Abstract
The lignin content of biomass can impact the ease and cost of biomass processing. Lignin reduction through breeding and genetic modification therefore has potential to reduce costs in biomass-processing industries (e.g. pulp and paper, forage, and lignocellulosic ethanol). 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 hydroxycinnamoyltransferase (HCT). We investigated whether the difference in reactivity during lignification of 4-coumaryl alcohol (H) monomers versus the naturally dominant sinapyl alcohol and coniferyl alcohol lignin monomers alters the lignin structure. Sequential base extraction readily reduced the H monomer content of the transgenic lines, leaving a residual lignin greatly enriched in H subunits; the extraction profile highlighted the difference between the control and transgenic lines. Gel permeation chromatography of isolated ball-milled lignin indicated significant changes in the weight average molecular weight distribution of the control versus transgenic lines (CTR1a, 6000; C3H4a, 5500; C3H9a, 4000; and HCT30a, 4000).
Highlights
The advent of large-scale liquid fuel production from biomass has served to highlight how difficult it is to commercially process biomass effectively and efficiently
We investigated whether the difference in reactivity during lignification of 4-coumaryl alcohol (H) monomers versus the naturally dominant sinapyl alcohol and coniferyl alcohol lignin monomers alters the lignin structure
Extractions—Methanol extraction was first used to ensure the removal of small molecules that are structurally unrelated to the plant cell wall
Summary
The advent of large-scale liquid fuel production from biomass has served to highlight how difficult it is to commercially process biomass effectively and efficiently. Accessibility to the cell wall is influenced by lignin, which provides structural integrity to the cell wall Both total lignin content and lignin monomer composition may impact the ease with which biomass is processed. In addition to the core H/G/S polymer, lignin includes a range of other structures commonly bonded to the main lignin structure through ether or ester linkages Such modifications are especially frequent in herbaceous species and usually involve the addition of p-coumaric acid, ferulic acid, and occasionally sinapic acid. Transgenic lines of alfalfa (Medicago sativa) with antisense down-regulation of the expression of p-coumarate 3-hydroxylase (C3H) and hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase (HCT) were used as models Both C3H and HCT genes are involved in the production
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