Combining tensile testing and microscopy to address a diverse range of questions.

Both plants and animals sense and respond to mechanical stresses that arise internally or are externally imposed. In many cases, tissues respond by changing their gene expression or their mechanical properties, which has an impact on how they develop. Many tools have been developed to measure mechanical properties and to investigate responses to mechanical stress. Here we review the state of microscope-coupled tensile testing at the single-cell and tissue scale and give a view on future opportunities for extending the technology. Uniaxial tensile testing involves quantifying the deformation of a sample when a force is applied. By varying the amount of force, the speed at which the force is applied or the length of time that it is applied for, many characteristics of the mechanical properties of the sample can be calculated. Tensile testing has been used extensively to measure the mechanical properties of whole tissues or organs. The need for higher resolution data resulted in more researchers using indentation tests to measure mechanical properties instead. Indentation tests provide information at a different scale and are not suitable for answering the same type of questions as tensile testing. Here we discuss that by coupling tensile-testing machinery with microscopes such as is the case for the Automated Confocal Micro-Extensometer (ACME) it is possible to obtain tissue-scale measurements of mechanical properties with cellular resolution. Moreover, to understand and identify the biological processes cells and tissues use to respond to mechanical stress, we need to be able to apply mechanical perturbations to plant samples while recording the induced biological changes with microscopy. LAY DESCRIPTION: Plants, like most living organisms, are sensitive to their environment. This includes mechanical stresses imposed upon them by gravity or wind. Mechanical stress can also arise from internal tissue tension, which can build up if different parts of a tissue grow at different rates. In many cases, the cells respond to mechanical stress by changing their mechanical properties, which can affect their growth and their final shape. There is thus a critical need to develop tools for measuring mechanical properties and the response to mechanical stress. Mechanical properties cannot be visualised directly but must be inferred by looking at how a tissue deforms when a force is applied or vice versa. This is more challenging when one wishes to achieve this at the cellular scale, as the forces and deformations are much smaller. There are a range of methods available that have advantages and disadvantages. Here we review some of these methods. In particular, we focus on methods that cause deformation in the main axis of the tissue. This type of test can be coupled with conventional and state-of-the-art microscopes. Coupling with microscopes increases the resolution of the tests that can be performed and facilitates the simultaneous observation of responses to the mechanical stresses.