Blue light but not red light induces a calcium transient in the moss Physcomitrella patens (Hedw.) B., S. & G.

In caulonemal filaments of the moss, Physcomitrella patens. which had been incubated ill darkness, 3 s irradiation with blue light (lambda(max) 450 nm) at fluence rates of 100 mu mol m(-2) s(-1) and above caused a transient increase in cytosolic calcium ion concentration, [Ca2+](cyt), which was both intensity- and time-dependent. Measurements of [Ca2+] were made using moss transformed with the cDNA for apoaequorin and reconstituting the Ca2+-dependent photoprotein aequorin in the cytosol by incubation in coelenterazine. In response to blue light at fluence rates of 100-1000 mu mol photons m(-2) s(-1) [Ca2+](cyt) increased transiently from a basal level of approximately 50 nM to between 200 and 700 nM. Irradiation with red light did not evoke any measurable change in [Ca2+](cyt). The presence of calcium in the incubating medium was not required for the increase in [Ca2+](cyt) to occur. A mutant strain, gad-139, was identified which required an irradiance of only I s to evoke a response. The kinetics showed a delay of approximately 6 s from the beginning of illumination before the beginning of the increase in [Ca2+](cyt). The data suggest that the activation of a photoreceptor rather than the direct opening of calcium channels is involved in this blue-light response.