Wood is a complex biological structure providing trees with the mechanical support needed to ensure upright growth whilst simultaneously facilitating the transport of nutrients and water (Groover et al., 2010). An important role of mechanical reinforcement in wood is performed by cellulose microfibrils (CMFs) which are hierarchical structures of β(1→4) linked D-glucose units which form glucan chains of nanoscale semi-crystalline fibrils in a bottom-up fashion (Fernandes et al., 2011). The CMFs are positioned throughout the wood cell walls where they are embedded in a hydrated matrix comprising primarily of hemicellulose and lignin (Knox, 2008). The resulting natural nanocomposite structure gives rise to the distinct properties of wood (Gibson, 1992).
The prospect of extracting CMFs as a value-added product from wood has gained attention over the past two decades. CMFs that have been subjected to isolation and purification processes are referred to as cellulose nanofibrils (CNFs) and inherit many of the distinct properties of CMFs, including high tensile strength (Wu et al., 2014), stiffness (Iwamoto et al., 2009), aspect ratio (Varanasi et al., 2013) and low coefficient of thermal expansion (Fukuzumi et al., 2009).
Development of various processing techniques have further enabled successful isolation of CNFs that closely resembles the most basic crystalline structures of the CMFs (Saito and Isogai, 2004; Saito et al., 2009; Wågberg et al., 2008). Through careful processing it is also possible to retain high degree of polymerization (DP) of the cellulose (Saito et al., 2009), which is manifested as longer CNFs (Henriksson et al., 2008; Shinoda et al., 2012). These process developments have further led to the prospect where the characteristics of CMFs are potentially influencing the isolated CNFs. Thus, understanding of how to alter CMFs in the original feedstock opens for the possibility to control final CNF characteristics.
The understanding of how CMFs are synthesized in plants has been described as one of the main challenges in plant biology (Saxena and Brown Jr, 2005). Research of the CMF synthesis machinery has revealed multiple proteins that are involved in this process (Polko and Kieber, 2019). One of the cellulose biosynthesis associated proteins that plays a crucial role in the alignment of the nascent CMFs into the cell wall is the cellulose synthase interactive 1 (CSI1) protein (Gu et al., 2010; Li et al., 2012). The plasma membrane localized cellulose synthase complex (CSC) moves in the plasma membrane during cellulose biosynthesis (Paredez et al., 2006). CSI1 guides the CSC along the cortical microtubules (cMTs) to align CMFs during primary cell wall biosynthesis (Bringmann et al., 2012; Li et al., 2012), and during the initial phase of secondary cell wall formation (Schneider et al., 2017). From a study of transgenic trees with reduced expression of CSI1 it was observed that both stiffness and strength of the wood was decreased, as well as the cellulose DP (Bünder et al., 2020). There were no apparent structural or compositional changes in the wood cell wall of the transgenic lines which led to the hypothesis that a reduction of CSI1 may impact the mechanical properties of the wood by a reduction of the cellulose DP and thus altered CMF characteristics. It is thus of interest to isolate CNFs from these transgenic trees with a reduced level of CSI1 and assess the possible influence of the genetic modification on CNF properties.
To compare CNFs isolated from wild type (WT) and transgenic wood, mild (pH = 6.8) direct oxidation using the catalytic system 2,2,6,6-tetramethylpiperidin-1-yl) oxyl (TEMPO) was employed together with high-pressure homogenization (Saito et al., 2009). This process has the benefit of allowing for direct oxidation of lignin and cellulose in one experimental step whilst preserving the cellulose DP (Jonasson et al., 2020; Kaffashsaie et al., 2021; Ma et al., 2012; Puangsin et al., 2017; Saito et al., 2009). Two transgenic lines with reduced expression of CSI1 were studied together with the control tree (WT) with normal CSI1 expression. The isolated CNFs were characterized in the dispersion state using viscosity, conductimetric titration, atomic force microscopy, yield- and surface area estimations. Networks were then manufactured from the dispersions and tested for DP, water-uptake and mechanical behavior. The characteristics of the CNFs and their networks are discussed in the context of initial wood properties.