Abstract
Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.
Highlights
Lignocellulosic biomass from softwood and hardwood trees is most commonly used for construction, pulp and paper products and for biorefinery applications that entail separating the biomass into its individual components to produce various bioproducts (Xavier et al, 2010; Pei et al, 2016; Zhu et al, 2016)
Additional unknowns include the genes which code for the Golgi localized GlcA and SAM transporters, the exact functions of IRX15/IRX15L, the full complement of xylan synthase complex (XSC) and reducing end sequence (RES) synthesizing proteins, and whether there are any more xylan associated proteins
Work in monocots suggest that several other proteins are involved in xylan biosynthesis and interact with the biosynthetic machinery (Zeng et al, 2010; Jiang et al, 2016)
Summary
Lignocellulosic biomass from softwood and hardwood trees is most commonly used for construction, pulp and paper products and for biorefinery applications that entail separating the biomass into its individual components to produce various bioproducts (Xavier et al, 2010; Pei et al, 2016; Zhu et al, 2016). After chemical or enzymatic pretreatment, the cellulosic and hemicellulosic component of lignocellulosic biomass can be subjected to saccharification and fermentation; a process which employs chemicals, enzymes and microbes to convert the polysaccharide components into ethanol for second generation biofuels and various bioproducts (Ragauskas et al, 2014) Product value in these industries is driven by high product quality and purity, but the physical properties of the SCW biopolymers themselves impede the efficiency of deconstructing the biomass (Gübitz et al, 1998; Himmel et al, 2007; DeMartini et al, 2013; McCann and Carpita, 2015). Despite these studies highlighting certain aspects of xylan biosynthesis, a xylan-centric systems biology analysis still needs to be performed to gain a holistic understanding of the process
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