Rhamnogalacturonan‐I is a determinant of cell–cell adhesion in poplar wood

Abstract

Summary The molecular basis of cell–cell adhesion in woody tissues is not known. Xylem cells in wood particles of hybrid poplar (Populus tremula × P. alba cv. INRA 717‐1B4) were separated by oxidation of lignin with acidic sodium chlorite when combined with extraction of xylan and rhamnogalacturonan‐I (RG‐I) using either dilute alkali or a combination of xylanase and RG‐lyase. Acidic chlorite followed by dilute alkali treatment enables cell–cell separation by removing material from the compound middle lamellae between the primary walls. Although lignin is known to contribute to adhesion between wood cells, we found that removing lignin is a necessary but not sufficient condition to effect complete cell–cell separation in poplar lines with various ratios of syringyl:guaiacyl lignin. Transgenic poplar lines expressing an Arabidopsis thaliana gene encoding an RG‐lyase (AtRGIL6) showed enhanced cell–cell separation, increased accessibility of cellulose and xylan to hydrolytic enzyme activities, and increased fragmentation of intact wood particles into small cell clusters and single cells under mechanical stress. Our results indicate a novel function for RG‐I, and also for xylan, as determinants of cell–cell adhesion in poplar wood cell walls. Genetic control of RG‐I content provides a new strategy to increase catalyst accessibility and saccharification yields from woody biomass for biofuels and industrial chemicals.

UDP‐Api/UDP‐Xyl synthases affect plant development by controlling the content of UDP‐Api to regulate the RG‐II‐borate complex

“…Both calcium cross‐links and pectin esters are required for cell–cell adhesion in fresh sugar beet ( Beta vulgaris ) root parenchyma (Marry et al ., 2006). However, in poplar ( Populus ) wood, RG‐I, but not HG, is a determinant of cell–cell adhesion (Yang et al ., 2020). RG‐II is regarded as decorated HG.…”

A TEMPO-catalyzed oxidation–reduction method to probe surface and anhydrous crystalline-core domains of cellulose microfibril bundles

“…The enhancement of sacchari cation potential observed after catalytic deligni cation is more likely a result of alterations of nanoscale and macroscale properties than in the molecular scale interactions of lignin and cellulose. As high S-lignin content results of more facile cell separation and comminution of poplar wood particles (Yang et al 2020), enhanced sacchari cation in the CDL-treated materials might re ect enhanced access of cellulases into the biomass despite the increased cellulose bundling after deligni cation. Cellulose bundling is still a major factor of recalcitrance to digestibility.…”

Redesigning plant cell walls for the biomass-based bioeconomy

“…A relatively minor wall constituent called rhamnogalacturonan-I is also a determinant of cell-cell adhesion. Oxidation of lignin by sodium chlorite and extraction of hemicelluloses and pectins by alkali were both required to achieve fiber cell separation in wood particles from poplar (26). Xylanase alone was ineffective in permitting cell separation of delignified materials, but addition of a rhamnogalacturonan-I lyase caused dissolution of the compound middle lamella.…”

“…Xylanase alone was ineffective in permitting cell separation of delignified materials, but addition of a rhamnogalacturonan-I lyase caused dissolution of the compound middle lamella. When an Arabidopsis gene encoding a rhamnogalacturonan-I lyase was expressed in poplar, disruption of cell-cell adhesion, reduction in particle size upon grinding, and saccharification yields were all enhanced over wild-type controls without reducing biomass yield (26). The transgenic poplar wood was more easily fragmented under mechanical stress, potentially reducing energy inputs for comminution.…”

“…1A). Pectins comprise a class of acidic polysaccharides including homogalacturonans, polymers of galacturonic acid, and rhamnogalacturonan-I, a pectic polymer of alternating rhamnose and galacturonic acid residues substituted with side-chains of arabinans and galactans (26). In grasses and related monocot species, including bioenergy crops, such as sorghum and switchgrass, glucuronoarabinoxylans are the most abundant hemicelluloses that, together with cross-linking phenylpropanoid networks, form the primary walls (Fig.…”