STEWART, J. J., AKIYAMA, T., CHAPPLE, C., RALPH, J. & MANSFIELD, S. D. (2009) The Effects on Lignin Structure of Overexpression of Ferulate 5-Hydroxylase in Hybrid Poplar1. Plant Physiol., 150, 621-635.

Abstract: Poplar lignins with exceedingly high syringyl monomer levels are produced by over-expression of the ferulate 5-hydroxlase (F5H) gene driven by a cinnamate 4-hydroxylase (C4H) promoter. Compositional data derived from both standard degradative methods and NMR analyses of the entire lignin component (as well as isolated lignin fraction) indicated that the C4H::F5H transgenic lignin was comprised of as much as 97.5% syringyl units (derived from sinapyl alcohol), the remainder being guaiacyl units (derived from coniferyl alcohol); the syringyl level in the wild-type control was 68%. The resultant transgenic lignins are more linear, and display a lower degree of polymerization. Although the crucial β-ether content is similar, the distribution of other inter-unit linkages in the lignin polymer is markedly different, with higher resinol (β–β) and spirodienone (β–1) contents, but with virtually no phenylcoumarans (β–5, which can only be formed from guaiacyl units). p-Hydroxybenzoates, acylating the γ-positions of lignin sidechains, were reduced by over 50%, suggesting consequent impacts on related pathways. A model depicting the putative structure of the transgenic lignin resulting from the over-expression of F5H is presented. The altered structural features in the transgenic lignin polymer, as revealed here, support the contention that there are
significant opportunities to improve biomass utilization by exploiting the malleability of plant lignification processes.