Xylan Biosynthesis and Modification Characterisation of a Glycosyltransferase and a Glycoside Hydrolase in Hybrid Aspen

Wood is an important renewable material used by humans for a variety of downstream applications. The basic subcellular structure in wood is the cell wall, mainly consisting of the cross-linked polymers cellulose, hemicellulose, and lignin. Xylan is the main hemicellulose found in angiosperm wood, and its biosynthesis and effects on wood properties in hybrid aspen (Populus tremula x tremuloides) were the object of the present study. We identified a glycoside hydrolase in developing wood, PtxtXyn10A— a previously suggested xylanase—and found that it rather has xylan transglycosylase activity. This xylan transglycosylase probably rearranges xylan chains during secondary wall deposition. Interestingly, it affected the orientation of cellulose microfibrils, giving new insights into the interaction of cell wall polymers and their control during deposition into the wall. To study genes involved in secondary wall formation and to modify wood properties using transgenic approaches, it is essential to efficiently target transgene expression to the secondary wall forming cells. We isolated and tested a promoter, pGT43B, which proved to be very effective in altering gene expression and in generating intended chemical modifications in woody cells. We applied this GT43B promoter to modify xylan acetylation and polymerisation in hybrid aspen wood. Furthermore, it enabled the identification of the sucrose transporter SUT3, elucidating how sucrose is transported from ray to developing fiber and vessel cells. Glycosyltransferases (GTs) are enzymes involved in polysaccharide biosynthesis. We characterised the xylan biosynthetic GT43 gene family in hybrid aspen and found that reduced GT43 expression levels positively affected overall plant growth, wood mechanical strength, and saccharification efficiency, indicating potential applications for engineering bioenergy feedstocks. We conclude that cell specific modification of xylan properties in hybrid aspen can alter cellulose deposition and result in activation of cambial proliferation which has a positive impact on overall biomass yield and properties for downstream woodutilisation.