Calcium uptake by symbiosomes and the peribacteroid membrane vesicles isolated from yellow lupin root nodules

In order to test whether the symbiosomes of infected cells are able to actively take up calcium ions, preparations of these nitrogen-fixing units and the PBM vesicles isolated from yellow lupin (Lupinus luteus L.) root nodules were investigated to this end. Ca2+ uptake was recorded with the use of the metallochromic Ca2+ indicator arsenate III added to the incubation medium. It was found that the addition of ATP to symbiosomes suspended in the presence of Mg2+ and Ca2+ initiated a gradual removal of calcium from the incubation medium. This process was rapidly inhibited with addition of vanadate, but was resistant to protonophores, the Ca2+ ionophore A23187, valinomycin in the presence of potassium ions, and erythrosin B, and was greatly stimulated by nitrate anions. Qualitatively similar results were obtained with preparations of the PBMs, except that in this case the Ca2+ ionophore A23187 effectively facilitated the calcium release from the PBM vesicles after the uptake. The data obtained demonstrate primary active transport of calcium across the PBM, which is most likely caused by the activity of the Mg2+-dependent Ca2+-ATPase associated with this membrane.