Regulated expression of the calmodulin-related TCH genes in cultured Arabidopsis cells: induction by calcium and heat shock.

Expression of the calmodulin-related TCH genes of Arabidopsis is strongly and rapidly up-regulated in plants after a variety of stimuli, including touch. As an approach to investigating the mechanism(s) of TCH gene regulation, a manipulable cell culture system in which TCH gene expression is regulated has been developed. In response to increased external calcium or heat shock, TCH2, -3, and -4 mRNA levels significantly increased. Significantly, these two stimuli are known to result in cytoplasmic calcium increases, therefore implicating a role for calcium itself in the regulation of calmodulin-related genes. Further, external calcium is required for maximal heat-shock induction of expression of the TCH genes but not of the 70-kDa heat shock protein; therefore, there may exist at least two distinct mechanisms of heat shock induction of gene expression. Calcium ion regulation of genes encoding calcium-binding proteins may ensure the efficacy of calcium ion as a transient second messenger and the maintenance of cellular homeostasis. This possible regulatory circuit would likely be relevant not only for plant cells but also for the great variety of animal cells that transduce extracellular stimuli, such as hormones and electrical impulses, into calcium signals.