Xylan in the middle : understanding xylan biosynthesis and its metabolic dependencies toward improving wood fiber for industrial processing

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.Table S1. List of enzymes which catalyze the biosynthesis of sugar nucleotides, s-adenosyl methionine and acetyl-CoA. (a) The numbering of the enzymes corresponds to Figures 4–6. (b) Name of the enzyme. (c) Not all accessions were provided for each reaction, just one representative from each cellular compartment. Accessions and cellular localisation were obtained from TAIR and SUBA3, respectively.File S1. References pertaining to numbered items in Table 1.File S2. Biotechnology approaches that have scaled from Arabidopsis to Populus.This work was funded in part by the National Research Foundation (NRF) of South Africa – Bioinformatics and Functional Genomics Programme (BFG Grant UID 86936 and 97911), the Technology and Human Resources for Industry Programme (THRIP Grant UID 96413), the Department of Science and Technology (DST, Strategic Grant for the Eucalyptus Genomics Platform) and by Sappi Forest Research through the Forest Molecular Genetics (FMG) Programme at the University of Pretoria (UP). MW acknowledges postgraduate scholarship support from the NRF. VM acknowledges a postdoctoral fellowship support from UP.http://www.frontiersin.org/Plant_Scienceam2019BiochemistryForestry and Agricultural Biotechnology Institute (FABI)GeneticsMicrobiology and Plant Patholog