Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 23, 2004
Publication Date: May 1, 2005
Citation: Grabber, J.H. 2005. How do lignin composition, structure, and cross-linking affect degradability? A review of cell wall model studies. Crop Science. 45:820-831. Interpretive Summary: Grasses are a major source of nutrients for wild and domesticated livestock. Grass cereal crops like corn, rice, wheat, oats and rye dominate cultivated cropland, supplying most of the dietary energy needs of people and many types of livestock. Fiber is a major component of these plants, making up 25 to 80% of their dry weight. Fiber is composed mainly of complex carbohydrates. These carbohydrates are potentially an important source of digestible energy for livestock and they may also be converted into various chemicals for use in automobile fuels, plastics, and other products. Unfortunately, the enzymatic breakdown of complex carbohydrates into sugars is limited by an indigestible component in fiber known as 'lignin'. Because of the great complexity of fiber formation in plants, scientists have not understood how lignin limits fiber breakdown. To better understand how lignin limits fiber breakdown, we artificially formed lignin of various types in fiber isolated from corn (Zea mays L.). We found that varying the proportions of normal components that make up lignin or changing its three-dimensional shape did not influence the breakdown of complex carbohydrates by digestive enzymes. In contrast, we found that carbohydrate digestion was greatly enhanced if lignin was less waterproof and attached (cross-linked) to carbohydrates by fewer chemical bonds. In ongoing work, we are investigating additional ways making lignin less inhibitory to fiber digestion. These studies will help scientists to develop improved grasses and processing methods so that lignin is less of a barrier to fiber digestion. Ultimately these studies will lower the cost and environmental impact of converting grasses and cereal crop residues into food, fuel, and industrial products.
Technical Abstract: Due to the complexity of plant cell wall biosynthesis, the mechanisms by which lignin restricts fiber degradation are poorly understood. In a series of model studies, primary cell walls from Zea mays (L.) were artificially lignified to assess whether variations in lignin composition, structure, and cross-linking influence cell wall degradation by fungal enzymes. Each unit of lignin formed with varying ratios of p-coumaryl, coniferyl, and sinapyl alcohols reduced cell wall degradability by 2 units, indicating that normal variations in lignin composition do not influence cell wall degradation. Plants with perturbed lignin biosynthesis incorporate unusual units into lignin and one of these, coniferaldehyde, increased lignin hydrophobicity and further decreased cell wall degradability by 30%. The effects of other unusual and acylated monolignols (e.g. 5-hydroxyconiferyl alcohol, dihydroconiferyl alcohol, sinapyl acetate, and sinapyl p-coumarate) on lignin formation and cell wall degradation are currently under investigation. In other studies, lignins formed by gradual 'end-wise' or rapid 'bulk' polymerization had markedly different structures but similar effects on cell wall degradability. Reductions in cell wall cross-linking, via oxidative coupling of feruloylated xylans to lignin or nucleophilic addition of structural polysaccharides to lignin quinone-methide intermediates, increased the initial hydrolysis of cell walls by up to 40% and the extent of hydrolysis by up to 25%. Overall, these studies suggest that reductions in lignin concentration, hydrophobicity, and cross-linking will improve the enzymatic hydrolysis and utilization of plant structural polysaccharides for nutritional and industrial purposes.