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

Hojae Yi,Baris Kandemir,James Z. Wang,Yue Rui,Charles T. Anderson,Virendra M. Puri

doi:10.3389/fpls.2018.01566

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.

Highlights

Stomata function as osmotically tunable pores that control CO2 intake and water loss at the surface of plants
To (1) minimize the effects of idealized geometric assumptions on boundary conditions describing constraints on a stomatal complex, (2) accurately account for the degree of freedom at the stomatal junction area, and (3) pinpoint locations and areas where guard cells interact with neighboring cells, we modeled stomatal guard cells using the contours of actual stomata by computationally tracing 3D confocal images of guard cells and inputting these coordinates directly into our finite element models (FEMs)
To capture stomatal shape in three dimensions, we used propidium iodide (PI) to stain intact leaves of Col-0, cesa3je5, xxt1 xxt2, and POLYGALACTURONASE INVOLVED IN EXPANSION1 (PGX1) OE genotypes that had been treated with light or darkness to induce stomatal opening or closure, respectively

Summary

Introduction

Stomata function as osmotically tunable pores that control CO2 intake and water loss at the surface of plants. Stomatal opening is thought to be driven by the anisotropic deformation of the guard cell, and such anisotropy is hypothesized to be linked to the molecular construction and mechanical properties of the guard cell wall (Meckel et al, 2007; Amsbury et al, 2016; Rui and Anderson, 2016; Carter et al, 2017; Marom et al, 2017; Woolfenden et al, 2017). The role of hemicellulose, namely xyloglucan in Arabidopsis, has been highlighted by the finding that stomata of xxt xxt mutants lacking xyloglucan exhibit smaller pore widths in both open and closed states (Rui and Anderson, 2016). Several reports have found evidence for the role of pectins in controlling the elasticity of guard cell walls and the dynamic range of stomata (Jones et al, 2003, 2005; Amsbury et al, 2016; Rui et al, 2017)

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Results

Conclusion