ATP-dependent regulation of nuclear Ca(2+) levels in plant cells

Localised alterations in cytoplasmic Ca(2+) 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 Ca(2+) levels reflect the prevailing cytoplasmic Ca(2+) levels. Using nuclei prepared from carrot (Daucus carota L.) cells, we now show that Ca(2+) can be transported across nuclear membranes in an ATP-dependent manner and that over 95% of Ca(2+) is accumulated into a pool releasable by the Ca(2+) ionophore A.23187. ATP-dependent nuclear Ca(2+) uptake did not occur in the presence of ADP or ADPgammaS and was abolished by orthovanadate. Confocal microscopy of nuclei loaded with dextran-linked Indo-1 showed that the initial ATP-induced rise in [Ca(2+)] occurs in the nuclear periphery. The occurrence of ATP-dependent Ca(2+) uptake in plant nuclei suggests that alterations of intranuclear Ca(2+) levels may occur independently of cytoplasmic [Ca(2+)] changes.