Burning glass effect of water droplets triggers an ER-derived calcium response in the chloroplast stroma ofArabidopsis thalianaleaves

Plants require water and light for photosynthesis, but light, when focused by water droplets on leaves, can create high light intensity spots that are harmful to plants. As excessive light intensity can reduce growth or even induce cell death, it is vital for plants to detect and react to changes in light exposure and acclimate to high light stress. Ca 2+ signaling was previously implicated in high light acclimation. However, the dynamics of free Ca 2+ concentration in the chloroplast, the primary site of photosynthesis, or in the nucleus and in the cytoplasm, where transcription and translation for long-term acclimation occurs, remain unknown. Here we studied the dynamics and mechanism of the Ca 2+ response to high light exposure. Focusing light through a glass bead to mimic water droplets triggered an increase of the free Ca 2+ concentration in the chloroplast stroma of Arabidopsis thaliana . This finding was corroborated using established and newly developed genetically encoded calcium indicators, which revealed a biphasic increase in the stromal free Ca 2+ concentration when exposed to varying intensities and qualities of light. Among photosynthetic by-products, reactive oxygen and lipophilic species in particular, have been implicated in high light stress acclimation. A H 2 O 2 signature was induced, albeit with different dynamics than the Ca 2+ response, while chemical inhibition of the photosynthetic electron transport points towards singlet oxygen as a potential culprit of the high light-induced increase in stromal free Ca 2+ concentration. The observed dynamics differed from those of a heat-shock induced Ca 2+ signature, although temperature had a positive effect on the Ca 2+ response. Based on Ca 2+ inhibitor treatments and the free Ca 2+ concentration dynamics, we suggest that the high light-induced stromal Ca 2+ is derived from the endoplasmic reticulum rather than from the cytoplasm. In conclusion, inspired by the burning glass effect of water droplets on leaves, we uncovered a Ca 2+ response that implicates a novel mechanism for plants to acclimate to high light stress—a process that will become increasingly relevant in a changing climate.