Calcium Stores in Symbiosomes from Yellow Lupin Root Nodules

Abstract

Summary The PPA technique was used for the visualization of calcium in the symbiosomes of yellow lupin root nodule cells at the ultrastructural level both in situ and in vitro. Nodulated plants were grown in pots containing 6 kg of sand with nutrients and various calcium concentrations. At relatively low calcium level (without added calcium or 2 mmol of endogenous calcium per pot) the pyroantimonate calcium precipitates were detected on the cytoplasmic side of the PBM, in the bacteroid cytoplasm, to a much lesser extent within the PBS, and, in addition, in some other subcellular compartments of the infected cells. The observed features in symbiosome calcium distribution became much more profound after increasing the calcium concentration (up to 3 and 6 mmol/pot) in the nutrient medium. In addition, under these conditions numerous electron opaque particles associated with the inner surface of the PBM appeared as well. With the same technique it was found that isolated lupin symbiosomes incubated in the presence of ATP, Mg2+ and Ca2+ are able to accumulate Ca2+, as judged by clearly observed deposits of large size in the PBS in this case. In the experiments with calcium- and ΔpH-sensitive dyes (arsenazo III and acridine orange, respectively), the existence of a calcium pool within the symbiosomes was confirmed by their ability to release calcium ions into the suspension medium after permeabilization of the PBM with the Ca 2+ ionophore A23187. These results provide direct evidence for the hypothesis that the symbiosomes behave as calcium stores in infected cells and, in addition, may be involved in calcium homeostasis in the plant cytosol.