCA). This work describes characterization studies of the p-coumaroylation process in relation to corn stem development. Corn plants from three locations were harvested and tissues were analyzed from all nodes and even-numbered internodes above soil line. Changes in carbohydrates reflect a shift to lignification at the expense of structural polysaccharide synthesis. Accumulation of pCA paralleled the incorporation of lignin while ferulic acid remained relatively constant as a proportion of the cell wall (5-7g/kg cell wall). The p-coumaroyl transferase (pCAT), which is responsible for attaching pCA to lignin monomers, displayed maximum levels of activity in the middle of the stem (internodes 10-12, 2-3 nM min-1 mg-1). Syringyl content as a proportion of the total lignin did not change significantly with cell wall maturation, although there was a trend toward increased amounts of syringyl units in the more mature cell walls. Incorporation of pCA into corn cell walls not only mirrored lignification, but the pCAT activity as well. Levels of pCAT activity may be an indicator of rapid lignification, specifically for syringyl-type lignin.