Altered viscoelastic properties of the Nicotiana tabacum BY-2 suspension cell lines adapted to high concentrations of NaCl and mannitol

Plant cells have to maintain the cellular microenvironment within a certain range of its properties to stay alive and grow. The functioning of the cell requires a continuous exchange of mass and energy with an environment. Such system openness could lead to perturbation in cellular microenvironment and disorder in molecular processes. Hyperosmolarity significantly stresses cells, induces ROS, promotes water efflux, and decreases turgor, optionally up to plasmolysis and a cell's shrinkage. However, in time upon exposure to osmotic stress a cellular adaptation could be developed. How the biomechanical balance of the cell is established and maintained in an environment of osmotic stress remains a nurturing question. We show that changes of the viscoelastic properties of protoplasts are element of such adaptation. We used Brillouin light scattering (BLS) and BODIPY-based fluorescence molecular rotors to study Nicotiana tabacum suspension BY-2 cells adapted to high concentrations of NaCl and mannitol. We revealed an increased molecular crowd and viscoelasticity in adapted BY-2 cells, as well as an altered plasma membrane tension. Importantly, viscosity-related changes imposed on adapted BY-2 cells by salt stress were smaller than those observed for control cells. Cellular microenvironments were elucidated in on the background of their own environments, what revealed that alterations of cytoplasm and vacuoles viscoelasticity in adapted cells has also a relative dimension. This work provides the first semi-quantitative data, to our knowledge, for viscoelastic cellular changes in plant cells adapted to growth under salt and osmotic stress conditions and exposed to subsequent short-term stress. Applied methods provide evidence that adaptation to hyperosmotic stress leads to different ratio in the protoplast and environment qualities that helps to maintain cell integrity. Many challenges remain in the field of plant mechanobiology, to which our approach could provide valuable insight into plant cell responses to the environment and the further development of useful methods.