Calcium Signaling-Linked CBF/DREB1 Gene Expression was Induced Depending on the Temperature Fluctuation in the Field: Views from the Natural Condition of Cold Acclimation

Environmental adaptability is essential for plant survival. Though it is well known that a simple cooling or cold shock leads to Ca²⁺ signals, direct evidence has not been provided that plants use Ca²⁺ signals as a second messenger in the cold acclimation (CA) process in the field. By developing a technique to analyze Ca²⁺ signals using confocal cryomicroscopy, we investigated Ca²⁺ signals under several temperature conditions by combining the start temperature, cooling rate and cooling time duration. In both root and leaf cells, Ca²⁺ signals rapidly disappeared after cooling stopped, and thereafter under a constant low temperature no Ca²⁺ signal was observed. Interestingly, under the cooling regime from 2°C to −2°C, non-acclimated plants grown at 23°C hardly showed Ca²⁺ signals, but cold-acclimated plants at 2°C were able to form Ca²⁺ signals in root cells. These findings suggest that plants sense temperature decreases with Ca²⁺ signals while adjusting the temperature sensitivity to their own temperature environment. Furthermore, if the temperature is constant, no Ca²⁺ signal is induced even during CA. Then, we also focused on the CA under field conditions, rich in temperature fluctuations. In CA under field conditions, the expression patterns of CBF/DREB1 genes were distinctly different from those in artificial CA. Pharmacological studies with Ca²⁺ channel blockers showed that the Ca²⁺-induced expression of CBF/DREB1 genes was closely correlated with the amplitude of temperature fluctuation, suggesting that Ca²⁺ signals regulate CBF/DREB1 gene expression during CA under natural conditions.