Analyses of Ca2+ Accumulation and Dynamics in the Endoplasmic Reticulum of Arabidopsis Root Cells Using a Genetically Encoded Cameleon Sensor

In planta, very limited information is available about how the endoplasmic reticulum (ER) contributes to cellular Ca2+ dynamics and homeostasis. Here, we report the generation of an ER-targeted Cameleon reporter protein suitable for analysis of Ca2+ accumulation and dynamics in the lumen of the ER in plant cells. Using stably transformed Arabidopsis (Arabidopsis thaliana) plants expressing this reporter protein, we observed a transiently enhanced accumulation of Ca2+ in the ER in response to stimuli inducing cytosolic Ca2+ rises in root tip cells. In all experimental conditions, ER Ca2+ dynamics were substantially different from those monitored in the cytosol. A pharmacological approach enabled us to evaluate the contribution of the different ER-resident Ca2+-ATPase classes in the regulation of the ER Ca2+ homeostasis. Taken together, our results do not provide evidence for a role of the ER as a major source that releases Ca2+ for stimulus-induced increases in cytosolic Ca2+ concentration. Instead, our results show that the luminal ER Ca2+ elevations typically follow cytosolic ones, but with distinct dynamics. These findings suggest fundamental differences for the function of the ER in cellular Ca2+ homeostasis in plants and animals.