A PIIB-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 Ca2+-ATPase gene (PCA1) from the moss Physcomitrella patens. Functional analysis of PCA1 in a Ca2+ transport-deficient yeast mutant suggests that PCA1 encodes a PIIB-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 [Ca2+]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.