Calcium imaging shows differential sensitivity to cooling and communication in luminous transgenic plants

Imaging of a recombinant bioluminescent Ca2+ indicator, aequorin, in an entire organism showed three novel features of Ca2+ signals in plants. First, cooling the plant from 25 degrees C to 2 degrees C demonstrated differential sensitivities between organs, the roots firing a Ca2+ signal at some 8-10 degrees C higher than the cotyledons. Secondly, prolonged cooling provoked Ca2+ oscillations, but only in the cotyledons. These oscillations occurred with a frequency of 100 s and damped down within 800 s. Thirdly, cooling the roots of mature plants triggered a Ca2+ signal in the leaves, as a result of organ-organ communication. However, warming and then recooling the roots did not generate a second Ca2+ signal in these leaves. This desensitisation was not due to down-regulation in the leaf since this was able to generate a Ca2+ signal of its own when cooled directly. Thus a combination of a recombinant bioluminescent indicator with photon counting imaging reveals startling new aspects of signalling in intact organs and whole organisms.