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

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