Calcium status of yellow lupin symbiosomes as a potential regulator of their nitrogenase activity: The role of the peribacteroid membrane

The capacity of symbiosomes from yellow lupin root nodules for active Ca2uptake and the sensitivity of their nitrogenase activity to a disturbance o f the symbiotic Ca partition were investigated. The experiments carried out on the isolated symbiosomes and the peribacteroid membrane (PBM) vesicles, using Ca2+ indicators arsenate III and chlorotetracycline, and the cytoche mical Ca visualization with potassium pyroantimonate (PA) provided evidence that an Mg-ATP-energized pump, most likely Mg2+-dependent Ca2+ ATPase cata lyzing the active transport of Ca2+ from the cytosol of the plant cell into the symbiosomes across the PBM, functions on this membrane. Depleting the symbiosomes of Ca both in vivo and in vitro by treating the intact nodules of yellow lupin root or the purified symbiosomes isolated from the latter w ith EGTA and Ca2+-ionophore A23187 substantially decreased their nitrogenas e activity. The inhibitory effect of calcium deficit in the symbiosomes was not reversed by the addition of calcium to the incubation medium containin g the plant tissues under study and was even enhanced under these condition s. The nitrogenase activity of the isolated symbiosomes not experiencing ca lcium deficit was also inhibited by the addition of relatively high concent rations of exogenous calcium to the incubation medium. These results seem t o give evidence that the calcium status of nodule symbiosomes from yellow l upin roots controls their nitrogenase activity. The data obtained suggest t hat both Ca2+ transport on PBM and the low passive permeability of this mem brane for the given cation play the key role in such a control.