Engineering Non-cellulosic Polysaccharides of Wood for the Biorefinery

Abstract

Non-cellulosic polysaccharides constitute approximately one third of usable woody biomass for human exploitation. In contrast to cellulose, these substances are composed of several different types of unit monosaccharides and their backbones are substituted by various groups. Their structural diversity and recent examples of their modification in transgenic plants and mutants suggest they can be targeted for improving wood-processing properties, thereby facilitating conversion of wood in a biorefinery setting. Critical knowledge on their structure-function relationship is slowly emerging, although our understanding of molecular interactions responsible for observed phenomena is still incomplete. This review: (1) provides an overview of structural features of major non-cellulosic polysaccharides of wood, (2) describes the fate of non-cellulosic polysaccharides during biorefinery processing, (3) shows how the non-cellulosic polysaccharides impact lignocellulose processing focused on yields of either sugars or polymers, and (4) discusses outlooks for the improvement of tree species for biorefinery by modifying the structure of non-cellulosic polysaccharides.

Keywords: galactan; hemicellulose; non-cellulosic polysaccharides; pectin; secondary cell wall; tree genetic improvement; wood biorefining; woody biomass.

FIGURE 1

Schematic illustration of types of non-cellulosic polysaccharides of wood, including hemicelluloses (gray background), pectins (blue background), callose (yellow background) and AGs-II (orange background), and hardwood fibers and softwood tracheids (inset). Polymer structures were based on different sources: hardwood GX (Teleman, 2009; Smith et al., 2017), softwood arabinoglucuronoxylan (Teleman, 2009; Martínez-Abad et al., 2017; Smith et al., 2017), hardwood and softwood glucomannan (GM), softwood GGM, tension and compression wood galactans, callose (Teleman, 2009), xyloglucan (Carpita and McCann, 2000; Teleman, 2009), HG (Atmodjo et al., 2013), RG-I and -II (Edashige and Ishii, 1996, 1997, 1998; Atmodjo et al., 2013), AG-II (Carpita and McCann, 2000; Hijazi et al., 2014), softwood arabinogalactan (Ponder and Richards, 1997; Teleman, 2009). Polymer localization is based on the following sources: hardwood GX and mannans (Kim and Daniel, 2012; Gorshkova et al., 2015; Guedes et al., 2017), softwood arabinoglucuronoxylan (Altaner et al., 2010; Donaldson and Knox, 2012), callose (Altaner et al., 2010; Zhang et al., 2016), xyloglucan (Bourquin et al., 2002; Sandquist et al., 2010; Nishikubo et al., 2011; Donaldson and Knox, 2012; Kim and Daniel, 2013; Guedes et al., 2017), HG (Kim and Daniel, 2013), RG-I/compression wood galactan/tension wood galactan (Gorshkova et al., 2015; Zhang et al., 2016; Guedes et al., 2017), AG-II/softwood arabinogalactan (Altaner et al., 2010; Guedes et al., 2017). PM, pit membrane; CML, compound middle lamella; S, secondary wall layer (S-layer), G, gelatinous layer (G-layer); C, cavities; S_2i, inner S₂ layer; S_2L, outer lignified S₂ layer.