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Guard cells respond to cold and other stimuli with an oscillating Ca2+ signal, causing an opening or closing of the stomatal pore. However, the molecular basis for the regulation of Ca2+ transients in guard cells has so far not been elucidated. We have localized the expression of two closely related Ca2+-ATPases, ACA8 and ACA10, to guard cells in Arabidopsis. Whereas ACA8 expression was limited to guard cells and vascular tissues, ACA10 was found to be ubiquitously expressed. Upon cold treatment, the expression of ACA8 was found to be upregulated while that of ACA10 was downregulated. This finding was supported by the presence of the cold responsive C-repeat/dehydration-responsive element motif in the ACA8 promoter sequence.