Calcium transients in response to salinity and osmotic stress in the nitrogen‐fixing cyanobacterium Anabaena sp. PCC7120, expressing cytosolic apoaequorin

We show here that both salinity and osmotic stress trigger transient increases in intracellular free Ca 2+ concentration ([Ca 2+ ] i ) in cells of the nitrogen‐fixing filamentous cyanobacterium Anabaena sp. PCC7120, which constitutively expresses apoaequorin. Isoosmolar concentrations of salt (NaCl) and osmoticum (sucrose) induced calcium transients of similar magnitude and shape, suggesting that cells sense, via Ca 2+ signalling, mostly osmotic stress. The Ca 2+ transients induced by NaCl and sucrose were completely blocked by the calcium chelator ethylene glycol‐bis(b‐aminoethylether)N,N,N¢,N¢‐tetraacetic acid (EGTA) and were partially inhibited by the calcium channel blocker verapamil. Increased external Ca 2+ and the Ca 2+ ionophore calcimycin (compound A23187) enhanced Ca 2+ influx further, suggesting the involvement of extracellular Ca 2+ in the observed response to salinity and osmotic stress. However, the plant hormone abscisic acid (ABA) did not provoke any effect on the Ca 2+ transients induced by both stresses, indicating that it may not be acting upstream of Ca 2+ in the signalling of salinity and/or osmotic stress in Anabaena sp. PCC7120.