Table 1.

Xylan as a source of recalcitrance to woody biomass processing and improvements being made to reduce recalcitrance.

	Saccharification and Fermentation (S&F)	Dissolving pulp production (DPP)
Main product(s)	• Ethanol [1,2,3].	• Pure cellulose: nanocellulose viscose (textiles), rayon (tire strings), cellulose acetate films, methylcellulose, nanopaper, surgical stitches [2,4,5,6].
Value added Products from “waste”	• Bioplastics, fermentable lignin, pharmaceuticals, flavourants [2].	• Xylitol, lignosulphonates, bioinks, nanoparticles, pharmaceuticals, bioplastics [5,7,8,9].
Xylans Impact on the industrial process	• Blockage of glucanase access to cellulose as a result of xylan major domain’s tight association with cellulose reduces saccharification efficiency [10]. • Pre-treatment causes xylan present on the hydrophilic face of cellulose to slide to the hydrophobic face through the action of GlucA .groups, which facilitates additional chemical treatment to remove [12]. • Lignin carbohydrate complexes formed during pre-treatment block xylanases [13, 14, 15, 16] • Pre-treatment causes undesirable and toxic breakdown products which inhibit fermentation into ethanol [17,18]. • Acetyl groups blocking CWDE [19, 20, 21]. • Released acetyl groups alter pH of fermentation fluid thereby inhibiting fermentation [17,18]. • GlucA methylation affects xylose release [27]. • Yeast does not ferment xylose efficiently [17,18].	• Xylan major domain’s tight association with cellulose reduces separation and purity of biopolymers [11,12]. • Pre-treatment causes xylan present on the hydrophilic face of cellulose to slide to the hydrophobic face through the action of GlucA groups, which facilitates additional chemical treatment to remove [12]. • Lignin carbohydrate complexes formed during pre-treatment block efficient chemical removal [13,14,15,16] • Pre-treatment causes undesirable and toxic breakdown products which decrease the purity of the separated biopolymers [4] • Strong acid treatment decreases strength of cellulose fibers due to cellulose autohydrolysis [4,22,23]. • Released acetyl groups alter pH of alkaline pulping liquors [24,25,26]. • Calcium bridges and crystal formation around adjacent GlucA groups lead to the use of strong acids for xvlan removal [12]. • Stacking of multiple xylan chain increase stability xylans association with cellulose [12].
Improvements to techniques	• Ionic liquid and microwave assisted heating used during pretreatment increase biomass separation while decreasing toxin and inhibitor production [28,29,30,31]. • Multifunctional enzymes [32.33,34]. • Genetically engineered yeasts [38,39,40,41].	• Ionic liquids allow for improved biopolymer separation [28,29,30,31]. • Improved cellulose fibrillation with chemical treatment with 2,2,6,6- tetramethylpiperidine-l-oxyl (TEMPO) [35,36,37]. • WET spinning small cellulose fragments into large [42,43].
Plant biotechnology approaches	Down-regulation of recalcitrance associated genes [44,45,46,47]. Upregulation of recalcitrance reducing genes [48,49]. Knock-out mutagenesis of recalcitrance associated genes [27,50,51]. Vessel complementation of knock-out mutants [52]. Ectopic xylan modifications [53,54,55]. Metabolic engineering [56,57,58,59,60,61] CRISPR:CAS9,dCAS9, or activator and DNA methyltransferase fusion dCAS9 [62,63,64]. Endogenous expression of processing enzymes which can also become active only under specific conditions | 65,66.67,68,69]. Promoter feedback loops [70]. Gene stacking [71].

References pertaining to numbered items can be found in Supplementary File S2.