Hojae Yi

Architecture-Based Multiscale Computational Modeling of Plant Cell Wall Mechanics to Examine the Hydrogen-Bonding Hypothesis of the Cell Wall Network Structure Model

A primary plant cell wall network was computationally modeled using the finite element approach to study the hypothesis of hemicellulose (HC) tethering with the cellulose microfibrils (CMFs) as one of the major load-bearing mechanisms of the growing cell wall. A computational primary cell wall network fragment (10 × 10 μm) comprising typical compositions and properties of CMFs and HC was modeled with well-aligned CMFs. The tethering of HC to CMFs is modeled in accordance with the strength of the hydrogen bonding by implementing a specific load-bearing connection (i.e. the joint element). The introduction of the CMF-HC interaction to the computational cell wall network model is a key to the quantitative examination of the mechanical consequences of cell wall structure models, including the tethering HC model. When the cell wall network models with and without joint elements were compared, the hydrogen bond exhibited a significant contribution to the overall stiffness of the cell wall network fragment. When the cell wall network model was stretched 1% in the transverse direction, the tethering of CMF-HC via hydrogen bonds was not strong enough to maintain its integrity. When the cell wall network model was stretched 1% in the longitudinal direction, the tethering provided comparable strength to maintain its integrity. This substantial anisotropy suggests that the HC tethering with hydrogen bonds alone does not manifest sufficient energy to maintain the integrity of the cell wall during its growth (i.e. other mechanisms are present to ensure the cell wall shape).

Characterizing microscale biological samples under tensile loading: stress-strain behavior of cell wall fragment of onion outer epidermis.

UNLABELLED PREMISE OF THE STUDY The results of published studies investigating the tissue-scale mechanical properties of plant cell walls are confounded by the unknown contributions of the middle lamella and the shape and size of each cell. However, due to their microscale size, cell walls have not yet been characterized at the wall fragment level under tensile loading. It is imperative to understand the stress-strain behavior of cell wall fragments to relate the walls mechanical properties to its architecture. • METHODS This study reports a novel method used to characterize wall fragments under tensile loading. Cell wall fragments from onion outer epidermal peels were cut to the desired size (15 × 5 µm) using the focused ion beam milling technique, and these fragments were manipulated onto a microelectromechanical system (MEMS) tensile testing device. The stress-strain behavior of the wall fragments both in the major and minor growth directions were characterized in vacuo. • KEY RESULTS The measured mean modulus, fracture strength, and fracture strain in the major growth direction were 3.7 ± 0.8 GPa, 95.5 ± 24.1 MPa, and 3.0 ± 0.5%, respectively. The corresponding properties along the minor growth direction were 4.9 ± 1.2 GPa, 159 ± 48.4 MPa, and 3.8 ± 0.5%, respectively. • CONCLUSIONS The fracture strength and fracture strain were significantly different along the major and minor growth directions, the wall fragment level modulus of elasticity anisotropy for a dehydrated cell wall was 1.23, suggesting a limited anisotropy of the cell wall itself compared with tissue-scale results.

Mechanical characterization of outer epidermal middle lamella of onion under tensile loading

UNLABELLED • PREMISE OF THE STUDY The cells in plant tissue are joined together by a distinct layer called the middle lamella (ML). Understanding the mechanical properties of the ML is crucial in studying how tissue-level mechanical properties emerge from the subcellular-level mechanical properties. However, the nanoscale size of the ML presents formidable challenges to its characterization as a separate layer. Consequently, the mechanical properties of the ML under tensile loading are as yet unknown.• METHODS Here, we characterize the ML from a subcellular sample excised from two adjacent cells and composed of two wall fragments and a single line of ML in between. Two techniques, cryotome sectioning and milling with a focused ion beam, were used to prepare ML samples, and tensile experiments were performed using microelectromechanical system (MEMS) tensile testing devices.• KEY RESULTS Our test results showed that even at a subcellular scale, the ML appears to be stronger than the wall fragments. There was also evidence that the ML attached at the corner of cells more strongly than at the rest of the contact area. The contribution of the additional ML contact area was estimated to be 40.6 MPa. Wall fragment samples containing an ML layer were also significantly stronger (p < 0.05) than the wall fragments without an ML layer.• CONCLUSIONS The tensile properties of the ML might not have a major impact on the tissue-scale mechanical properties. This conclusion calls for further study of the ML, including characterization under shear loading conditions and elucidation of the contributions of other extracellular parameters, such as cell size and shape, to the overall tissue-level mechanical response.

