Spatial and temporal specificity of Ca²⁺ signalling in Chlamydomonas reinhardtii in response to osmotic stress

Summary Ca 2+ ‐dependent signalling processes enable plants to perceive and respond to diverse environmental stressors, such as osmotic stress. A clear understanding of the role of spatiotemporal Ca 2+ signalling in green algal lineages is necessary in order to understand how the Ca 2+ signalling machinery has evolved in land plants. We used single‐cell imaging of Ca 2+ ‐responsive fluorescent dyes in the unicellular green alga Chlamydomonas reinhardtii to examine the specificity of spatial and temporal dynamics of Ca 2+ elevations in the cytosol and flagella in response to salinity and osmotic stress. We found that salt stress induced a single Ca 2+ elevation that was modulated by the strength of the stimulus and originated in the apex of the cell, spreading as a fast Ca 2+ wave. By contrast, hypo‐osmotic stress induced a series of repetitive Ca 2+ elevations in the cytosol that were spatially uniform. Hypo‐osmotic stimuli also induced Ca 2+ elevations in the flagella that occurred independently from those in the cytosol. Our results indicate that the requirement for Ca 2+ signalling in response to osmotic stress is conserved between land plants and green algae, but the distinct spatial and temporal dynamics of osmotic Ca 2+ elevations in C. reinhardtii suggest important mechanistic differences between the two lineages.