Subcellular calcium localization in the root nodules of legumes

Electron microscopic cytochemical assay with the use of potassium pyroantim onate was applied to localize intracellular Ca2+ in root nodules of yellow lupine (Lupinus luteus L., a calcium-phobic plant) and broad bean (Vicia fa ba L., a calcium-philic plant). Plants were grown at different levels of av ailable calcium under conditions of both effective and ineffective symbiosi s. The same plants were used for measuring nitrogenase activity of their no dules. For beans, the maximum value of this activity was observed at a high er level of calcium (7 mmol per pot) in the nutrient medium than for lupine (3 mmol per pot). At an optimal supply of Ca2+ to the plants, calcium was visualized in the matrix of mitochondria, nuclear euchromatin, cisternae of granular endoplasmic reticulum, on the plasma membrane, and in vacuoles. I n infected cells, Ca2+ was located on the peribacteroid membrane of symbios omes, in the cytoplasm of bacteroids, and within the peribacteroid space. T he size and number of precipitate granules formed during the cytochemical r eaction increased at a higher supply of calcium to the plants. At increased calcium doses (6 and 12 mmol per pot for lupine and beans, respectively), this cation was accumulated in symbiosomes, vacuoles, and other cellular or ganelles, as well as in the apoplast. Under conditions of ineffective symbi osis, calcium depletion of symbiosomes occurred, which was largely due to t he decrease in the calcium content of bacteroids. Destructive changes in th e symbiotic system were observed when ineffective symbiosis was combined wi th a high calcium level in the nutrient medium. These results suggest that the observed changes in subcellular calcium distribution in the infected no dules reflect the involvement of symbiosomes in the regulation of intracell ular calcium homeostasis.