Quantifying plant cell-wall failure in vivo using nanoindentation
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Quantifying plant cell-wall failure in vivo using nanoindentation Elham Forouzesh, Ashwani K. Goel, and Joseph A. Turner, Mechanical and Materials Engineering, University of Nebraska-Lincoln, W342 Nebraska Hall, Lincoln, Nebraska 68588-0526 Address all correspondence to Joseph A. Turner at [email protected] (Received 6 May 2014; accepted 11 August 2014)
Abstract Nanoindentation experiments have been carried on Arabidopsis thaliana using spherical tungsten tips. Load–displacement plots obtained from experiments suggest that there is an optimum diameter of tip size which can be used to safely penetrate the tip through the cell wall. Based on the exact tip size used in the experiments and the measured load–displacement response, the failure stress was calculated using the experimental data in conjunction with a computational model. The value of failure stress was investigated in hypertonic (plasmolyzed), isotonic, and hypotonic (turgid) samples.
Introduction The plant cell wall surrounds the cell and protects it against environmental stresses, fungi, pathogens, and insects such as aphids that have to penetrate through the cell wall for food.[1] Therefore, the effect of cell disease or genetic modifications on cell-wall composition and structure can be related to the consequential cell-wall failure properties.[2] In addition, studies of the failure properties of cell walls are important for bioprocessing applications in which it is necessary to break down the cell wall.[3] Furthermore, an understanding of the failure properties of the cell wall is crucial for successful delivery of materials inside the cells. Conventional drug delivery systems suffer from inefficiency or non-specific effects.[4,5] Thus nanosystems are being introduced for targeted drug delivery.[6] For this purpose, the carrier must be able to enter the cell wall without causing death or damage to the cell while at the same time penetrating through the wall. Therefore, it is important to understand the biomechanics of the cell wall to estimate the force needed to penetrate it while keeping the cell alive. Underestimation or overestimation of the force may lead to inadequate and unsuccessful delivery in the first case or damage to the cell in the second case.[7,8] Failure stress represents the stress required for the cell wall to rupture. Different single-cell methods have been used to quantify these properties.[9] These methods include cell poking,[10] compression testing,[11,12] micro-indentation,[13,14] and nanomanipulation.[15,16] In these methods single cells are prepared by culturing. Application of these methods to very small cells and visualization of cell separation and breakage is very difficult with an optical microscope due to insufficient depth of focus and low resolution. The scanning electron microscope (SEM) has better resolution and depth of field. However, to study cells an environmental SEM (ESEM) should
be used because this instrument permits the presence of a gas, such as water vapor. It has been shown that ESEM-based nano
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