Mapping the subcellular mechanical properties of live cells in tissues with fluorescence emission–Brillouin imaging

Fluorescence-Brillouin imaging reveals that plants regulate the mechanical properties of the extracellular matrix in response to light. Seeing mechanical properties of living cells Mechanical properties of cells and the matrix that surrounds them contribute to cell shape, control cell migration, and regulate cell growth. Elsayad et al. engineered a microscope system that integrated fluorescence emission detection with detection of a light-scattering process called the Brillouin frequency shift and called the method fluorescence emission–Brillouin scattering imaging (FBi). With this optical approach, the authors showed that the mechanical properties of live plants can be visualized at the submicrometer scale and demonstrated that this approach can be used to investigate regulatory events that alter cellular and extracellular mechanical properties of living cells within tissues. This work also revealed that the cytoplasm near the cell membrane and the extracellular matrix are regions of locally increased stiffness and showed that the sides parallel to the growth axis of an expanding plant hypocotyl, but not root, cells are “stiffer” than the sides perpendicular to the growth axis. Thus, FBi is another tool in the microscopy toolkit for exploring properties of cells and tissues. Extracellular matrices (ECMs) are central to the advent of multicellular life, and their mechanical properties are modulated by and impinge on intracellular signaling pathways that regulate vital cellular functions. High spatial-resolution mapping of mechanical properties in live cells is, however, extremely challenging. Thus, our understanding of how signaling pathways process physiological signals to generate appropriate mechanical responses is limited. We introduce fluorescence emission–Brillouin scattering imaging (FBi), a method for the parallel and all-optical measurements of mechanical properties and fluorescence at the submicrometer scale in living organisms. Using FBi, we showed that changes in cellular hydrostatic pressure and cytoplasm viscoelasticity modulate the mechanical signatures of plant ECMs. We further established that the measured “stiffness” of plant ECMs is symmetrically patterned in hypocotyl cells undergoing directional growth. Finally, application of this method to Arabidopsis thaliana with photoreceptor mutants revealed that red and far-red light signals are essential modulators of ECM viscoelasticity. By mapping the viscoelastic signatures of a complex ECM, we provide proof of principle for the organism-wide applicability of FBi for measuring the mechanical outputs of intracellular signaling pathways. As such, our work has implications for investigations of mechanosignaling pathways and developmental biology.