In vivo analysis of the calcium signature in the plant Golgi apparatus reveals unique dynamics

The Golgi apparatus is thought to play a role in calcium homeostasis in plant cells. However, the calcium dynamics in this organelle is unknown in plants. To monitor the [Ca²⁺]_Golgi in vivo, we obtained and analyzed Arabidopsis thaliana plants that express aequorin in the Golgi. Our results show that free [Ca²⁺] levels in the Golgi are higher than in the cytosol (0.70μM vs. 0.05μM, respectively). Stimuli such as cold shock, mechanical stimulation and hyperosmotic stress, led to a transient increase in cytosolic calcium; however, no instant change in the [Ca²⁺]_Golgi concentration was detected. Nevertheless, a delayed increase in the [Ca²⁺]_Golgi up to 2-3μM was observed. Cyclopiazonic acid and thapsigargin inhibited the stimuli-induced [Ca²⁺]_Golgi increase, suggesting that [Ca²⁺]_Golgi levels are dependent upon the activity of Ca²⁺-ATPases. Treatment of these plants with the synthetic auxin analog, 2,4-dichlorophenoxy acetic acid (2,4-D), produced a slow decrease of free calcium in the organelle. Our results indicate that the plant Golgi apparatus is not involved in the generation of cytosolic calcium transients and exhibits its own dynamics modulated in part by the activity of Ca²⁺ pumps and hormones.