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 1: Round and Angular Monosize and Binary Mixtures

The bulk mechanical properties of two different types of rootzone sands (round and angular) were measured using a cubical triaxial tester. Two monosize sands (d 50 = 0.375 mm and 0.675 mm) and their 50:50 binary mixtures (d 50 = 0.500 mm) were studied. The compression, shear, and failure responses of the above-mentioned six compositions were analyzed, compared, and modeled. Two elastic parameters (bulk and shear moduli) and two elastoplastic parameters (swelling and consolidation indices) of the six sand compositions were also calculated and compared. The angular sand was more compressible than round sand during isotropic compression. In addition, the angular sands tended to have lower initial bulk density and high porosity values. Among the three different size fractions, the 0.375 mm mixture was least compressible for both sand shapes. The failure strength and shear modulus of the angular sand were higher than the round sands. In addition, due to their simplicity, phenomenological models were developed to predict the compression and shear behavior of the sands. The prediction models were validated using subangular and subround sands. Average relative difference values were calculated to determine the effectiveness of the prediction models. The mean average relative difference values for compression profiles, i.e., volumetric stress vs. volumetric strain, were from 16 % to 39 %, except for the initial load-response portion (< 1 % volumetric strain). The predictive models were effective in reproducing the failure responses: at 17.2 kPa confining pressure, the mean of average relative difference was 23 %; at 34.5 kPa , the mean difference was 24 %.

Contributions of the mechanical properties of major structural polysaccharides to the stiffness of a cell wall network model

PREMISE OF THE STUDY The molecular mechanisms regulating the expansive growth of the plant cell wall have yet to be fully understood. The recent development of a computational cell wall model allows quantitative examinations of hypothesized cell wall loosening mechanisms. METHODS Computational cell wall network (CWN) models were generated using cellulose microfibrils (CMFs), hemicelluloses (HCs), and their interactions (CMF-HC). For each component, a range of stiffness values, representing various situations hypothesized as potential cell-wall-loosening mechanisms, were used in the calculation of the overall stiffness of the computational CWN model. Thus, a critical mechanism of the loosening of the primary cell wall was investigated using a computational approach by modeling the molecular structure. KEY RESULTS The increase in the stiffness equivalent of the CMF-HC interaction results in an increase in the Youngs modulus of the CWN. In the major growth direction, the CWN stiffness is most sensitive to the CMF-HC interaction (75%). HC stiffness contributes moderately (24%) to the change in the CWN stiffness, whereas the CMF contribution is marginal (1%). Minor growth direction exhibited a similar trend except that the contributions of CMFs and HCs are higher than for the major growth direction. CONCLUSIONS The stiffness of the CMF-HC interaction is the most critical mechanical component in altering stiffness of the CWN model, which supports the hypothesized mechanism of expansins role in efficient loosening of the plant cell wall by disrupting HC binding to CMFs. The comparison to experiments suggests additional load-bearing mechanisms in CMF-HC interactions.

POLYGALACTURONASE INVOLVED IN EXPANSION3 Functions in Seedling Development, Rosette Growth, and Stomatal Dynamics in Arabidopsis thaliana

The polygalacturonase PGX3 functions in tissue growth and stomatal dynamics in Arabidopsis thaliana, revealing how controlled pectin degradation influences stomatal function. Plant cell separation and expansion require pectin degradation by endogenous pectinases such as polygalacturonases, few of which have been functionally characterized. Stomata are a unique system to study both processes because stomatal maturation involves limited separation between sister guard cells and stomatal responses require reversible guard cell elongation and contraction. However, the molecular mechanisms for how stomatal pores form and how guard cell walls facilitate dynamic stomatal responses remain poorly understood. We characterized POLYGALACTURONASE INVOLVED IN EXPANSION3 (PGX3), which is expressed in expanding tissues and guard cells. PGX3-GFP localizes to the cell wall and is enriched at sites of stomatal pore initiation in cotyledons. In seedlings, ablating or overexpressing PGX3 affects both cotyledon shape and the spacing and pore dimensions of developing stomata. In adult plants, PGX3 affects rosette size. Although stomata in true leaves display normal density and morphology when PGX3 expression is altered, loss of PGX3 prevents smooth stomatal closure, and overexpression of PGX3 accelerates stomatal opening. These phenotypes correspond with changes in pectin molecular mass and abundance that can affect wall mechanics. Together, these results demonstrate that PGX3-mediated pectin degradation affects stomatal development in cotyledons, promotes rosette expansion, and modulates guard cell mechanics in adult plants.

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 2: Effect of Moisture on Continuous Sand Mixtures

The compression and failure responses of four rootzone sand mixtures (with different types of particle shapes) were analyzed, compared, and modeled at two different moisture states (air dried and 30 cm tension). Differences in particle packing characteristics arising from particle shape and moisture were quantified. The air-dried and moist samples of the sand mixtures had initial bulk density (IBD) values ranging from 1.55 to 1.67g/cc and 1.23 to 1.48g/cc, respectively. The low IBD values observed for moist mixtures were attributed to the particle-particle agglomeration effects that take place in the presence of moisture. In addition, it was observed that the sand mixtures porosity increased with decreasing particle sphericity. During compression testing, moist samples underwent a greater volumetric deformation compared to the air-dried samples for the same pressure levels, e.g., at 69kPa, the volumetric strain of moist round sand mixtures was 8% higher than that of the air-dried round sand mixtures. Therefore, moisture acted as lubricant during volumetric compression of sand mixtures. Also, the bulk modulus values decreased with increasing moisture content and decreasing particle sphericity. During shear testing, the moist samples underwent a larger amount of strain deformation compared to the air-dried samples for the same stress difference values. This suggests that the presence of moisture makes the sand mixtures ductile during shear testing, unlike the usual brittle response in air-dried state. Shear modulus values linearly increased with the increase in mean pressure for the air-dried samples, whereas, for moist samples, the shear modulus values increased gradually or remained practically constant. The effect of pressure, moisture, and particle shape was also quantified for two elastoplastic parameters (consolidation and swelling indices). It was generally observed that the average consolidation index values decreased with pressure but increased with moisture and particle angularity. On the other hand, average swelling index values increased with pressure, moisture, and particle angularity. Overall, it was concluded that the moisture and particle shape had a decisive influence on the compression and shear profiles of continuous rootzone sand mixtures.

