Glucuronoyl Esterase Expressed in Aspen Xylem Affects γ‐Ester Linkages Between Lignin and Glucuronoxylan Reducing Recalcitrance and Accelerating Growth

Wood is the most abundant renewable natural resource composed of different polysaccharides and lignin, but its utilisation is hampered by intermolecular linkages between these components forming lignin-carbohydrate complexes (LCCs) causing recalcitrance. The links between glucuronoxylan and the γ-C of lignin (γ-ester linkages) are thought to contribute to one-third of LCCs, but direct evidence for their natural occurrence and their role in recalcitrance has been scarce so far. To address these issues, Phanerochaete carnosa glucuronoyl esterase (PcGCE), hydrolysing γ-ester linkages, was expressed in cell walls of developing wood in hybrid aspen (Populus tremula L. × tremuloides Michx.). The enzyme reduced HSQC 2D NMR signals corresponding to the γ-esters and xylan in dioxane-extracted LCCs without altering glucuronoxylan content or structure. This increased acid solubility of lignin and decreased lignin content. Reduced wood recalcitrance was shown by increased sugar yields and glucose production rates (by approx. 20%) in saccharification without pretreatment and increased xylan extractability by subcritical water (by approx. 70%). Moreover, trees expressing PcGCE exhibited greater primary and secondary growth. Transcriptomics and metabolomics analyses in developing wood suggested that growth could have been induced by a higher transcription of SMR2 and RPOTmp, which was likely triggered by the secondary cell wall integrity signalling. The results provide evidence for the natural existence of LCC γ-esters and their significant contribution to lignocellulose recalcitrance. Furthermore, they show that reducing γ-ester linkages could increase plant productivity.