Inositol 1,4,5-trisphosphate releases Ca2+ from vacuolar membrane vesicles of oat roots.

In plant cells, transient changes in cytoplasmic Ca2+ levels can modulate numerous developmental processes. Ca2+ is accumulated in the vacuole via a H+/Ca2+ antiport system that is energized by the tonoplast H+-pumping ATPase. Inositol 1,4,5-triphosphate (InsP3), but not inositol 1,4-bisphosphate, myo-inositol 1-phosphate, or fructose 2,6-bisphosphate, caused a transient reduction of Ca2+ levels in tonoplast vesicles. The decrease was dependent on InsP3 concentration (Km apparent = 0.6 microM). The InsP3-induced Ca2+ release was blocked by the Ca2+ antagonist, 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl. These results suggest that the vacuolar membrane is one target site for InsP3 action and that InsP3 may operate as a second messenger in the mobilization of intracellular Ca2+ in plant cells.