Verapamil-sensitive calcium transporter in the peribacteroid membrane of symbiosomes from Vicia faba root nodules

Based on electron microscopic studies and visualization of calcium with the Ca indicator pyroantimonate, it was established that a prolonged incubation of the bean (Vicia faba L.) root nodules and isolated symbiosomes in EGTA-containing buffer depletes calcium in these nitrogen-fixing units. Other experiments demonstrated that the induction of calcium deficit in symbiosomes both in vivo and in vitro substantially decreases their nitrogenase activity. The addition of verapamil and ruthenium red, well-known inhibitors of Ca channels, to the suspension of root nodules largely prevented both the EGTA-induced calcium efflux from the symbiosomes and the decrease in their nitrogenase activity. Similar effects of verapamil were also observed on isolated symbiosomes. The treatment of isolated symbiosomes with valinomycin in the presence of K+ induced a rapid efflux of Ca2+ from symbiosomes; this efflux was strongly inhibited by verapamil. The results present evidence for the existence in the peribacteroid membrane of a Ca2+-transporting system that exports Call from the symbiosomes.