Abstract
Cellulose, the most abundant biopolymer on earth, is a versatile, energy rich material found in the cell walls of plants, bacteria, algae, and tunicates. It is well established that cellulose is crystalline, although the orientational order of cellulose crystallites normal to the plane of the cell wall has not been characterized. A preferred orientational alignment of cellulose crystals could be an important determinant of the mechanical properties of the cell wall and of cellulose-cellulose and cellulose-matrix interactions. Here, the crystalline structures of cellulose in primary cell walls of onion (Allium cepa), the model eudicot Arabidopsis (Arabidopsis thaliana), and moss (Physcomitrella patens) were examined through grazing incidence wide angle X-ray scattering (GIWAXS). We find that GIWAXS can decouple diffraction from cellulose and epicuticular wax crystals in cell walls. Pole figures constructed from a combination of GIWAXS and X-ray rocking scans reveal that cellulose crystals have a preferred crystallographic orientation with the (200) and (110)/(1bar 10) planes preferentially stacked parallel to the cell wall. This orientational ordering of cellulose crystals, termed texturing in materials science, represents a previously unreported measure of cellulose organization and contradicts the predominant hypothesis of twisting of microfibrils in plant primary cell walls.
Highlights
Background correctionBackground correction for χ-pole figures was done by subtracting a local background from the azimuthal integration of the cellulose (110)/(110) reflection in both grazing incidence wide angle X-ray scattering (GIWAXS) and rocking scan data
This is likely due to the polylamellate cell wall, where cellulose microfibrils have a preferred direction along each lamella, but the orientation of cellulose microfibrils varies between different lamellae[13]
We speculate that this in-plane orientation is a consequence of layered wax structures lying on the plane of the cell wall, which have been previously identified through electron and XRD studies[29]
Summary
Background correctionBackground correction for χ-pole figures was done by subtracting a local background from the azimuthal integration of the cellulose (110)/(110) reflection in both GIWAXS and rocking scan data. An azimuthal integration along polar angle (χ) of a region outside the (110)/(110) reflection was selected as the local background. The sector with q-range from 0.5 to 0.6 Å−1 was selected as local background for GIWAXS data for onion. The sector with q-range of 0.6–0.7 Å−1 was selected for Arabidopsis and moss. The intensity of the local background at each polar angle was subtracted from the intensities of azimuthal cuts over the (110)/(110) reflection at the corresponding polar angle. The regions selected as backgrounds for rocking scans have a q-range of 2–2.2 Å−1 for onion samples, 1.8–1.9 Å−1 for Arabidopsis samples, and 0.7–0.8 Å−1 for moss samples. The integrated intensities of these regions were subtracted from the intensities of the azimuthal cuts over the (110)/(110) reflection in rocking scan data at the corresponding polar angle
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