Stronger cellulose microfibril network structure through the expression of cellulose-binding modules in plant primary cell walls

In plant primary cell walls, cellulose microfibrils (CMFs) act as a load bearing component embedded in a hydrated matrix of hemicelluloses and pectins (Brown 2004 ; Carpita and Gibeaut 1993 ; Somerville 2006 ). CMFs are synthesized at the surface of the plasma membrane by large cellulose synthase complexes designated as rosettes. Cellulose nanofibers or cellulose nanofibrils (CNFs) are three orders of magnitude smaller than intact plant fiber cells, and have attracted particular attention in nanotechnology and material sciences recently. CNFs can be prepared from diverse sources, such as wood pulp, bacterial cellulose, and food processing waste streams. Extensive efforts have been devoted to their preparation and characterization, and to the fabrication of new CNF-based composite structures, including films, fibers, aerogels, and hydrogels (Klemm et al. 2011 ; Moon et al. 2011 ). The preparation of CNFs generally requires several steps, typically an enzymatic pretreatment and/or a chemical modification step followed by mechanical homogenization. For example, CNFs with uniform width similar to that of individual CMFs spun by single rosettes (ca. 3.5 nm) have been successfully prepared from wood pulp fibers using chemical pretreatment and subsequent mechanical disintegration (Pei et al. 2013 ; Saito et al. 2006 ). Each of the preparation steps influences the particle shape, size and size distribution, and interfacial properties of the CNFs obtained. In order to exploit improved preparation methods to decrease the internal damage in CNFs and lower the energy consumption required, an increased understanding of the mechanism involved in plant cell wall formation and its recalcitrance to degradation is essential. To make biomass less recalcitrant to processes for energy production, such as biofuel generation, the cell wall can be modified by altering its chemical composition or properties through genetic engineering (Albersheim et al. 2010 ). The modification of the biosynthetic process of cellulose in plant cell walls has great potential to alter the structure of cellulose microfibrils and decrease their resistance against chemical and mechanical treatments. However, the impact of such genetic engineering approaches on the extraction of cellulose nanomaterials from biomass, including nanofibrils and nanocrystals, has not been addressed previously.