Arabidopsis transcriptional response to extraceliular Ca^2＋ depletion involves a transient rise in cytosolic Ca^2

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.