Wood is an essential natural and renewable resource for human activities; e.g. paper and pulp industries, house construction and energy production. Wood cells such as fibers are fundamentally important cells whose morphology and chemical components influence the wood quality. They are formed in the vascular cambium and differentiate to maturity through cell elongation/expansion and deposition of secondary cell wall during the highly organized process of wood formation. Final cell morphology is largely determined by plasticity of its primary cell walls, while cell wall chemical composition is mainly determined during the secondary cell wall formation. These features are directly regulated by cell-wall residing enzymes, which modulate the cell wall components. Here I describe the functions of selected carbohydrate-active extracellular hydrolases including cellulases, a xylanase and a xyloglucan endotransglycosylase (XET), which are identified to be highly expressed at specific phases of wood formation in hybrid aspen (Populus tremula L. x Populus tremuloides Michx.). The XET PttXET16-34 is expressed during the primary cell wall stage and regulates cell growth by strengthening or weakening xyloglucan-cellulose microfibril networks. A putative xylanase, PttXyn10A, and a membrane anchored cellulase, PttCel9A1, are highly expressed during the secondary wall stage of xylem cell development. PttXyn10A may assist with the remaining fiber elongation at the early stage of secondary cell wall deposition by softening the walls by degrading xylans cross-linking to lignins. PttCel9A1 facilitates cellulose biosynthesis in a way that decreases cellulose crystallinity in cell walls, which is of great importance for the properties of cell wall structural framework. Thus, the elaboration of wood cells is performed through the well-coordinated biosynthesis and modification of chemical components, and through the diverse and dynamic actions of specific carbohydrate-active hydrolases. The understanding of these enzyme actions will lead to the improvement of wood characteristics to create biomaterials more applicable for different aspects of the forest industry
