ATP‐dependent regulation of nuclear Ca2+ levels in plant cells

Localised alterations in cytoplasmic Ca2+ levels are an integral part of the response of eukaryotic cells to a plethora of external stimuli. Due to the large size of nuclear pores, it has generally been assumed that intranuclear Ca2+ levels reflect the prevailing cytoplasmic Ca2+ levels. Using nuclei prepared from carrot (Daucus carota L.) cells, we now show that Ca2+ can be transported across nuclear membranes in an ATP-dependent manner and that over 95% of Ca2+ is accumulated into a pool releasable by the Ca2+ ionophore A.23187. ATP-dependent nuclear Ca2+ uptake did not occur in the presence of ADP or ADPγS and was abolished by orthovanadate. Confocal microscopy of nuclei loaded with dextran-linked Indo-1 showed that the initial ATP-induced rise in [Ca2+] occurs in the nuclear periphery. The occurrence of ATP-dependent Ca2+ uptake in plant nuclei suggests that alterations of intranuclear Ca2+ levels may occur independently of cytoplasmic [Ca2+] changes.