Lignin Manipulation in Plants and Inspirations from Nature toward 鈥楧esigning鈥 Biomass Lignins for the Biorefinery
Abstract
Lignin remains one of the most significant barriers to the efficient utilization of lignocellulosic substrates, in processes ranging from ruminant digestibility to indus颅trial pulping, and in the current focus on biofuels production. Inspired largely by the recalcitrance of lignin to biomass processing, plant en颅gineering has routinely sought to alter lignin quantity, composition, and structure by exploiting the inherent plasticity of lignin biosynthesis.
More recently, researchers are attempting to strategically design plants for increased degradability by incorporating monomers that lead to a lower degree of polymerization, reduced hydrophobicity, fewer bonds to other cell wall constituents, or novel chemically labile linkages in the polymer backbone.1,2 The incorporation of value-added structures could also help valorize lignin. Designer lignins may satisfy the biological requirement for lignification in plants while improving the overall efficiency of biomass utilization. Although possibilities abound, maintaining plant health is paramount and, ultimately, the plants themselves will dictate which of these approaches can be tolerated.
One such method, via the so-called 鈥榸ip-lignin鈥 approach, is showing particular promise.3-5 Poplar has been engineered to incorporate monolignol ferulate conjugates into the lignification process, by using an exotic transferase gene, FMT, and a xylem-specific promoter.5 This results in the introduction of readily cleavable ester linkages into the backbone of the polymer, and delivers significantly improved processing. Various applications for which these altered trees appear superior are only just beginning to be explored. Now that we have sensitive methods for determining if/when/whether plants are making monolignol ferulate conjugates and using them for lignification, it appears that Nature herself may have already been exploring this avenue.6 In attempting to modify monocot lignins, another transferase appears to be useful for improving cell wall digestibility. This involves p-coumaroylation of the lignin. Although this occurs naturally in monocots, it is not evident in dicots, but providing dicots with that pathway has interesting implications.7,8
Nature continues to inspire us with new avenues toward lignin modification that have potential value for various processes. For example, the ramifications of finding that grasses are using tricin, a flavone from beyond the monolignol biosynthetic pathway, to start lignin chains are interesting; tricin itself is valuable, and plants with tricin knocked out have higher lignin and lower CW digestibility.9-13
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13. Eloy NB, Voorend W, Lan W, Cesarino I, Vanholme R, de Lyra Soriano Saleme M, Goeminne G, Pallidis A, Morreel K, Nicomedes J, Ralph J, & Boerjan W (2017) Silencing chalcone synthase in maize impedes the incorporation of tricin into lignins and increases lignin content. Plant Physiology 173:998-1016.
