Increase in 4-Coumaryl Alcohol (H) Units During Lignification in Alfalfa (Medicago sativa) Alters Extractability and Molecular Weight of Lignin

The lignin content of biomass can impact the ease biomass processing and therefore genetic modification of the lignin biosynthetic pathway has the potential to reduce costs in biomass processing industries (e.g. pulp and paper, forage, and biofuels). We investigated compositional changes in two low lignin alfalfa (Medicago sativa) lines with antisense down-regulation of p-coumarate 3-hydroxylase (C3H) or hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT). Gel permeation chromatography (GPC) of isolated ball milled lignin indicated significant changes in the weight averaged molecular weight distribution of the control versus the transgenic lines. Sequential base extraction greatly reduced the lignin content of the transgenic lines leaving a residual H lignin component. We hypothesize the increased extractability of the lignin-like material is due to a decrease in molecular weight of the polymer. The decrease in molecular weight is explained by the reduced stability of the coumaryl alcohol monomeric building block during radical coupling compared to the more prevalent sinapyl and coniferyl alcohol. Our findings suggest that the lignin molecular weight, and as a result, the ease with which it can be removed during chemical processing, can be influenced by altering monomer ratio.