Chloroplasts require a fine-tuned control of their internal Ca²⁺ concentration, which is crucial for many aspects of photosynthesis and for other chloroplast-localized processes. Increasing evidence suggests that calcium regulation within chloroplasts also may influence Ca²⁺ signaling pathways in the cytosol. To investigate the involvement of thylakoids in Ca²⁺ homeostasis and in the modulation of chloroplast Ca²⁺ signals in vivo, we targeted the bioluminescent Ca²⁺ reporter aequorin as a YFP fusion to the lumen and the stromal surface of thylakoids in Arabidopsis (Arabidopsis thaliana). Thylakoid localization of aequorin-based probes in stably transformed lines was confirmed by confocal microscopy, immunogold labeling, and biochemical analyses. In resting conditions in the dark, free Ca²⁺ levels in the thylakoid lumen were maintained at about 0.5 μm, which was a 3- to 5-fold higher concentration than in the stroma. Monitoring of chloroplast Ca²⁺ dynamics in different intrachloroplast subcompartments (stroma, thylakoid membrane, and thylakoid lumen) revealed the occurrence of stimulus-specific Ca²⁺ signals, characterized by unique kinetic parameters. Oxidative and salt stresses initiated pronounced free Ca²⁺ changes in the thylakoid lumen. Localized Ca²⁺ increases also were observed on the thylakoid membrane surface, mirroring transient Ca²⁺ changes observed for the bulk stroma, but with specific Ca²⁺ dynamics. Moreover, evidence was obtained for dark-stimulated intrathylakoid Ca²⁺ changes, suggesting a new scenario for light-to-dark-induced Ca²⁺ fluxes inside chloroplasts. Hence, thylakoid-targeted aequorin reporters can provide new insights into chloroplast Ca²⁺ storage and signal transduction. These probes represent novel tools with which to investigate the role of thylakoids in Ca²⁺ signaling networks within chloroplasts and plant cells.

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