Calcium in the Life Cycle of Legume Root Nodules

The present review highlights both the fundamental questions of calcium localization, compartmentation, and its participation in symbiosome signaling cascades during nodule formation and functioning. Apparently, the main link of such signaling is the calmodulin…calcium- and calmodulin-dependent protein kinases…CYCLOPS…NIN…target genes cascade. The minimum threshold level of calcium as a signaling agent in the presence of intracellular reserves determines the possibility of oligotrophy and ultraoligotrophy in relation to this element. During the functioning of root nodules, the Ca2+-ATPases activity maintains homeostasis of low calcium concentrations in the cytosol of nodule parenchyma cells. Disturbation of this homeostasis can trigger the root nodule senescence. The same reasons determine the increase in the effectiveness of symbiosis with the help of seed priming with sources of calcium. Examples of calcium response polymorphism in components of nitrogen fixing simbiosis important in practical terms are shown.

Emerging evidence for potential role of Ca2+-ATPase-mediated calcium accumulation in symbiosomes of infected root nodule cells

Symbiosomes are organelle-like compartments responsible for nitrogen fixation in infected nodule cells of legumes, which are formed as a result of symbiotic association of soil bacteria rhizobia with certain plant root cells. They are virtually the only source of reduced nitrogen in the Earth's biosphere, and consequently, are of great importance. It has been proven that the functioning of symbiosomes depends to a large extent on the transport of various metabolites and ions - most likely including Ca2+ - across the symbiosome membrane (SM). Although it has been well established that this cation is involved in the regulation of a broad spectrum of processes in cells of living organisms, its role in the functioning of symbiosomes remains obscure. This is despite available data indicating both its transport through the SM and accumulation within these compartments. This review summarises the results obtained in the course of studies on the given aspects of calcium behaviour in symbiosomes, and on this basis gives a possible explanation of the proper functional role in them of Ca2+.