Examination of biological hotspot hypothesis of primary cell wall using a computational cell wall network model

Computational modeling reveals that a cell wall network whose mechanical integrity is dominated by a small mass fraction of “hotspot” linkers between microfibrils can sit close to a percolation threshold, across which mechanical integrity is very sensitive to the number of hotspots. In the model, the mechanical properties of cell wall fragments consisting of cellulose microfibrils and xyloglucan linkers with different levels of disorder were examined under progressive decimation of the network, modeling enzymatic degradation. The percolation limit so obtained is close to mass fraction of xyloglucan that must be removed to induce creep experimentally. Greater disorder in the interconnectivity of the network raises the number of hotspot linkers per fibril necessary to reach the percolation threshold. To maintain the required mechanical stiffness with a sparse network of hotspot connections, either each xyloglucan linker must be much stiffer than a single polymeric strand or an additional cell wall component, i.e. pectin, must carry substantial load with a sensitive non-linear mechanical response, such as that associated with a glass transition.

Measurement of Bulk Mechanical Properties and Modeling the Load Response of Rootzone Sands. Part 3: Effect of Organics and Moisture Content on Continuous Sand Mixtures

The interactions between organics and sand particles at different moisture contents are important in understanding the general mechanical behavior of rootzone sand mixtures. Towards this end, eight rootzone sand mixtures (4 shapes 2 2 moisture contents) used in golf green construction were tested using the cubical triaxial tester (CTT). These eight mixtures consist of sphagnum peat as the organic source and four sands of varying particle shape (round, subround, subangular, and angular). The sand-peat mixtures were tested at two moisture contents (air-dried and 30 cm tension). Of all the test samples, air-dried round sand with peat had the highest initial bulk density (IBD) value (1.49 g/cc), while moist angular sand with peat had the lowest IBD value (1.23 g/cc). These values influenced the compression behavior of samples, for example, the air-dried round sand with peat was least compressible while moist angular sand with peat was most compressible. Generally, moisture enhanced the compressibility of test specimens. At an isotropic pressure of 100 kPa, the volumetric strain value of moist round sand with peat was 47% higher than the volumetric strain value of the air-dried round sand with peat. Consequently, moisture and peat in bulk sand samples act as lubricants and assist in the compression process. In addition, bulk modulus values decreased with moisture. Due to the dominant effect of peat, there were no large differences between bulk modulus values of different particle shapes. The shear and failure responses of the above-mentioned eight compositions were also analyzed, compared, and modeled. Of all sand mixtures tested, air-dried angular sands with peat had the highest brittle-type failure stress value, 181 kPa at 34.5 kPa confining pressure, and moist subangular sand with peat had the lowest ductile-type failure stress value, 141 kPa at the same confining pressure. Shear modulus values increased with the increase of mean pressure, but in the case of sands containing both moisture and peat, shear modulus values increased gradually. Overall, peat and moisture content have a dominant effect on the compression and failure behavior of the rootzone sands. rootzone sand mixtures moisture effect particle shape effect organics effect mechanical behavior compression response shear/failure response prediction models

A multiscale FEA framework for bridging cell-wall to tissue-scale mechanical properties: the contributions of middle lamella interface and cell shape

Plant tissue represents cellular material with multiple structural hierarchies enabling a wide range of multifunctionalities and extraordinary mechanical properties. However, it is yet to be elucidated how subcellular-scale mechanical properties and cell-to-cell interactions by a middle lamella (ML) layer are translated to larger scale responses. In this study, we examined an onion epidermal cell wall profile as a representative multicellular material system and developed a novel framework for a multiscale finite element analysis (FEA) model that allows two-scale coupling in a commercial FEA package. The core of this multiscale approach is a 3D repetitive volume element (RVE), which is composed of four cell wall fragments from four adjacent cells attached by a distinct ML layer. We parameterized ML mechanical properties and cell shape anisotropy at RVE to investigate resulting mechanical responses, which were then scaled up to the tissue level. It was observed that, within the elastic limit, the RVE- and tissue-scale mechanical responses are barely affected by ML modulus value; however, they are moderately affected by cell shape factor. The detailed 3D feature of ML interface was found critical for creating anisotropic mechanical behavior and localized stress concentration at RVE scale. Based on the observed results, a soft nanoscopic ML layer with its specified 3D architecture was suggested as the key mediator for attributing multifunctionality in plant cellular material system. The reported computational model framework offers new insight into how different length scales may affect the material properties of multicellular materials exhibiting hierarchical multiscale structures.