Compartment-specific Ca 2+ imaging in the green alga Chlamydomonas reinhardtii reveals high light-induced chloroplast Ca 2+ signatures.

To investigate the role of intracellular Ca 2+ signaling in the perception and response mechanisms to light in unicellular microalgae, the genetically encoded ratiometric Ca 2+ 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 Ca 2+ signaling was investigated in different conditions and different genotypes, including the photoreceptors mutants phot and acry. A genetically encoded H 2 O 2 sensor was also adopted to investigate the possible role of H 2 O 2 formation in light-dependent Ca 2+ signaling. Light-dependent Ca 2+ 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 Ca 2+ response, while the absence of the blue photoreceptor PHOT had no significant effects. A correlation between high light-induced chloroplast H 2 O 2 gradients and Ca 2+ transients was drawn, supported by H 2 O 2 -induced chloroplast Ca 2+ transients in the dark. In conclusion, different triggers are involved in the light-induced chloroplast Ca 2+ signaling as saturation of the photosynthetic electron transport, H 2 O 2 formation, and aCRY-dependent light perception.