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

Abstract

Summary 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 H2O2 sensor was also adopted to investigate the possible role of H2O2 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 H2O2 gradients and Ca2+ transients was drawn, supported by H2O2‐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, H2O2 formation, and aCRY‐dependent light perception.