Designer lignins: inspirations from Nature

Abstract

Lignin remains one of the most significant barriers to the efficient utilization of lignocellulosic substrates, in processes ranging from ruminant digestibility to indus-trial pulping, and in the current focus on biofuels production. Inspired largely by the recalcitrance of lignin to biomass processing, plant engineering efforts have routinely sought to alter lignin quantity, composition, and structure by exploiting the inherent plasticity of lignin biosynthesis. More recently, researchers are attempting to strategically designplants for increased degradability by incorporating monomers that lead to a lower degree of polymerisation, reduced hydrophobicity, fewer bonds to other cell wall constituents, or novel chemically labile linkages in the polymer backbone.[1]In addition, the incorporation of value-added structures could help valorise lignin. Designer lignins may satisfy the biological requirement for lignification in plants while improving the overall efficiency of biomass utilisation.Researchers are now already beginning todesignlignins, by introducing novel phenolic precursors into the plant lignification process, to improve the ease with which the resulting lignins can be removed from the cell wall. Although possibilities abound, maintaining plant health is paramount and, ultimately, the plants themselves will dictate which of these approaches can be tolerated. Onesuch method, via the so-called ‘zip-lignin’ approach, is showing particular promise.[2-4]Poplar trees have been engineered to incorporate monolignol ferulate conjugates into the lignification process, resulting inthe introduction of readily cleavable ester linkages into the backbone of the polymer, and resulting in significantly improved processing. Various applications for which these altered trees seem well suited will be discussed. We’ll describe recent advances, including getting the monolignol ferulate conjugates into grasses, in which we were concerned that the natural p-coumaroylation of mono-lignols might compete. In addition, now that we have sensitive methods for determin-ing if/when/whether plants are making monolignol ferulate conjugates and using them for lignification (methods that have not previously been available), it appears that Nature herself may have already been exploring this avenue. We’ll provide insight into the plants that seem to be doingthis and try to elucidate how. [The question of why is likely to require a lot more time, research, and insight]. Finally, we’ll note other avenues, inspired by Nature, for lignin modification that have potential value for various processes.For example,the ramifications of finding that grasses are using aphenolic, tricin, a flavonoid from beyond the monolignol biosynthetic pathway to start lignin chainsare interesting indeed.[5,6] [1] Y. Mottiar, R. Vanholme, W. Boerjan, J. Ralph, S. D. Mansfield, Curr. Opin. Biotechnol. 2016, 37, 190-200. [2] J. H. Grabber, R. D. Hatfield, F. Lu, J. Ralph, Biomacromolecules 2008, 9, 2510-2516. [3] J. Ralph, Phytochem. Rev. 2010, 9, 65-83.[4] C. G. Wilkerson, S. D. Mansfield, F. Lu, ...,D. Padmakshan, F. Unda, J. Rencoret, J. Ralph, Science 2014, 344, 90-93. [5] J. C. del Río, J. Rencoret, P. Prinsen, Á. T. Martínez, J. Ralph, A. Gutiérrez, J. Agric. Food Chem. 2012, 60, 5922-5935. [6] W. Lan,F. Lu, M. Regner, Y. Zhu, J. Rencoret, S. A. Ralph, U. I. Zakai, K. Morreel, W. Boerjan, J. Ralph, Plant Physiol. 2015, 167, 1284-1295