How do lignin composition, structure, and cross-linking affect degradability? A review of cell wall model studies

Because of the complexity of plant cell wall biosynthesis, the mechanisms by which lignin restrict fiber degradation are poorly understood. Many aspects of grass cell wall lignification and degradation are successfully modeled by dehydrogenation polymer-cell wall (DHP-CW) complexes formed with primary walls of corn Zea mays L. This system was used to assess how variations in lignin composition, structure, and cross-linking influence the hydrolysis of cell walls by fungal enzymes. Altering the normal guaiacyl, syringyl, and p-hydroxyphenyl makeup of lignin did not influence cell wall degradability; each unit of lignin depressed cell wall degradability by two units. Plants with perturbed lignin biosynthesis often incorporate unusual precursors into lignin and one of these, coniferaldehyde, increased lignin hydrophobicity and further depressed degradability by up to 30%. In other studies, lignin formed by gradual "bulk" or rapid "end-wise" polymerization of monolignols had markedly different structures but similar effects on degradability. Reductions in cell wall cross-linking, via oxidative coupling of feruloylated xylans to lignin or nucleophilic addition of cell wall sugars to lignin quinone-methide intermediates, increased the initial hydrolysis of cell walls by up to 46% and the extent of hydrolysis by up to 28%. Overall, these studies suggest that reductions in lignin concentration, hydrophobicity, and cross-linking will improve the enzymatic hydrolysis and utilization of structural polysaccharides for nutritional and industrial purposes. In ongoing work, we are developing a DHP-CW system for dicots and are investigating how cross-linking and various acylated and unusual monolignols influence the formation of lignin and the degradation of cell walls by rumen microflora.