Arabidopsis transcriptional response to extracellular Ca2+ depletion involves a transient rise in cytosolic Ca2+

Ecological evidence indicates a worldwide trend of dramatically decreased soil Ca2+ levels caused by increased acid deposition and massive timber harvesting. Little is known about the genetic and cellular mechanism of plants' responses to Ca2+ depletion. In this study, transcriptional profiling analysis helped identify multiple extracellular Ca2+ ([Ca2+](ext)) depletion-responsive genes in Arabidopsis thaliana L., many of which are involved in response to other environmental stresses. Interestingly, a group of genes encoding putative cytosolic Ca2+ ([Ca2+](cyt)) sensors were significantly upregulated, implying that [Ca2+](cyt) has a role in sensing [Ca2+](ext) depletion. Consistent with this observation, [Ca2+](ext) depletion stimulated a transient rise in [Ca2+](cyt) that was negatively influenced by [K+](ext), suggesting the involvement of a membrane potential-sensitive component. The [Ca2+](cyt) response to [Ca2+](ext) depletion was significantly desensitized after the initial treatment, which is typical of a receptor-mediated signaling event. The response was insensitive to an animal Ca2+ sensor antagonist, but was suppressed by neomycin, an inhibitor of phospholipase C. Gd3+, an inhibitor of Ca2+ channels, suppressed the [Ca2+](ext)-triggered rise in [Ca2+](cyt) and downstream changes in gene expression. Taken together, this study demonstrates that [Ca2+](cyt) plays an important role in the putative receptor-mediated cellular and transcriptional response to [Ca2+](ext) depletion of plant cells.