Calcium Dynamics in Chloroplasts

Transient changes in intracellular free Ca²⁺ concentration ([Ca²⁺]) are involved in the sensing of a wide variety of abiotic and biotic stimuli iIn plants. The unique spatiotemporal patterns in [Ca²⁺] that result enable specific stimulus-response coupling. Several intracellular compartments of the plant cell participate in Ca²⁺ homeostasis, including the vacuole, endoplasmic reticulum, mitochondria, and plastids. Numerous studies indicate that chloroplasts require a fine-tuned control of the organellar Ca²⁺ concentration. It is known that free Ca²⁺ modulates crucial aspects of photosynthesis, including the assembly and function of PSII, the regulation of stromal enzymes of the Calvin cycle, as well as other plastid-localized processes such as the import of nucleus-encoded proteins and organelle division. However, little is known about the mechanisms that underlie the generation and dissipation of Ca²⁺ transients and the transduction of environmental signals within plastids or their localization inside the organelle. To investigate the involvement of thylakoids in Ca²⁺ homeostasis and in the modulation of chloroplast Ca²⁺ signals in vivo, Sello et al. (10.1104/pp.18.00027) targeted the bioluminescent Ca²⁺ reporter aequorin (fused with Yellow Fluorescent Protein) to the lumen and the stromal surface of thylakoids in Arabidopsis. In resting conditions in the dark, free Ca²⁺ levels in the thylakoid lumen were maintained at about 0.5 mM, 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. Evidence was also obtained for dark-stimulated intrathylakoid Ca²⁺ changes. The novel toolkit of thylakoid-targeted Ca²⁺ reporters reported upon advances our current understanding of Ca²⁺ regulation in chloroplasts and paves the way for future investigations on plant organellar Ca²⁺ signaling.