Chiral cellulose fibril organization in a plant cell wall as liquid crystal confined in a cylindrical boundary

Abstract

The order of cellulose microfibrils in xylem cell walls is identified using polarized optical microscopy. The line, helix, ring, crossed helix, and twisted helix organizations in a cylindrical shell are the equilibrium states of balanced elastic deformation and surface anchoring. A computational model of the five organizations is established to simulate their birefringent-colored profiles. These five textures present unique optical textures, which are clearly recognized in the cross sections of Eucalyptus grandis. The libriform fiber cell for support has a twisted helical cell wall. The vessel cell for high-speed water uptake consists of a helical layer covered by a layer of vortex array. The ray cell for radial transportation is crossed helical. The microfibril angles versus the radius of the cell wall were measured utilizing the distribution of birefringence colors. In fiber cell walls, the MFA is significantly correlated with the curvature, bending, and surface anchoring, respectively. In vessel cell walls, the vortex array includes focal conic domains of chiral order and topological defects of nematic order, suggesting that the phase transition of cellulose fibrils leads to pattern formation. Liquid crystal phases and patterns in the cell walls reveal how the cell wall thickens and how cells differentiate. Out of the frustration of long, stiff, twisting fibrils packed in slender tubes, trees generate the helical channel networks, transforming the brittle lamina into an elastic, tear-resistant, self-healing tissue.