Mechanical Effects of Cellulose, Xyloglucan, and Pectins on Stomatal Guard Cells of Arabidopsis thaliana

Abstract

Stomata function as osmotically tunable pores that facilitate gas exchange at the surface of plants. Stomatal opening and closure are regulated by turgor changes in guard cells that result in mechanically regulated deformations of guard cell walls. However, how the molecular, architectural, and mechanical heterogeneities that exist in guard cell walls affect stomatal dynamics is unclear. In this work, stomata of wild type Arabidopsis thaliana plants or of mutants lacking normal cellulose, hemicellulose, or pectins were experimentally induced to close or open. Three-dimensional images of these stomatal complexes were collected using confocal microscopy, images were landmarked, and three-dimensional finite element models (FEMs) were constructed for each complex. Stomatal opening was simulated with a 5 MPa turgor increase. By comparing experimentally measured and computationally modeled changes in stomatal geometry across genotypes, anisotropic mechanical properties of guard cell walls were determined and mapped to cell wall components. Deficiencies in cellulose or hemicellulose were both predicted to stiffen guard cell walls, but differentially affected stomatal pore area and the degree of stomatal opening. Additionally, reducing pectin molecular mass altered the anisotropy of calculated shear moduli in guard cell walls and enhanced stomatal opening. Based on the unique architecture of guard cell walls and our modeled changes in their mechanical properties in cell wall mutants, we discuss how each polysaccharide class contributes to wall architecture and mechanics in guard cells. This study provides new insights into how the walls of guard cells are constructed to meet the mechanical requirements of stomatal dynamics.

The stomatal flexoskeleton: how the biomechanics of guard cell walls animate an elastic pressure vessel

“…An equally important aspect is the overall shape and size (geometry) of the stomatal complex. As demonstrated in Yi et al (2018), even with identical mechanical properties in the cell wall, differences in the overall shape and size of stomatal complexes result in different stomatal opening under the same amount of turgor increase.…”

“…In computational modeling studies, guard cell-neighbor interactions are modeled as distributed loads (pressure) from neighboring cells on the guard cells (Cooke et al, 1976;Woolfenden et al, 2017;Yi et al, 2018) or are not explicitly considered (Marom et al, 2017). Thus, there is no consensus on the magnitude and importance of the biomechanical influence of neighboring cells on guard cells during stomatal opening and closure.…”

“…Thus, there is no consensus on the magnitude and importance of the biomechanical influence of neighboring cells on guard cells during stomatal opening and closure. Woolfenden et al (2017) state that epidermal pressure has a minor effect on stomatal opening, whereas Yi et al (2018) showed that the same cell wall properties as those used in Woolfenden et al (2017) result in different amounts Fig. 1.…”

“…For example, more recent computational modeling studies have used symmetric and well-defined geometric shapes (Cooke et al, 2008;Carter et al, 2017;Marom et al, 2017;Woolfenden et al, 2017Woolfenden et al, , 2018. However, close observation of reconstructed 3D images of stomatal complexes reported in Zhao and Sack (1999), Meckel et al (2007), and Yi et al (2018) reveals that stomatal complexes are not all identical and are neither perfectly symmetrical nor perfectly elliptical or toroidal. Recently, Yi et al (2018) developed computational models directly from 3D confocal images of stomatal complexes that accurately reflect their irregular shapes.…”

“…As pointed out by Yi et al (2018) and Marom et al (2017), understanding the geometric configuration of a stomatal complex and its interaction with neighboring cells is important for explaining how the specific deformation patterns of stomatal guard cells facilitate the opening and closure of the stomatal pore. Acknowledging that stomatal behavior is a collective response of multiple elements such as stomatal complex shape, guard cell shape, cell wall morphology, interactions between sister guard cells and their neighboring subsidiary or pavement cells, and the mechanical properties of guard cell walls that reflect their molecular structures, it is imperative to account for the implications of any assumptions, idealizations, and/or simplifications employed during analysis of experimental and modeling data.…”