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 increa ses in intracellular free Ca2+ concentration ([Ca2+]i) in cells of the nitr ogen-fixing filamentous cyanobacterium Anabaena sp, PCC7120, which constitu tively expresses apoaequorin, Isoosmolar concentrations of salt (NaCl) and osmoticum (sucrose) induced calcium transients of similar magnitude and sha pe, suggesting that cells sense, via Ca2+ signalling, mostly osmotic stress . The Ca2+ transients induced by NaCl and sucrose were completely blocked b y the calcium chelator ethylene glycol-bis(beta -aminoethylether)N,N,N',N'- tetraacetic acid (EGTA) and were partially inhibited by the calcium channel blocker verapamil. Increased external Ca2+ and the Ca2+ ionophore calcimyc in (compound A23187) enhanced Ca2+ influx further, suggesting the involveme nt of extracellular Ca2+ in the observed response to salinity and osmotic s tress. However, the plant hormone abscisic acid (ABA) did not provoke any e ffect on the Ca2+ transients induced by both stresses, indicating that it m ay not be acting upstream of Ca2+ in the signalling of salinity and/or osmo tic stress in Anabaena sp, PCC7120.