Compartment‐specific Ca2+ imaging in the green alga Chlamydomonas reinhardtii reveals high light‐induced chloroplast Ca2+ signatures

To investigate the role of intracellular Ca2+ signaling in the perception and response mechanisms to light in unicellular microalgae, the genetically encoded ratiometric Ca2+ indicator Yellow Cameleon (YC3.6) was expressed in the model organism for green algae Chlamydomonas reinhardtii, targeted to cytosol, chloroplast, and mitochondria. Through in vivo single-cell confocal microscopy imaging, light-induced Ca2+ signaling was investigated in different conditions and different genotypes, including the photoreceptors mutants phot and acry. A genetically encoded H2 O2 sensor was also adopted to investigate the possible role of H2 O2 formation in light-dependent Ca2+ signaling. Light-dependent Ca2+ response was observed in Chlamydomonas reinhardtii cells only in the chloroplast as an organelle-autonomous response, influenced by light intensity and photosynthetic electron transport. The absence of blue and red-light photoreceptor aCRY strongly reduced the light-dependent chloroplast Ca2+ response, while the absence of the blue photoreceptor PHOT had no significant effects. A correlation between high light-induced chloroplast H2 O2 gradients and Ca2+ transients was drawn, supported by H2 O2 -induced chloroplast Ca2+ transients in the dark. In conclusion, different triggers are involved in the light-induced chloroplast Ca2+ signaling as saturation of the photosynthetic electron transport, H2 O2 formation, and aCRY-dependent light perception.