A P_IIB-type Ca²⁺-ATPase is essential for stress adaptation in Physcomitrella patens

Transient cytosolic Ca²⁺ ([Ca²⁺]_cyt) elevations are early events in plant signaling pathways including those related to abiotic stress. The restoration of [Ca²⁺]_cyt to prestimulus levels involves ATP-driven Ca²⁺ pumps, but direct evidence for an essential role of a plant Ca²⁺-ATPase in abiotic stress adaptation is missing. Here, we report on a stress-responsive Ca ²⁺-ATPase gene (PCA1) from the moss Physcomitrella patens. Functional analysis of PCA1 in a Ca²⁺ transport-deficient yeast mutant suggests that PCA1 encodes a P_IIB-type Ca²⁺-ATPase harboring an N-terminal autoinhibitory domain. In vivo localizations identified membranes of small vacuoles as the integration site for a PCA1:GFP fusion protein. PCA1 mRNA levels are up-regulated by dehydration, NaCl, and abscisic acid, and PCA1 loss-of-function mutants (ΔPCA1) exhibit an enhanced susceptibility to salt stress. The ΔPCA1 lines show sustained elevated [Ca ²⁺]_cyt in response to salt treatment in contrast to WT that shows transient Ca²⁺ elevations, indicating a direct role for PCA1 in the restoration of prestimulus [Ca²⁺]_cyt. The altered Ca²⁺ response of the ΔPCA1 mutant lines correlates with altered expression levels of stress-induced genes, suggesting disturbance of a stress-associated signaling pathway. We propose that PCA1 is an essential component for abiotic stress adaptation in Physcomitrella involved in the generation of a specific salt-induced Ca²⁺ signature.