Lignin Engineering in Forest Trees

Abstract

Wood is a renewable resource that is mainly composed of lignin and cell wall polysaccharides. The polysaccharide fraction is valuable as it can be converted into pulp and paper, or into fermentable sugars. On the other hand, the lignin fraction is increasingly being considered a valuable source of aromatic building blocks for the chemical industry. The presence of lignin in wood is one of the major recalcitrance factors in woody biomass processing, necessitating the need for harsh chemical treatments to degrade and extract it prior to the valorization of the cell wall polysaccharides, cellulose and hemicellulose. Over the past years, large research efforts have been devoted to engineering lignin amount and composition to reduce biomass recalcitrance toward chemical processing. We review the efforts made in forest trees, and compare results from greenhouse and field trials. Furthermore, we address the value and potential of CRISPR-based gene editing in lignin engineering and its integration in tree breeding programs.

Accurate Determination of Genotypic Variance of Cell Wall Characteristics of a Populus trichocarpa Pedigree Using High-Throughput Pyrolysis-Molecular Beam Mass Spectrometry

“…In particular, total lignin content and monomer composition can impact the technical and economic feasibility of using lignocellulosic biomass such as wood as a feedstock for biofuels or other goods (1)(2)(3). Modifying lignin content or composition could be accomplished in several ways, including genetic engineering (4)(5)(6)(7)(8) or environmental priming (9,10).…”

“…Lignin content can also be modified through breeding approaches, taking advantage of natural variation to obtain significant trait gains, using well-established protocols and experimental designs (11,12). Moreover, recent sophisticated techniques such as genomic selection allow for accelerated breeding with considerable reduction in costs (7,13,14).…”

SNPeffect: identifying functional roles of SNPs using metabolic networks

“…It is a major contributor to woody biomass recalcitrance as it prevents the access of extracellular enzymes to the degradable sugar moieties. Thus, there is considerable interest in developing plants with reduced lignin content that do not incur a growth penalty (Chanoca et al , 2019) and SNPeffect‐identified SNPs can serve as potential tunable levers for controlling flux through this pathway. Indeed, growth‐related phenotypes have been reported for lignin biosynthetic genes flagged as having functional SNPs.…”

Compensatory guaiacyl lignin biosynthesis at the expense of syringyl lignin in4CL1-knockout poplar

“…Furthermore, silencing lignin-associated 4CLs can have different effects on syringyl-to-guaiacyl lignin (S/G) ratio depending on the species, ranging from increases in tobacco, Arabidopsis , and switchgrass (Kajita et al, 1996; Lee et al, 1997; Xu et al, 2011), to little change in alfalfa (Nakashima et al, 2008), to decreases in and rice (Gui et al, 2011). Variable effects on S/G ratio have even been reported among closely related poplar species (Hu et al, 1999; Voelker et al, 2010; Chanoca et al, 2019 and references therein). Such variation has been attributed both to differing degrees of 4CL downregulation, as well as to 4CL multiplicity (Boerjan et al, 2003; Saballos et al, 2012).…”