Cell- and Stimulus Type-Specific Intracellular Free Ca2+ Signals in Arabidopsis

Appropriate stimulus-response coupling requires that each signal induces a characteristic response, distinct from that induced by other signals, and that there is the potential for individual signals to initiate different downstream responses dependent on cell type. How such specificity is encoded in plant signaling is not known. One possibility is that information is encoded in signal transduction pathways to ensure stimulus-and cell type-specific responses. The calcium ion acts as a second messenger in response to mechanical stimulation, hydrogen peroxide, NaCl, and cold in plants and also in circadian timing. We use GAL4 transactivation of aequorin in enhancer trap lines of Arabidopsis (Arabidopsis thaliana) to test the hypothesis that stimulus-and cell-specific information can be encoded in the pattern of dynamic alterations in the concentration of intracellular free Ca2+ ([Ca2+](i)). We demonstrate that mechanically induced increases in [Ca2+](i) are largely restricted to the epidermal pavement cells of leaves, that NaCl induces oscillatory [Ca2+](i) signals in spongy mesophyll and vascular bundle cells, but not other cell types, and detect circadian rhythms of [Ca2+](i) only in the spongy mesophyll. We demonstrate stimulus-specific [Ca2+](i) dynamics in response to touch, cold, and hydrogen peroxide, which in the case of the latter two signals are common to all cell types tested. GAL4 transactivation of aequorin in specific leaf cell types has allowed us to bypass the technical limitations associated with fluorescent Ca2+ reporter dyes in chlorophyll-containing tissues to identify the cell-and stimulus-specific complexity of [Ca2+](i) dynamics in leaves of Arabidopsis and to determine from which tissues stress-and circadian-regulated [Ca2+](i) signals arise.