Ammonium uptake and cellular alkalisation in roots of Arabidopsis thaliana: The involvement of cytoplasmic calcium

Using roots from Al Arabidopsis thaliana expressing the recombinant calcium indicator aequorin, we show that NH3, uptake and alkalisation of plant cel ls act as a stimulus which induces transient elevations of the cyloplasmic free calcium concentration ([Ca2+](c)), The magnitudes of these [Ca2+](c) e levations are dependent on the concentration of the membrane permeable form , NH3,, and hence, particularly dependent on the pH in the external medium. EGTA and La3+ are able to significantly suppress the [Ca2+](c) transients shelving that Ca2+ influx through the plasma membrane is likely to be invol ved, Verapamil and nifedipine had no inhibitory effects, which suggests tha t Ca2+ release from internal stores might not contribute significantly to t he NH3-triggered [Ca2+], response. Pre-incubation in L-methionine-DL-sulpho ximine - an inhibitor of the glutamine synthetase - did not alter the NH3-i nduced [Ca2+](c) responses at all. These results are consistent with previo us studies where NH3-induced changes of cytoplasmic and vacuolar pH were in vestigated in maize. roots. Furthermore, the similarity between the kinetic s of NH3 driven cellular pH changes demonstrated in previous studies and th e [C2+], transients shown here suggests a direct relationship between [Ca2], and cellular alkalisation (cytoplasmic pH and/or vacuolar pH, However, t he mechanism behind this possible causal relation remains to be elucidated.