Calcium Status of Yellow Lupin Symbiosomes as a Potential Regulator of Their Nitrogenase Activity: The Role of the Peribacteroid Membrane

Abstract

The capacity of symbiosomes from yellow lupin root nodules for active Ca2+uptake and the sensitivity of their nitrogenase activity to a disturbance of the symbiotic Ca partition were investigated. The experiments carried out on the isolated symbiosomes and the peribacteroid membrane (PBM) vesicles, using Ca2+indicators arsenazo III and chlorotetracycline, and the cytochemical Ca visualization with potassium pyroantimonate (PA) provided evidence that an Mg-ATP-energized pump, most likely Mg2+-dependent Ca2+-ATPase catalyzing 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 vivoandin vitroby treating the intact nodules of yellow lupin root or the purified symbiosomes isolated from the latter with EGTA and Ca2+-ionophore A23187 substantially decreased their nitrogenase activity. The inhibitory effect of calcium deficit in the symbiosomes was not reversed by the addition of calcium to the incubation medium containing the plant tissues under study and was even enhanced under these conditions. The nitrogenase activity of the isolated symbiosomes not experiencing calcium deficit was also inhibited by the addition of relatively high concentrations of exogenous calcium to the incubation medium. These results seem to give evidence that the calcium status of nodule symbiosomes from yellow lupin roots controls their nitrogenase activity. The data obtained suggest that both Ca2+transport on PBM and the low passive permeability of this membrane for the given cation play the key role in such a control.