Severe inhibition of maize wall degradation by synthetic lignins formed with coniferaldehyde

Although the enzymatic or ruminal degradability of plants deficient in cinnamyl alcohol dehydrogenase (CAD) is often greater than their normal counterparts, factors responsible for these degradability differences have not been identified. Since lignins in CAD deficient plants often contain elevated concentrations of aldehydes, we used a cell-wall model system to evaluate what effect aldehyde-containing lignins have on the hydrolysis of cell walls by fungal enzymes. Varying ratios of coniferaldehyde and coniferyl alcohol were polymerised into non-lignified primary walls of maize (Zea mays L) by wall-bound peroxidase and exogenously supplied H₂O₂. Coniferaldehyde lignins formed fewer cross-linked structures with other wall components, but they were much more inhibitory to cell wall degradation than lignins formed with coniferyl alcohol. This suggests that the improved degradability of CAD deficient plants is not related to the incorporation of p-hydroxycinnamaldehyde units into lignin. Degradability differences were diminished if enzyme loadings were increased and if hydrophobic aldehyde groups in lignins were reduced to their corresponding alcohols by ethanolic sodium borohydride. © 1998 Society of Chemical Industry.