Designer biomass for next-generation biorefineries: leveraging recent insights into xylan structure and biosynthesis

Abstract

Xylans are the most abundant noncellulosic polysaccharides in lignified secondary cell walls of woody dicots and in both primary and secondary cell walls of grasses. These polysaccharides, which comprise 20–35% of terrestrial biomass, present major challenges for the efficient microbial bioconversion of lignocellulosic feedstocks to fuels and other value-added products. Xylans play a significant role in the recalcitrance of biomass to degradation, and their bioconversion requires metabolic pathways that are distinct from those used to metabolize cellulose. In this review, we discuss the key differences in the structural features of xylans across diverse plant species, how these features affect their interactions with cellulose and lignin, and recent developments in understanding their biosynthesis. In particular, we focus on how the combined structural and biosynthetic knowledge can be used as a basis for biomass engineering aimed at developing crops that are better suited as feedstocks for the bioconversion industry.

Identification of an algal xylan synthase indicates that there is functional orthology between algal and plant cell wall biosynthesis

“…Xylan is an abundant and complex cell wall component in plants, particularly in commelinid primary walls and in secondary cell walls of all angiosperms (Scheller & Ulvskov, 2010). It consists of a polymeric backbone of 1,4-b-linked D-xylose (Xyl) decorated mainly with acetyl groups, and is further substituted by L-arabinofuranose (Araf ) in commelinids or methylated or unmethylated D-glucuronic acid (GlcA) in noncommelinid angiosperms (Scheller & Ulvskov, 2010;Smith et al, 2017;Fig. 2).…”

Designing xylan for improved sustainable biofuel production

“…These enzymes catalyse the biosynthesis of the xylan backbone, transferring nucleotide sugars to the growing AX chain within the Golgi apparatus. Two members of the glycosyltransferase family 43 (GT43), IRREGULAR XYLEM9 (IRX9) and IRX14 proteins, and one member of GT47, IRX10, are implicated in the biosynthesis of the xylan backbone, but the specific role is not completely established (Brown et al ., ; Smith et al ., ). Subsequent works confirmed that IRX10 is the β‐1,4‐xylan xylosyl transferase responsible for xylan polymer extension, transferring xylosyl residues from UDP‐xylose to xylooligosaccharides at the reducing end, whereas IRX9 and IRX14 are accessory proteins involved in the elongation of the xylan backbone and are structural components of the functioning xylan synthase complex (XSC).…”

“…Subsequent works confirmed that IRX10 is the β‐1,4‐xylan xylosyl transferase responsible for xylan polymer extension, transferring xylosyl residues from UDP‐xylose to xylooligosaccharides at the reducing end, whereas IRX9 and IRX14 are accessory proteins involved in the elongation of the xylan backbone and are structural components of the functioning xylan synthase complex (XSC). Additionally, glucuronosyltransferases (GUX) from the GT8 family and arabinosyltransferases (XAT) from the GT61 family are responsible for the addition of glucuronosyl and arabinosyl on the xylan backbone, respectively (Smith et al ., ).…”

“…For more background information of xylan biosynthesis and modifications, see the review by Smith et al . ().…”

“…Arabinoxylan also influences the enzymatic hydrolysis of cellulose, and it requires enzymes different from those used to hydrolyse cellulose. Lowering AX content and/or its decorations (with ara f and FA) reduces the cross‐linkages among AX, lignin and cellulose in plant cell walls, decreasing biomass recalcitrance (Smith et al ., ). Research aiming at elucidating the genes required for xylan biosynthesis, the way they are controlled and how changes in these genes influence plant development has been boosted by the potential of plant biomass as a source of renewable energy.…”