The primary cell wall determines many physical aspects of growing plant cells, including size, shape, growth, turgor relations and mechanical strength. Here we took a ‘deep dive’ into the cell wall mechanics of the cross-ply epidermal wall of onion. Cyclic tensile stretching experiments showed complex mechanics: an initial phase of nonlinear elasticity where the modulus increased with strain (stiffening), followed by onset of plastic deformation where the modulus declined and a final linear phase of constant stress-strain slope. The stress-strain curves were largely unaffected by a 100X change in strain rate and 20<sup>o</sup>C change in temperature. We interpret these results to mean that pectin viscoelasticity does not contribute substantially to this wall behavior, which is largely elasto-plastic in character and lacks evidence of a viscous component. Previous work showed that xyloglucanase and pectinase digestions had little or no effect on tensile tests. Thus we infer that the cellulose network is the dominant determinant of epidermal wall mechanics. To understand these behaviors in a quantitative conceptual framework, we developed a coarse-grained molecular dynamics model of the cross-ply onion wall and carried out stress-strain experiments <i>in silico</i>. The model consists of spring-and-bead chains tuned to the physical properties of cellulose, pectin and xyloglucan. In simulations, these components spontaneously assembled into a structure that resembled AFM images of the onion wall. Cellulose microfibrils assembled into a strong 2D network within each lamella, xyloglucans bound to cellulose in conformations similar to those observed by nanoscale imaging, and pectin formed a dynamic network in contact with much of the cellulose surface. In simulations of tensile stretching experiments, the model captured much of the wall’s actual (experimental) behavior and provided insights in the molecular basis for the wall’s complex behavior, including stress-dependent elasticity, stiffening, and plasticity above a yield threshold. Plasticity originates from fibril-fibril sliding in aligned cellulose networks. The steep slope of the initial part of the retract curve comes from elastic unloading of axially-aligned cellulose microfibrils, which are stretched up to ~2% in the model. This study provides quantitative insight into fundamental questions of plant mechanobiology and reveals design principles of biomaterials that combine stiffness with yielding and extensibility.

Role of cellulose in the nonlinear tensile mechanics of plant primary cell walls

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.