Abstract text: Xylan–cellulose interactions play a central role in plant cell walls, contributing to wall mechanics and forming a protective sheath around cellulose microfibrils that can impede enzymatic degradation. Understanding how xylanolytic enzymes facilitate access for cellulose-active enzymes is therefore key to deciphering plant fungal biomass decomposition. Lytic polysaccharide monooxygenases (LPMOs) are mono-copper enzymes that oxidatively cleave diverse polysaccharides and are recognised as key contributors to plant biomass deconstruction. A subgroup of cellulose-active AA9 LPMOs can also cleave cellulose-bound xylan. These xylan-active AA9 LPMOs vary in their preference towards xylan or cellulose and in their sensitivity to xylan substitution patterns.

In this work, we demonstrate that xylan-active AA9 LPMOs can cleave acetylated glucuronoxylan that is naturally associated with cellulose fibrils using individualised holocellulose nanofibrils (hCNFs) prepared from native plant cell wall material. Furthermore, xylan-acting AA9s (and xylanases) enhance cellulose depolymerization by cellulases and cellulose-active LPMOs, supporting the idea that they increase cellulose accessibility. Our findings refine the understanding of the biological role of AA9 LPMOs and support the hypothesis that the multiplicity of AA9 genes in saprophytic and plant pathogenic fungi reflects evolutionary adaptation to the structural complexity of plant cell walls.