The calcium transporter ANNEXIN1 mediates cold-induced calcium signaling and freezing tolerance in plants.

The transient elevation of cytosolic free calcium concentration ([Ca ] ) induced by cold stress is a well-established phenomenon; however, the underlying mechanism remains elusive. Here, we report that the Ca -permeable transporter ANNEXIN1 (AtANN1) mediates cold-triggered Ca influx and freezing tolerance in Arabidopsis thaliana. The loss of function of AtANN1 substantially impaired freezing tolerance, reducing the cold-induced [Ca ] increase and upregulation of the cold-responsive CBF and COR genes. Further analysis showed that the OST1/SnRK2.6 kinase interacted with and phosphorylated AtANN1, which consequently enhanced its Ca transport activity, thereby potentiating Ca signaling. Consistent with these results and freezing sensitivity of ost1 mutants, the cold-induced [Ca ] elevation in the ost1-3 mutant was reduced. Genetic analysis indicated that AtANN1 acts downstream of OST1 in responses to cold stress. Our data thus uncover a cascade linking OST1-AtANN1 to cold-induced Ca signal generation, which activates the cold response and consequently enhances freezing tolerance in Arabidopsis.