Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2007
Publication Date: 4/25/2007
Citation: Grabber, J.H., Lu, F. 2007. Formation of syringyl-rich lignins in primary maize walls as influenced by cell wall feruloylation and p-coumaroylated monolignols. Planta. 226:741-751. Interpretive Summary: Forage grasses are a major feedstuff for livestock. Grain from grass cereal crops like maize, rice, and wheat also supplies most of the dietary energy needs of people and many types of livestock. Fiber makes up 30 to 80% of the dry weight of grasses and it is composed primarily of polysaccharides (polymers of sugar molecules) cemented together with a polymer known as lignin. Unfortunately, lignin hinders the enzymatic breakdown of polysaccharides into sugars and this impairs the value of forage grasses as feedstuffs for livestock and as renewable feedstocks for the industrial production of ethanol fuels, plastics, and other products. For some unknown reason, grass lignins are covered with small molecules known as p-coumarate. To try and understand the role p-coumarates in grasses, we artificially lignified fiber from corn (Zea mays L.) with and without the addition of sinapyl p-coumarate, the probable precursor of p-coumarate units in lignin. Adding sinapyl p-coumarate with other lignin precursors had little effect on the amount of lignin formed in cell walls. Quite unexpectedly, we found that sinapyl p-coumarate inactived peroxidase (the enzyme responsible for lignin formation) and it interfered with the attachment of lignin to polysaccharides by ferulates. (Ferulates are small molecules attached polysaccharides and they act to knit together fiber in grasses.) Thus, the functional role of p-coumarates on grass lignins remains a mystery. In future work, we will further examine how p-coumarates influence lignin formation in grasses and we will determine whether their presence on lignin affects the enzymatic breakdown of polysaccharides in fiber. These studies will help scientists to develop improved grasses and processing methods so that lignin is less of a barrier to polysaccharide utilization.
Technical Abstract: Grass cell walls are atypical because their xylans are acylated with ferulate and lignins are acylated with p-coumarate. To probe the role and interactions of these p-hydroxycinnamates during lignification, feruloylated primary cell walls isolated from maize cell suspensions were lignified with coniferyl and sinapyl alcohols and with varying levels of p-coumarate esters. Ferulate xylan esters enhanced the formation of wall-bound syringyl lignin more than methyl p-coumarate, however, maximal concentrations of syringyl lignin were only one-third that of guaiacyl lignin. Including sinapyl p-coumarate, the presumed precursor of p-coumaroylated lignins, with monolignols unexpectedly accelerated peroxidase inactivation, interfered with ferulate copolymerization into lignin, and had minimal or adverse effects on cell wall lignification. Free phenolic groups of p-coumarate esters in isolated maize lignin and pith cell walls did not undergo oxidative coupling with each other or with added monolignols. Thus, the extensive formation of syringyl-rich lignins and the functional role of extensive lignin acylation by p-coumarate in grasses remains a mystery.