Spatial and temporal specificity of Ca(2+) signalling in Chlamydomonas reinhardtii in response to osmotic stress

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.