Pivato, M.; Costa, A.; Wheeler, G. L.; Ballottari, M.

Calcium (Ca2+)-dependent signalling plays a well-characterized role in the perception and response mechanisms to environmental stimuli in plant cells. In the context of a constantly changing environment, it is fundamental to understand how crop yield and microalgal biomass productivity are affected by external factors. Ca2+ signalling is known to be important in different physiological processes in microalgae but many of these signal transduction pathways still need to be characterized. Here, the role of compartment-specific Ca2+ signalling was investigated in Chlamydomonas reinhardtii in response to environmental stressors such as nutrient availability, osmotic stress, temperature fluctuations and carbon sensing. An in vivo single-cell imaging approach was adopted to directly visualize signalling processes at the level of specific subcellular compartments, using Chlamydomonas reinhardtii lines expressing a genetically encoded ratiometric Ca2+ indicator. Hyper-osmotic shock caused cytosolic and chloroplast Ca2+ elevations, whereas high temperature and inorganic carbon availability primarily induced Ca2+ transients in the chloroplast. In contrast, hypo-osmotic stress only induced Ca2+ elevations in the cytosol. The results herein reported show that compartment-specific signalling pathways are likely to play an important role in the response of Chlamydomonas to these stimuli providing new understanding of the mechanisms exploited by microalgae to respond to specific natural conditions.