Characteristics of Cellulose Nanofibrils from Transgenic Trees with Reduced Expression of Cellulose Synthase Interacting 1.

Cellulose nanofibrils can be derived from the native load-bearing cellulose microfibrils in wood. These microfibrils are synthesized by a cellulose synthase enzyme complex that resides in the plasma membrane of developing wood cells. It was previously shown that transgenic hybrid aspen trees with reduced expression of CSI1 have different wood mechanics and cellulose microfibril properties. We hypothesized that these changes in the native cellulose may affect the quality of the corresponding nanofibrils. To test this hypothesis, wood from wild-type and transgenic trees with reduced expression of CSI1 was subjected to oxidative nanofibril isolation. The transgenic wood-extracted nanofibrils exhibited a significantly lower suspension viscosity and estimated surface area than the wild-type nanofibrils. Furthermore, the nanofibril networks manufactured from the transgenics exhibited high stiffness, as well as reduced water uptake, tensile strength, strain-to-break, and degree of polymerization. Presumably, the difference in wood properties caused by the decreased expression of CSI1 resulted in nanofibrils with distinctive qualities. The observed changes in the physicochemical properties suggest that the differences were caused by changes in the apparent nanofibril aspect ratio and surface accessibility. This study demonstrates the possibility of influencing wood-derived nanofibril quality through the genetic engineering of trees.

The Effect of High Lignin Content on Oxidative Nanofibrillation of Wood Cell Wall.

Wood from field-grown poplars with different genotypes and varying lignin content (17.4 wt % to 30.0 wt %) were subjected to one-pot 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl catalyzed oxidation and high-pressure homogenization in order to investigate nanofibrillation following simultaneous delignification and cellulose oxidation. When comparing low and high lignin wood it was found that the high lignin wood was more easily fibrillated as indicated by a higher nanofibril yield (68% and 45%) and suspension viscosity (27 and 15 mPa·s). The nanofibrils were monodisperse with diameter ranging between 1.2 and 2.0 nm as measured using atomic force microscopy. Slightly less cellulose oxidation (0.44 and 0.68 mmol·g-1) together with a reduced process yield (36% and 44%) was also found which showed that the removal of a larger amount of lignin increased the efficiency of the homogenization step despite slightly reduced oxidation of the nanofibril surfaces. The surface area of oxidized high lignin wood was also higher than low lignin wood (114 m2·g-1 and 76 m2·g-1) which implicates porosity as a factor that can influence cellulose nanofibril isolation from wood in a beneficial manner.

Sucrose synthase determines carbon allocation in developing wood and alters carbon flow at the whole tree level in aspen.

Despite the ecological and industrial importance of biomass accumulation in wood, the control of carbon (C) allocation to this tissue and to other tree tissues remain poorly understood. We studied sucrose synthase (SUS) to clarify its role in biomass formation and C metabolism at the whole tree level in hybrid aspen (Populus tremula × tremuloides). To this end, we analysed source leaves, phloem, developing wood, and roots of SUSRNAi trees using a combination of metabolite profiling, 13 CO2 pulse labelling experiments, and long-term field experiments. The glasshouse grown SUSRNAi trees exhibited a mild stem phenotype together with a reduction in wood total C. The 13 CO2 pulse labelling experiments showed an alteration in the C flow in all the analysed tissues, indicating that SUS affects C metabolism at the whole tree level. This was confirmed when the SUSRNAi trees were grown in the field over a 5-yr period; their stem height, diameter and biomass were substantially reduced. These results establish that SUS influences C allocation to developing wood, and that it affects C metabolism at the whole tree level.

CELLULOSE SYNTHASE INTERACTING 1 is required for wood mechanics and leaf morphology in aspen.

Cellulose microfibrils synthesized by CELLULOSE SYNTHASE COMPLEXES (CSCs) are the main load-bearing polymers in wood. CELLULOSE SYNTHASE INTERACTING1 (CSI1) connects CSCs with cortical microtubules, which align with cellulose microfibrils. Mechanical properties of wood are dependent on cellulose microfibril alignment and structure in the cell walls, but the molecular mechanism(s) defining these features is unknown. Herein, we investigated the role of CSI1 in hybrid aspen (Populus tremula × Populus tremuloides) by characterizing transgenic lines with significantly reduced CSI1 transcript abundance. Reduction in leaves (50-80%) caused leaf twisting and misshaped pavement cells, while reduction (70-90%) in developing xylem led to impaired mechanical wood properties evident as a decrease in the elastic modulus and rupture. X-ray diffraction measurements indicate that microfibril angle was not impacted by the altered CSI1 abundance in developing wood fibres. Instead, the augmented wood phenotype of the transgenic trees was associated with a reduced cellulose degree of polymerization. These findings establish a function for CSI1 in wood mechanics and in defining leaf cell shape. Furthermore, the results imply that the microfibril angle in wood is defined by CSI1 independent mechanism(s).