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Certain aspects of Ca-dependent regulation of symbiosomes functioning mediated by the operation of Ca2+-translocating ATPase on the symbiosome membrane (SM) were considered. As follows from the results recently obtained by us, the mechanism underlying the action of this calcium pump exhibits a certain functional feature caused by that transfer by it of calcium ion through the SM is closely coupled with transmembrane translocation through the same membrane but in opposite direction of H+ ions. The data here outlined show that such the cation-exchanged mechanism of the Ca2+-ATPase operation can putatively lead to corresponding modulation or maintenance of both ionic and redox homeostasis of the symbiosome space (SS) around bacteroids at a proper level and thereby to the regulation of symbiosomes functioning. It is suggested that based on the same mechanism inherent function of the enzyme in question is associated with its involvement in symbiotic partner cells signaling as well. All these potential activities of the Ca2+-ATPase lead to the important question whether it is the factor determining substantial functions of symbiosomes. Although it is evident that research an answer to this question is a complex enough experimental problem, its solution may give significant contribution to elucidating mechanisms of Ca-dependent regulation of the processes occurring in mature symbiosomes. [ABSTRACT FROM AUTHOR]