Structure and Biomechanics during Xylem Vessel Transdifferentiation in Arabidopsis thaliana.

Eleftheria Roumeli,Chiara Daraio,Leah Ginsberg,Giada Spigolon,Rodinde Hendrickx,Misato Ohtani,Guruswami Ravichandran,Taku Demura,Robin McDonald

doi:10.3390/plants9121715

Abstract

Individual plant cells are the building blocks for all plantae and artificially constructed plant biomaterials, like biocomposites. Secondary cell walls (SCWs) are a key component for mediating mechanical strength and stiffness in both living vascular plants and biocomposite materials. In this paper, we study the structure and biomechanics of cultured plant cells during the cellular developmental stages associated with SCW formation. We use a model culture system that induces transdifferentiation of Arabidopsis thaliana cells to xylem vessel elements, upon treatment with dexamethasone (DEX). We group the transdifferentiation process into three distinct stages, based on morphological observations of the cell walls. The first stage includes cells with only a primary cell wall (PCW), the second covers cells that have formed a SCW, and the third stage includes cells with a ruptured tonoplast and partially or fully degraded PCW. We adopt a multi-scale approach to study the mechanical properties of cells in these three stages. We perform large-scale indentations with a micro-compression system in three different osmotic conditions. Atomic force microscopy (AFM) nanoscale indentations in water allow us to isolate the cell wall response. We propose a spring-based model to deconvolve the competing stiffness contributions from turgor pressure, PCW, SCW and cytoplasm in the stiffness of differentiating cells. Prior to triggering differentiation, cells in hypotonic pressure conditions are significantly stiffer than cells in isotonic or hypertonic conditions, highlighting the dominant role of turgor pressure. Plasmolyzed cells with a SCW reach similar levels of stiffness as cells with maximum turgor pressure. The stiffness of the PCW in all of these conditions is lower than the stiffness of the fully-formed SCW. Our results provide the first experimental characterization of the mechanics of SCW formation at single cell level.

Highlights

Plantae and plant-based materials are specialized conglomerates of plant cells
This loosening should be occurring in all osmotic conditions, but we propose that it is only distinguishable in hypotonic conditions because in these conditions the primary cell wall (PCW) is under the highest amount of stress since it is subjected to the highest turgor pressure
The mechanical data at different scales and in different osmotic conditions in combination with the proposed two spring model, allow us to decouple the contributions from each structural element of the cell as it responds to changes in turgor pressure at various stages of the differentiation process

Summary

Introduction

Plantae and plant-based materials are specialized conglomerates of plant cells. studying the mechanical properties of single cells and resolving further sub-cellular contributions provides a Plants 2020, 9, 1715; doi:10.3390/plants9121715 www.mdpi.com/journal/plantsPlants 2020, 9, 1715 basis for further analysis of the heterogeneous tissue and plant-level biomechanics. The micro-structure and composition of secondary cell wall (SCW) governs, to a large extent, the mechanical properties of the entire tissue [1,2]. Plant cells have two key structural elements that collectively govern their mechanical properties: the cell wall and the cytoskeleton. The support provided to plant cells by the cell wall allows them to hold water at high pressures (p = 0.3–1.0 MPa), mainly through swelling of the vacuole [5]. This phenomenon in plants is known as turgor pressure, and it is essential to the structural integrity and rigidity of the cell.

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