Abstract text: Xylan is the most abundant hemicellulose in eudicot secondary cell walls. The xylan backbone interacts with cellulose, while its glucuronic acid substitutions crosslink to lignin. Although defects in xylan and other hemicelluloses lead to a broad range of phenotypes, understanding their molecular roles has been limited by the difficulty in analysing the cell wall architecture at the nanoscale in planta.

Here, using 13C solid-state NMR, we show that xylan is required to maintain cellulose and lignin in close spatial association. Solid-state NMR enables the characterisation of polymer conformation, mobility, and intermolecular proximities within intact cell walls. Analysis of xylan-deficient mutants revealed that short xylan chains fail to associate effectively with cellulose, and that xylan binding alters the conformation of cellulose glucosyl residues. Furthermore, removal of glucuronic acid substitutions from short xylan resulted in stunted plant growth and produced xylan that failed to associate with either cellulose or lignin. This disruption led to a cell wall architecture in which cellulose, xylan, and lignin show minimal interaction.

These results provide new molecular insights into the organisation of secondary cell walls and highlight the power of solid-state NMR to resolve polymer interactions in planta, with implications for improving lignocellulosic biomass utilisation.