The mechanical properties of plant cell walls soft material at the subcellular scale: the implications of water and of the intercellular boundaries

Abstract

Subcellular mechanical characterization of the cell wall can provide important insights into the cell wall's functional organization, especially if the characterization is not confounded by extracellular factors and intercellular boundaries. However, due to the technical challenges associated with the microscale mechanical characterization of soft biological materials, subcellular investigations of the plant cell wall under tensile loading have yet to be properly performed. This study reports the mechanical characterization of primary onion epidermal cell wall profiles using a novel cryosection-based sample preparation method and a microelectromechanical system-based tensile testing protocol. At the subcellular scale, the cell wall showed biphasic behavior similar to tissue samples. However, instead of a transition zone between the linear elastic or viscoelastic and linear plastic zones, the subcellular-scale samples showed a plateau-like trend with a sharp drop in the modulus value. The critical ranges of stress (20-40 MPa) and strain (5-12 %) of the plateau zone were identified. A strain energy of 1.3 MJ m -3 was calculated at the midpoint of the critical stress-strain range; this value was in accordance with the previously estimated hydrogen bond energy of the cell wall. Subcellular-scale samples showed very large lateral/axial deformations (0.8 ± 0.13) at fracturing. In addition, investigating the cell wall's mechanical properties at three different water states showed that water is critical for the flow-like behavior of cell wall matrix polymers. These results at subcellular scale provide new insights into biological materials that possess a structural hierarchy at different length scales; which cannot be obtained from tissue-scale experiments.

The middle lamella—more than a glue

“…The thawed samples were then mounted onto the MEMS device using cyanoacrylate glue and stretched till fracture ( figure 2(c)). Remarkably, in both dry and hydrated state, the excised subcellular strips fractured within the primary wall region, not at the ML [79,80]. Since geometrical complications were eliminated in this experimental set-up, the results confirm that the ML is as strong as, if not stronger than the primary cell wall material.…”

“…The second type of molecular linkages are hydrogen bonds that may mediate the interaction of ML pectin to the cellulose and hemicellulose of the PW. Pectin is reported to strongly [79], and the left image of 2C is from figure (a) of Zamil et al [80] and is reproduced with permission from Springer.…”

Disentangling loosening from softening: insights into primary cell wall structure

“…Recent simulations of cell wall growth based on elastic deformations of finite element models (Yanagisawa et al ., ; Majda et al ., ; Bidhendi and Geitmann, ; Sapala et al ., ; Oliveri et al ., ) assume the wall is faithfully represented as a simple fiber‐reinforced elastic hydrogel, but such a material lacks many of the material properties reported here. Results in this study, along with previous work on the same material (Kim et al ., ; Zamil et al ., ; Zhang et al ., , ), provide key data for development of more realistic molecular models to connect cell wall structure with wall mechanics and to elucidate the molecular basis of wall‐loosening processes underlying cell growth.…”

Modeling the nonlinear elastic behavior of plant epidermis

“…This is generally due to technical challenges associated with sample preparation and testing at miniature scales. As a result, except for a few studies that carried out tensile tests on cell wall fragments (Toole et al 2001;Wei et al 2006;Zamil et al 2013Zamil et al , 2015, tensile testing has remained limited to tissue scale tests. In uniaxial tensile testing, the sample is gripped and stretched in one direction while the force required to stretch the sample is registered.…”