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

Abstract

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.

Keywords: Chlamydomonas; calcium; cell signaling; chloroplast; photoreceptor; photosynthesis; reactive oxygen species; single-cell imaging.