Cells in ecstasy: Molecular probes to map mechano-chemical patterns in walled cells

Biological processes in cells result from a complex interplay between gene expression, molecular interactions, and mechanical forces. Yet, no generalized frameworks exist to explain how chemical, genetic and mechanical influences on living systems are intertwined. This is the central goal of the emerging field of mechanobiology, which aims at understanding how mechanical signals are transduced in cells, and converted into biochemical signals that interact with the genetic machinery of the organism to regulate complex biological function. Most of the efforts in this field have been focused on the animal kingdom, while the study of mechanobiological processes in plants or other walled systems (e.g. fungi and oomycetes) has received much less attention. This is in part due to a lack of methods to probe mechano-chemical effects in these species with sufficient resolution. In this thesis, we harnessed the mechanochemistry of fluorescent molecular rotors to enable the study walled cell mechanobiology, as a stepping stone to better understand how walled cells and their tissues cope with mechanical stress. We developed new molecular mechanoprobes to target and stain various compartments of walled cells and tissues (i.e. cell wall, plasma membrane, cytoplasm, vacuole). Their implementation in plant and oomycete cells, combined with quantitative imaging, allowed us to unveil and visualize mechanical patterns, in-vivo and with subcellular resolution.

In Chapter 2 we provide a technical overview of the various synthesis routes used in this thesis to design and make BODIPY-based molecular rotors that are functionalized to target and report microviscosity patterns in the different cellular compartments studied.

In Chapter 3 we implement a set of BODIPY-based molecular rotors in Arabidopsis seedlings. We use Fluorescence Lifetime Imaging to image spatial variations in the rotors’ fluorescence lifetime, in order to construct so-called ‘microviscosity’ maps. This approach opens up new ways to understand the role of mechanics in the regulation of biological processes. In particular, it could provide valuable insights on the role of mechanical stress in cell polarisation and differentiation, and on the adaptation of local mechanics during important stages in the life cycle of the cell.

We critically reflect and refine the notion of ‘microviscosity’ in Chapter 4, by discussing the potential factors controlling the molecular rotation rate, and subsequently, the fluorescence lifetime of BODIPY-based molecular rotors in cells. We conclude that fluorescence lifetime mapping with these rotors gives information on the relative confinements and crowding density within cells, but that direct translation of lifetime into absolute microviscosity values requires cautious calibration.

In Chapter 5, we show that our dye toolbox is not restricted to use in plants, but can be readily adapted for micromechanical mapping in other walled organisms, such as oomycetes. Using the cell-wall targeting molecular rotor in combination with a plasma membrane chemical polarity probe, we are able to visualize effects of mechanical and chemical stress on the mechano-chemical properties of Phytophthora infestans cell wall and plasma membrane during invasion. The generated insight can be used to understand the mechanisms of plant-pathogen mechanical interaction, as well as the mode of action of fungicides developed to inhibit growth and host invasion.

The work in this thesis highlights how a physico-chemical approach, utilizing mechanochemistry tools, can help shed light on the complex biological processes occurring in walled cells. Through direct visualization of mechanical patterns at the microscopic scale, we can gain understanding of a wide variety of processes. In the General Discussion, we put our work in a broader context, provide routes to develop the work further, and suggest approaches, on the basis of preliminary results, to perform a probe calibration and develop a ratiometric equivalent to the lifetime-based probes.