Article AddendumCa²⁺ signatures The role of Ca ²⁺-ATPases

Calcium ions (Ca²⁺) are well known signaling molecules in plant signal transduction pathways including the response to abiotic stress. Particular stimuli cause specific transient elevations in cytosolic Ca ²⁺ ([Ca²⁺]_cyt) with a stimulus-dependent amplitude and temporal pattern. These Ca²⁺ transients, known as Ca²⁺ signatures, rely on the counteractive activities of Ca ²⁺-permeable channels and Ca²⁺-transporting proteins. Whereas the channels causing an increase in [Ca²⁺]_cyt were partly identified, direct evidence for the role of Ca²⁺-transporting proteins in the determination of Ca²⁺ signatures was missing. We recently reported on the functional characterization of a stress-responsive PIIB-type Ca²⁺-ATPase (PCA1) from the moss Physcomitrella patens. This study revealed an essential role of this Ca²⁺ pump in the adjustment of salt stress tolerance. Most strikingly, the generation of a specific Ca²⁺ signature in response to salt was abolished in Physcomitrella mutant lines lacking PCA1 activity. Thus, we can provide a direct link for the function of a Ca²⁺-ATPase in the generation of a specific Ca²⁺ signature in plants.